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Measurement of groundwater recharge on eastern Long Island, New York, U.S.A.
Journal o f Hydrology, 79 (1985) 145--169
145
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
[5]
MEASUREMENT OF GROUNDWATER RECHARGE ON EASTERN LONG ISLAND, NEW YORK, U.S.A.
TAMMO S. STEENHUIS 1 , CRAIG D. JACKSON 1 , SAMUEL K.J. KUNG 1 and WILFRIED B R U T S A E R T 2
1Department o f Agricultural Engineering, Cornell University, Riley Robb Hall, Ithaca, N Y 14853-031 7 (U.S.A.) 2 School o f Civil and Environmental Engineering, Cornell University, Ithaca, N Y 14853-0317 (U.S.A.) (Received February 10, 1984; revised and accepted January 15, 1985)
ABSTRACT Steenhuis, T.S., Jackson, C.D., Kung, S.K.J. and Brutsaert, W., 1985. Measurement of groundwater recharge on eastern Long Island, New York, U . S . A . J . Hydrol., 79: 145--169. Two methods were tested for their suitability to provide improved estimates of recharge in the region of eastern Long Island. The two methods tested consist, first, of measuring recharge with a direct application of Darcy's law in the vadose zone and, second, of calculating recharge by closure of the hydrologic budget equation with evaporation computed from micrometeorologic data. The recharge figure, now in general use, of 50% of the annual precipitation is a longterm average at best. Our measurements of recharge, which were performed during a three-year period, showed that the vertical flux past the 1 m depth was strongly dependent on both the time of the year and the precipitation amount. In late fall, winter and early spring a high percentage of the precipitation became recharge. During the summer months there was a small net upward movement of water past the 1 m depth. Precipitation during these months did not contribute to the annual recharge. It may be concluded from our measurements that in order to estimate recharge, special attention should be given to precipitation during the winter months. A better estimate for annual recharge than the current 50% of annual precipitation might be to take approximately 75--90% of the precipitation from October 15 until May 15. The two methods used for estimating recharge were labour intensive and required experienced technicians. Currently, one method cannot be recommended above the other. Both methods give a good estimate during the year except for the winter. The closure method using micrometeorological data gives a slightly higher estimate than the direct measurement method based on Darcy's law.
INTRODUCTION
G r o u n d w a t e r is a very i m por t a nt source of fresh water t h r o u g h o u t the world. In the United States of America, 20 of the 100 largest cities use
146
groundwater as their sole supply of potable water and an additional 13 depend on groundwater to augment surface water supplies. Underground water resources also supply domestic demand for 95% of the rural population in the U.S.A. On eastern Long Island, New York, where this study was performed, groundwater is the sole source of potable water. Because of this dependence on groundwater, water management officials and the public are becoming increasingly concerned about contamination of underground water resources. These concerns have a strong foundation. There have been many d o c u m e n t e d cases of groundwater contamination by agricultural and other chemicals (Josephson, 1976; La Fleur, 1976; Hebb and Wheeler, 1978). On Long Island, New York, tests of potable water wells have revealed a considerable multiple source pollution problem. As a result, between 10 and 20% of the wells tested have been closed. McLendon (1971) summarized many of the region's water problems, including the presence of high concentrations of nitrate, synthetic detergents, and even mercury. Recent investigations have also discovered carbamate pesticides in varying amounts. Most of this pollution has been detected in the upper 2 0 - - 3 0 m , a layer consisting of glacial deposits that lies predominately above sea level and is in direct contact with the ground surface through the vadose zone. It is obvious that to prevent further contamination of groundwater resources, it will be necessary to employ the best possible management models and operation techniques. Among the more important improvements needed in the water management scheme is an accurate statement of the hydrologic balance. A major uncertainty in the balance is the natural rate of replenishment of the groundwater reservoir. This replenishment, or recharge, is that water which percolates deep into the soft, travelling through the unsaturated zone to the water table. Recharge is a complex physical phenomenon.. Some of the factors that affect recharge include intensity, duration, and seasonal distribution of precipitation, topography, vegetative cover, soil texture, layering of the deposits, animal burrows, and other determinants of permeability. At present, estimates of recharge over an area may be obtained from the following hydrologic budget equation: R
= P--E--RE--AS
(1)
where, for a certain time period, expressed as a height of water; R = recharge or deep percolation; P = precipitation; E - - a c t u a l evapotranspiration; R F = runoff, both surface and subsurface; and AS = change in soil moisture storage. The measurement of the variables in eqn. (1)is often subject to substantial error. Although the most accurately measured variable is rainfall, standard rain gages are now recognized to be subject to a b o u t 10% error. Direct runoff on Long Island has been estimated to be only 2% of precipitation, b u t this figure is the result of only one study (Cohen et al., 1968). Soil
147
moisture storage is very difficult to quantify due to problems inherent in the soil moisture measurement techniques and due to soil variability. Evapotranspiration can be estimated from climatological measurements, but results are usually understood to be accurate only over long periods of time. It is easy to see that estimates of recharge are not reliable. The present "rule of t h u m b " of annual recharge in Suffolk County is 50% of the annual precipitation (Cohen et al., 1968). The objective of this research study was to test the suitability of two separate methods for providing improved calculations of recharge in the region of eastern Long Island, New York. The two methods tested consist, first, of measuring recharge with a direct application of Darcy's equation, and second, of calculating recharge from the hydrologic budget equation (1) with evapotranspiration c o m p u t e d from micrometeorologic data. By encompassing two separate methodologies in this objective an independent check was provided for validation of recharge results.
METHODS OF ESTIMATING RECHARGE
Two methods were used to obtain the recharge term. The first method (A) is based on measurement of moisture flux in the unsaturated soil above the aquifer. The second m e t h o d (B) calculates recharge from the water balance equation. M e t h o d A : Vertical soil w a t e r s e e p a g e
Two methods are currently in use to measure the vertical flux directly in the vadose zone -- the zero flux plane method and the Darcian flux method. These t w o methods are based on a simultaneous measurement of the matric potential (with tensiometers) and moisture content (with a neutron probe). The zero plane flux m e t h o d (Arya et al., 1975; Cooper, 1980) is based on locating the point of zero hydraulic gradient in the soil profile and then summing up the changes in water content above this point. The m e t h o d has the disadvantage that during periods of high recharge and severe drought the hydraulic gradient is positive or negative but n o t zero and the method cannot be used under these conditions. Thus this m e t h o d is inadequate for the purpose of this study. The flux in the Darcian method is calculated as the product of the unsaturated hydraulic conductivity and the hydraulic gradient. The advantage of the Darcian m e t h o d is that it can be used throughout the year provided that the unsaturated hydraulic conductivity can be measured with great enough precision. Thus a successful application depends on the success of determining hydraulic conductivity. The unsaturated hydraulic conductivity is related to matric potential (or water pressure) and moisture content. The relationship between conductivity
148 and pressure, k(~), is not unique, but depends on the history of soil wetting and drying. This p h e n o m e n o n is referred to as hysteresis. For situations where hysteresis is negligible, the following relationship has been suggested by Brooks and Corey (1964): k(~)
= ks
(2)
where: ks = saturated hydraulic conductivity; ~ = matrie potential or water pressure, ~h = a constant, also referred to as air entry pressure; and b = a constant. This equation has been found to fit data well (e.g., Bresler et al., 19'/8). There is no evidence in the literature that the relationship between conductivity and moisture content, k ( O ) , exhibits significant hysteresis (e.g., Jackson et al., 1976) so that it is safe to assume that conductivity is a unique function of moisture eontent. In a review of previous studies (Brutsaert, 1967) it was found that the following relationship has been used widely: k(0) = k s ( ~ ) n
(3)
where: n = constant which depends on pore size distribution; co = effective saturation (dimensionless), defined by: -
0 --0~ 0s --0~
(4)
where 0 = soil moisture content; 0r = a constant, also referred to as residual moisture content; and 0s = saturated moisture content. The hydraulic conductivity can also be expressed as a function of count ratio of the neutron probe (an instrument which measures moisture content) by simply substituting the neutron count ratio-moisture content relationship: =
~x(CR--CRr)
(5)
into eqn. (3) viz.: k(O)
=
ksm (CR -- CRr) n
(6)
where C R -- n e u t r o n count ratio; C R ~ = residual neutron count ratio (neutron count ratio when there is no moisture in the soil); and m = a n. The count ratio can be measured very precisely over a relatively large volume of soil with a neutron probe. In an experiment in the laboratory the m a x i m u m deviation between ten measurements of moisture content was less than 0.002 cm 3 cm -3. Therefore, the hydraulic conductivity based on count ratio satisfies the condition of great precision necessary to use Darcy's law to calculate fluxes in the soft. Field determinations of unsaturated hydraulic conductivity can be made non
149 evaporation. Water pressure and moisture content are measured simultaneously at several depths during the water redistribution phase. Water flux at any point may be calculated as the rate of change in water content in the profile above it. Unsaturated conductivity can be obtained as the quotient of flux and hydraulic pressure gradient viz.: /e(0) =
- - - ~ dz
(7)
0
where t = time, H = hydraulic potential or the sum of the matric and gravity potential and z = depth coordinate. The instrumentation used for the instantaneous profile m e t h o d is also used for calculating the recharge by Darcy's flux on the same site. Laboratory measurements were used to check the unsaturated conductivity obtained from the instantaneous profile method. For these measurements, the crust m e t h o d (Veneman, 1974) was selected. The crust m e t h o d requires undisturbed field cores, and it yields conductivity values in the range in which most water movement occurs naturally. Method B: Hydrologic budget The second m e t h o d for calculating recharge at the experimental site utilizes the hydraulic budget equation (1) with recharge as the only u n k n o w n quantity. This m e t h o d is widely applied but, because of the nature of routinely available data, does not usually provide very accurate results. In this research study, however, accurate micrometeorological data with high temporal resolution were recorded and used to obtain an accurate description of precipitation (P) and evapotranspiration (E). With precipitation and evapotranspiration known, recharge was calculated with the assumption that runoff ( R F ) and the change in soil moisture storage (AS) were negligible in the highly permeable and porous soil at the Long Island experimental site. Evapotranspiration was c o m p u t e d by means of relatively simple, routine methods that were tested and calibrated against results obtained from a detailed energy budget analysis. Three separate methods were tested against the energy budget results, including Penman's (1948) equation for potential evaporation, the equilibrium evaporation concept introduced by Slatyer and McIlroy (1961), and the advection--aridity m e t h o d introduced by Brutsaert and Stricker (1979). The energy budget m e t h o d which served as a reference was similar to the one employed by Stricker and Brutsaert (1978). For a thin layer at the earth's surface the amount of energy gained must be equal to the amount of energy lost. This conservative property provides the energy budget equation, which is a good approximation and can be written as follows: LeE = Rn - - H - - G
(8)
150 where: Le = latent heat of vaporization; E = rate of evaporation; Rn = net radiation transfer rate between the ground surface and the atmosphere; H = sensible heat transfer into the atmosphere; and G = heat transfer rate into the ground. The net radiation term, Rn, is the dominant term on the right hand side (RHS) of eqn. (8) and is easily measured. The ground heat flux, G, is difficult to measure accurately and is usually quite small. In addition, ground heat flux exhibits a strong diurnal pattern that tends to cancel itself over a 24 h period. For these reasons it was assumed that G = 0. The remaining term on the RHS of eqn. (8) is the sensible heat flux, H, which can be large in proportion to Rn. In order to quantify the sensible heat flux the assumption of the uniform atmospheric boundary layer was used. This means that horizontal gradients and vertical velocities are assumed to be negligible when compared to the vertical gradients and horizontal velocities. Hence atmospheric fluxes of m o m e n t u m , and sensible and latent heat occur only in the vertical direction and horizontal advection is not present. With these simplifying assumptions and on the basis of the similarity hypothesis of Monin and Obukhov it is possible to derive relationships between the vertical gradients of wind speed and air temperature near the ground surface and the respective vertical fluxes of m o m e n t u m and sensible heat. These Monin-Obukhov relationships are well-known and have been presented in detail elsewhere (e.g., Brutsaert, 1982; eqns. (4.34) and (4.35)). The stability functions for m o m e n t u m and sensible heat ~sm and ~sh used in this study were taken to be those suggested by Dyer (1974). These were also given by Brutsaert (1982) as eqns. (4.50) and (4.51) for unstable conditions, and (4.56) and (4.57) for stable conditions. Because of the implicit relationship between U., E, H, Rn and L in the energy budget [eqn. (8)] and in the Monin-Obukhov formulation, this was done by iteration, as explained elsewhere (Brutsaert, 1982, p. 198). The routine methods tested against the energy budget results include Penman's (1948) equation for potential evaporation. The wind function f ( U ) in this equation has been given various forms in the past, but in this study the form originally proposed by Penman (1948) was used. The equilibrium evaporation concept introduced by Slatyer and McIlroy (1961) was also tested in this study. Equilibrium evaporation has been found to be a reasonably good measure of actual evaporation (i.e., Denmead and McIlroy, 1970; Wilson and Rouse, 1972) under certain conditions. The third routinely applicable m e t h o d is the so-called advection--aridity m e t h o d introduced by Brutsaert and Stricker (1979). This method results from a combination of concepts already used in several other methods (Penman, 1948; Bouchet, 1963; Priestley and Taylor, 1972) and it has the advantage that, at least conceptually, it is able to respond to varying conditions of surface moisture availability. For the constant in this equation a consensus value of a = 1.27 (e.g., Brutsaert, 1982, p. 220) was used in this study.
151 EXPERIMENTALPROCEDURE The data for this research were recorded on the grounds of Cornell University's Horticultural Research Farm, located 4 km north of Riverhead, N.Y. and 1 km south of the Long Island Sound. The farm lies on a glacial out-wash plain with soils typical of the region. Soil at the site is a Haven sandy loam. The soil profile has a sharp distinction between the top 30 cm and the rest of the profile. The plow layer has a higher percentage of the clay and silt than the subsoil (Table 1) and is compacted by traffic. The soil below 30 cm is a yellowish-brown gravelly sand with loamy reddish-brown spots. The sand has a high saturated conductivity of over 10 m per day. Below the 150 cm depth, clay lenses are found consisting of clay and sand. Clay lenses are not continuous and water at low tension may pass between the lenses. The lens structure within 1 0 m from the recharge measurement sites is sketched in Fig. 1. Red streaks of ferric material as well as iron modules were f o u n d throughout the profile. The instrumentation for determining recharge with Method A was arranged in two circular plots, each 3 m in diameter, and designated A and B. In plot A water-mercury tensiometers with millibar-calibrated scales were installed in duplicate at each of depths 30.5, 61, 91, and 1 2 2 c m (1ft. intervals). Holes were dug by auger and a slurry of a texture finer than the original soil was poured around each tensiometer shaft. A moisture gage access hole was augered in the center of plot A and a 1.5 m section of steel tubing was snugged in. Moisture readings were taken with a Troxler TM 1257 Depth Moisture Gage, a neutron-emitting device. Auger samples from nearby soil were taken for a comparison of neutron gage and gravimetric moisture determinations. Plot B was instrumented with triplicate tensiometers at depths of 30.5, 61, 91, 122 cm and duplicates at 152, 193 and 213 cm. During the winter tensiometers were filled with a one-to-one mixture of ethylene glycol (antifreeze) and water to avoid equipment damage. Glycol in tensiometers was first used by McKim et al. (1976). A 2.5 m aluminum access tube for the neutron probe moisture gauge was also installed. Unsaturated conductivity measurements were made in situ for plots A and B with the instantaneous profile method. For plot A the unsaturated hydraulic conductivities were made during May and June of 1978. Initially during the draining phase hourly readings were taken of moisture content and soil water pressure for 12 h. Readings continued at 1--4 day intervals for one month. At the end of the measurement period, plot A was dug up to obtain soil samples for gravimetric soil moisture determination in order to calibrate the neutron probe. During August 1979 an adapted instantaneous profile m e t h o d was performed on plot B. The soil in the plot was not disturbed to obtain gravimetric samples and the plot was not covered with a plastic sheet. Readings of water pressure and moisture content were stopped after four days before evaporation became significant.
TABLE 1
0--10 10--20 20--30 45--75 75--105
(cm)
Depth
250--147
147--75
2000--75
8 9 8 15 16
9 11 11 13 14
27 32 30 50 53
7 7 7 7 6
6 6 6 2 2
57 65 62 87 91
8 10 8 3 3
50--25
589--250
2000--850
850--589
Silt
Sand
Fractions (pm)
T e x t u r a l analysis o f t h e Haven s a n d y l o a m at t h e r e c h a r g e m e a s u r e m e n t site
11 9 9 4 3
25--5
18 15 18 5 4
<5
Clay
Cgn
153
i~
PLOWLAYER ~i~i~ ii~ !i~i~ li~ li!~ i~i!~i~i!~i~i~i~ !i!~ i~ !~iii~i~i!~i~i~i~ii~ !i!i~i!~ i!]~i~ i~ i!i~ !i!ii:
SAND
-lO0
/CLAY LENSES
..... I--
:
"-':
. . . :.: .
"
: ~; ~; ~~!~>i!i~!ii:i!!ii~¸~:'~ ~'~' ':
"~
,
.
•
•
.:
:
:
.
"
:
.....
~ ;:. :..::
Fig. 1. Sketch of soil profile at recharge site.
Unsaturated hydraulic conductivity was also determined from laboratory m e a s u r e m e n t s u s i n g t h e c r u s t m e t h o d . T h i s m e t h o d , p e r f o r m e d in d u p l i c a t e , u t i l i z e d c o r e s a m p l e s 1 0 c m in d i a m e t e r a n d 3 0 c m l o n g t a k e n b y p u s h i n g
154 metal cylinders into the soft. Cores were taken at depths 0--30, 30--60 and 60--90 cm. F o r the crust m e t h o d , a ceramic disk under different constant water pressures was placed on t op of the soft. The column was placed on a soil base to facilitate draining. Tensiometers located vertically at 5 cm intervals measured water pressure. Measurements were taken when the flux through the soil became constant. The undisturbed cores were then fully saturated to determine the saturated conductivity of each core. The data needed for c om put i ng recharge by means of m e t h o d B utilized micrometeorological measurements recorded at the experimental site. T hey were recorded at one-hour intervals, by means of an automatic data acquisition system. Maintenance was p e r f o r m e d on a regular basis and as needed. A calibration of the instruments was p e r f o r m e d prior to the initiation o f the project and again after data acquisition was terminated. Precipitation measurements were made with three tipping bucket gauges. One gauge was located within a pit slightly deeper than the gauge, one gauge was surrounded by a wind shield, and one gauge was located in the open w i t h o u t shielding. The wind shield used is identical to those in standard use by U.S. National Weather Service stations. Wind speeds were measured at 1 and 3 m by lightweight cup anemometers to the nearest 0 . 1 5 m h -1 with a threshold of 0 . 5 m h -1. The non-linear o u t p u t o f each a n e m o m e t e r was linearized by sensor-matched electronic translators. The o u t p u t recorded at each ho ur interval is the total wind run for the preceding hour. Wind direction was measured by a flat vaned sensor and match ed translator. Air t e m p e r a t u r e was measured to the nearest 0.2°C at l m above ground level with a thermistor m o u n t e d in an aspirated radiation shield. O u t p u t from this thermistor was linearized with a m at ched translator. The temperature difference between 1 and 3 m , was measured to the nearest 0.01°C with a thermistor m o u n t e d at 3 m in an aspirated radiation shield and c o n n e c t e d in a bridge circuit to the thermistor at 1 m. O u t p u t from the bridge circuit was linearized by the same translator used for the air temperature at 1 m. Tem pe r at ur e and t e m p e r a t u r e difference o u t p u t for each hour were provided by single instantaneous samples. Since the time constants for the thermistors are short, a hollow aluminum cylinder was inserted over each thermistor to dampen the response time of the measurement. For c o m p u t a t i o n a l purposes, the mean temperatures representing each h o u r were th en obtained by averaging the recordings taken at the beginning and end o f each hour. Net radiation flux was measured using an aspirated Fritschen-type net radiometer m o u n t e d at I m. O u t p u t from the net radiometer was integrated over the h o u r with an integrating translator. Dew p o in t temperatures to the nearest 0.5°C at 1.68 and 2 . 6 8 m above the soil surface were measured using self-heated lithium chloride dew cells m o u n t e d in aspirated radiation shields. O u t p u t from each dew cell was linearized with m a t c he d translators and recorded from single instantaneous
155 samples taken at hourly intervals. For c o m p u t a t i o n a l purposes the dew point for each h o u r was averaged from recordings taken at the beginning and end of each hour. Total atmospheric p r e s s u r e was measured using a stacked diaphragm sensor and matched translator.
RECHARGE CALCULATION -- RESULTS
Method A The d eter min a t i on of the unsaturated hydraulic conductivity is an integral part o f the recharge measurements. Initially the analysis of the results of the two instantaneous profile m e t h o d s for plots A and B was complicated by the fact that residual neut r on c o u n t ratios (CRr) varied with dept h and between b o t h plots. Residual n e u t r o n c o u n t ratio is the background reading at zero moisture content. Thus absolute moisture contents are n o t know n but differences in moisture c o n t e n t in time at the same depth still can be calculated. Fluxes and resulting unsaturated hydraulic conductivities can be calculated as usual but the unsaturated hydraulic conductivities could n o t be expressed directly as a f unc t i on of moisture content. Instead, to obtain this expression and the residual n e u t r o n c o u n t ratios the k(O ) vs. c o u n t ratio (CR) relationship for the 30.5--61, 61--91 and the 9 1 - - 1 2 2 c m depths for b o th plots were regressed according to eqn. (6) with various trial values of residual n e u t r o n c o u n t ratios. The set of residual n e u t r o n c o u n t ratios was selected such th a t the k(O ) vs. (CR -- CRr) was identical for the three depths in b o th plots and when substituted in eqn. (5) the moisture contents determined with the n e u t r o n probe agreed with the observed gravimetric moisture contents. The selected set of residual c o u n t ratios is shown in Table 2. Values o f unsaturated hydraulic conductivity calculated with the crust m e t h o d (overlapping the instantaneous profile m e t h o d in the 1 5 - - 8 0 m b range) agreed well with the measurements in situ as exemplified in Fig. 2 for the 60--90 cm depth. The unsaturated hydraulic conductivity values for the 30--120 cm depth according to the relationships expressed in eqns. (2), (3) and (6) were f o u n d to be: k(~P) = 7.7 x 106(~t) -2"7
r 2 -- 75%
(9)
k(O)
r 2 = 97%
(10) (11)
= 5 . 0 x 106 x ( 0 - - 0 . 0 0 5 ) s'22
k(CR) = 1 . 4 x 1 0 3 ( C R - C R r ) s'22
r 2 = 97%
where the conductivity k is expressed in cm per day; the water pressure in cm and the moisture c o n t e n t in cm 3 cm -3 . The unsaturated hydraulic conductivity as a function of the matric potential has, as expect ed due to hysteresis, the lowest correlation coefficient. It is of interest to not e t hat
156 TABLE 2 R e s i d u a l c o u n t r a t i o s ( C R r ) for p l o t s A a n d B Depth
plot A
plot B
0.185 0.053 0.109
0.196 0.174 0.053
(cm) 60 90 120
15
o x JL
LABORATORY (CRUST METHOD) FIELD (INST. METHOD 1978) FIELD (INST. METHOD 1979)
12 @ >I--.
-I -
@
9
,,.,, Z 0 u
6
n,,,,
>,',r"
3
@
lip
°o
I
i
ao
J'~A I
.L
** % 2 . . . . . . . ~0
MATRIC
SUCTION
:
60
do
i
,;o
(MBARS)
Fig. 2. U n s a t u r a t e d c o n d u c t i v i t y as a f u n c t i o n o f m a t r i c s u c t i o n for a H a v e n loam at t h e 6 0 - - 9 0 cm d e p t h .
the value of the exponent in eqn. (9) approximates very closely the value of 2.6 proposed by Bresler et al. (1978) as a universal value. With the unsaturated hydraulic conductivity function known, the recharge to the aquifer was calculated from measurements of the matric potential and neutron count ratio. These measurements were made bi-weekly over the periods June through October 1978; March through June 1979; January 1980 through June 1980 and during November and December of 1980. Analysis of the instantaneous profile data showed that the values of hydraulic gradient varied linearly with time after a rainfall while the -
-
157 conductivity varied linearly with the square root of time. Therefore, the computation of recharge volume in the period between two measurements employed a geometric mean of conductivities and a straight average of the hydraulic gradients with time, viz.:
R =
(kt, kt2)ln
.1-2 "
~z~H tl +
-~z t2 ( t 2 - - t l )
(12)
where, R -- quantity of water that flowed past a certain depth over a time interval ( t 2 - - t l ); tl - - t i m e of first measurement; t2 = time of second measurement. Triplicate measurements of soil water pressure at the same depth were often different during prolonged dry periods. Differences of up to 3 0 m b were noted at these times. In comparison, differences of 0 - - 4 m b were observed during periods of downward flux. Tensiometer readings obviously in error were omitted. Averages of the remaining measurements were used in calculations. For days when measurements were available, the instantaneous recharge rate was calculated for the 90--120 cm depth. At this depth most rapid fluctuations of the flux rate at the surface were dampened out and therefore gave a better estimate with the bi-weekly measurement schedule. The hydraulic conductivity, k, was based on neutron probe count ratio, CR, when those data were available and on water pressure, 4 , whenever CR data were not available. The k (CR) relationship [eqn. (11)] is preferred over the k ( ~ ) relationship [eqn. (9)] because the former has a higher correlation coefficient and is hysteresis-free. Cumulative vertical flux at the 90--120 cm depth is shown with cumulative rainfall in Figs. 3--6. Figure 3 shows that there was, from June through the end of September 1978, equal up and downward movement at 90 cm resulting in zero net recharge. In the period of March 10 through June 1, 1979, a significant portion of the rainfall became recharge (Fig. 4). The higher recharge than cumulative rainfall in the beginning of the period was caused by drainage of the profile above the 120 cm depth of rainfall which fell before the data collection period started. The water movement below the 120 cm depth during the 75 days in spring amounted to 50--60% of the total rainfall. The plot was only sparsely vegetated during this period. An abundant, actively transpiring crop would be expected to allow a smaller proportion of the rainfall to become recharge. During January and February of 1980 the precipitation was below normal. Part of the precipitation which fell in the form of snow was removed from the site by wind. During February there was a net upwards m o v e m e n t of water to the frost front (Fig. 5). March and April 1980 were high recharge months. Starting in May the recharge leveled off. This was earlier than in 1979 resulting from lower precipitation in May 1980 than May 1979. Although not shown, there was a small upward flux during
158
5C
4O
=o 3O g
n-
.~
I_S
g IO o
u
C-
/
20
Rechorge
~
0
I
June
I
July
I
August
__Y I
September
I
October
Fig. 3. P r e c i p i t a t i o n a n d r e c h a r g e f r o m J u n e t h r o u g h O c t o b e r 1978.
25 Precipitotion 2O
Rechorge
~
~0
.&
~ 5
u o
I,/
Morch
I
I
April
I
May
Fig. 4. P r e c i p i t a t i o n a n d r e c h a r g e d u r i n g spring 1 9 7 9 .
159
600
Precipitolion ~ f o
S
40.C
g o
'&
Q.
20.0
3
E u
0
I Jonuory
I Februory
I Morch
I April
I Moy
June
Fig. 5. Precipitation and recharge from January through June 1980.
I ~ 15.0 ~' ~25 Precipitotion~ I0.0 i
I
7.5i 50
Rechorge
._o 2,5 u
I November
December
Fig. 6. Precipitation and recharge during November and December 1980.
July and August 1980 and a small downward flux during September and October. The soil was rather dry during the beginning of November. A portion of the rainfall during November was stored in the profile above 90 cm rather than being lost as vertical percolation (Fig. 6). The recharge pattern was similar to that observed by Rehm et al. (1982) in the prairie region in North Dakota. However, the quantity of recharge on Long Island is at least five times as high.
160 Method B
The hydrologic budget m e t h o d for determining recharge was applied to the experimental data for the period from December 27, 1979 through July 17, 1981. The data record for this period contains a period of 3½ months, July 16, 1980 through October 30, 1980 for which data were not recorded due to a malfunction in the automatic data recorder. Several other portions of the record did not contain all data parameters necessary for applying the energy budget analysis. The energy budget m e t h o d was also not applicable when the wind was incident between 10 ° (magnetic) and 235 °. This restriction is due to physical obstructions upwind in this general direction that could cause violation of the assumption requiring a large uniform surface. A perusal of all days for which data were available was performed to select days for which the energy budget analysis could be expected to produce valid evaporation results. This selection process provided 68 days to which the energy budget was applied. The routine methods, Penman's equation, the equilibrium evaporation equation and the advection-aridity method equation, were also applied to the 68 days. All four methods computed evaporation on an hourly basis, from which daily values were obtained. Results of the four evaporation methods are presented in Table 3. Figures 7--9 compare the results of the three routine methods with the energy budget results. These figures show that for eastern Long Island none of the routine methods are perfect predictors of evaporation. At low rates of evaporation the equilibrium evaporation and the Penman equation seem to provide the best results. At high evaporation rates the equilibrium method and the advection--aridity m e t h o d both appear to provide fairly good results. It can be seen from Table 4 that all three equations have reasonably good correlations with energy budget evaporation throughout the year. There is, however, a large degree of scatter during the winter and spring, as evidenced b y the high standard deviations. During the summer the standard deviations drop considerably. The advection--aridity method, with a = 1.27, does not appear to be a good predictor of evaporation during the winter and spring, but during the summer its performance was seen to improve substantially. The advection--aridity method gives the best overall prediction of the evaporation for the entire period. Still, Table 4 shows that on a seasonal basis, Ep and Ee can be used with the regression equations to give more reliable predictions of E than Eaa. To c o m p u t e a complete recharge time series for the experimental site, the routine methods which had the smallest standard deviation and the highest correlation, were applied to each day in the data base. Thus, for the winter months the regression relationship: E = 1.682Ee
(13)
161
I0 9 8 7
6
4
:5 2
0 -I -2 -2
!
!
I
!
!
i
|
I
|
I
[
-I
0
I
2
3
4
5
6
"/'
8
9
Energy Budget
I0
ET (mmldoy)
Fig. 7. Scatter plot showing comparison between evaporation rate computed with Penman equation and with the energy budget method.
was used to determine daily evaporation from hourly data, and similarly for the spring months: E = 0.706Ep
(14)
and for the summer months: E = 1.102Ee
(15)
These regression relationships were derived from data taken over a wide range of soil moisture conditions and should provide reasonable values for the study period. It should be noted, however, that precipitation during the study period was somewhat below normal. Therefore these regression relationships cannot be considered general and their use should be limited to the study period as well as to the area of study. Since the predictive ability of the routine methods seems to vary with the rate of evaporation, regressions were performed with the data separated into winter, spring and summer data. The results of the recharge calculations are presented as m o n t h l y sums in Table 5 along with measured and average m o n t h l y precipitation values. The recharge pattern is the same as for m e t h o d
162 TABLE 3 Computed values for energy budget (Eb) , potential evaporation (Ep), equilibrium evaporation (Ee) and evaporation based on advection--aridity method (Eaa) in nm per day Date
Eb
1/19/80 1/20/80 1/21/80
--0.470 0.167 1.742
3/4/80 3/6/80 3/9/80 3/16/80 3/19/80
Ep
Ee
Eaa
0.806 1.598 1.975
--0.467 --0.689 --0.531
--1.992 --3.349 --3.325
0.096 1.046 1.060 0.187 0.865
1.679 2.020 1.992 2.137 2.385
0.397 0.755 0.641 0.683 0.917
--0.671 --0.101 --0.365 --0.403 --0.056
1/4/81 1/5/81 I/7/81 1/19/81 1/23]81 1/24/81 1/26/81 1/27/81
--1.778 --1.525 --1.865 1.509 1.798 1.416 1.317 2.081
0.347 0.525 --0.355 1.219 1.661 1.612 1.188 2.030
--0.426 --0.495 --0.975 0.450 0.737 0.605 0.328 0.959
--1.430 --1.781 --2.121 --0.074 0.212 --0.076 --0.355 0.407
2/17/81 2/18/81
1.607 1.668
1.510 1.532
0.689 0.819
0.241 0.548
3/9/81 3/10/81 3/11/81 3/12/81 3/13/81 3/14]81 3/15/81 3/20/81 3/21/81 3/24/81 3/29/81 3/30/81 3/31/81
0.708 1.279 0.808 1.947 1.827 1.838 2.151 2.002 1.802 2.834 3.802 3.159 4.487
1.175 2.169 1.542 2.383 2.588 3.551 3.423 2.309 2.097 3.069 4.808 2.931 5.836
0.436 1.012 0.726 1.216 0.940 0.828 0.945 1.268 1.018 1.744 2.105 1.356 2.860
--0.067 0.403 0.302 0.706 --0.201 -- 1.447 --1.023 0.912 0.490 1.360 0.539 0.514 1.427
4]6]81 4/7/81 4/10/81 4/15/81 4/18181 4/19/81 4/20/81 4/21181 4122/81 4/25/81 4/26/81
2.982 4.162 4.830 3.595 4.392 3.543 2.145 3.361 2.572 2.553 3.857
3.407 4.869 5.828 4.855 5.669 5.057 3.658 5.189 4.396 3.066 4.760
1.598 2.261 2.725 2.182 2.851 2.706 1.606 2.332 2.401 1.616 3.040
0.652 0.873 1.092 0.687 1.573 1.817 0.421 0.735 1.703 1.039 2.962
5/13/81 5/17/81
3.889 4.625
6.497 6.325
3.866 4.107
3.324 4.108
(Continued)
163 TABLE 3 (Continued} Date
Eb
Ep
Ee
Eaa
5/23181 5/24/81 5/25/81 5/26/81
4.133 5.207 3.877 3.926
6.023 7.164 8.013 5.999
4.167 4.586 4.596 3.943
4.561 4.485 3.660 4.016
6/4/81 6/5181 6/6/81 617/81 6/9/81 6/10/81 6/11]81 6/17/81 6/26/81
4.894 5.628 3.578 5.828 4.781 2.669 5.710 5.395 6.300
5.719 6.867 5.316 7.779 6.955 4.206 7.293 7.193 7.738
4.167 5.158 3.338 5.038 3.449 1.960 4.908 5.118 5.049
4.864 6.234 3.164 5.018 1.805 0.771 5.172 5.806 5.086
7/5/81 7]6/81 7/7/81 7]8]81 7]9]81 7/11/81 7/13/81 7/14/81 7/15/81 7/16181 7/17/81
3.718 5.978 6.187 6.384 6.456 6.549 3.779 5.153 5.123 5.090 4.110
4.094 6.919 8.008 9.073 9.193 8.034 6.132 8.284 7.331 7.691 6.463
2.814 5.629 5.626 5.711 5.735 5.457 4.052 4.949 5.020 5.162 4.756
3.054 7.379 6.282 5.433 5.374 5.826 4.161 4.287 5.420 5.421 5.616
A, high during w i n t e r a n d negative during t h e s u m m e r . T h e negative evapor a t i o n values o b s e r v e d during s o m e w i n t e r m o n t h s are p r o b a b l y t h e result o f local m o i s t u r e a d v e c t i o n . L o n g Island is s u r r o u n d e d b y w a t e r a n d d u r i n g w i n t e r relatively w a r m m o i s t o c e a n winds b l o w i n g across t h e c o l d e r land surface m a y cause c o n d e n s a t i o n a n d dew. In a d d i t i o n , t h e local advective s i t u a t i o n resulting f r o m t h e p r o x i m i t y o f t h e s h o r e (1 k m ) , m a y on o c c a s i o n h a v e i n v a l i d a t e d t h e a s s u m p t i o n o f a large u n i f o r m surface. An e v a p o r a t i o n t i m e series was c o m p u t e d using t h e regressions d e f i n e d a b o v e . T h e t i m e series was t h e n s u m m e d o n a m o n t h l y basis a n d used to d e t e r m i n e m o n t h l y r e c h a r g e values using eqn. (1). F o r s o m e d a y s t h e d a t a r e c o r d was i n c o m p l e t e a n d t h e e v a p o r a t i o n t i m e series d i s c o n t i n u o u s . T o c o m p l e t e t h e t i m e series an average daffy evapor a t i o n r a t e was c o m p u t e d f o r each m o n t h f r o m t h e d a t a available f o r t h a t m o n t h . This average daily e v a p o r a t i o n was t h e n a p p l i e d t o t h e d a y s w i t h i n t h a t m o n t h f o r w h i c h d a t a w e r e u n a v a i l a b l e a n d t h e r e c h a r g e t i m e series was c o m p l e t e d . T h e m a x i m u m n u m b e r o f d a y s in o n e m o n t h t o w h i c h this t e c h n i q u e was a p p l i e d was f o u r during J u n e o f 1 9 8 1 . I t s h o u l d b e n o t e d t h a t t h e p r e c i p i t a t i o n r e c o r d was c o m p l e t e .
164 I0 9 8
7 O "O
6 I
E E
t
SI
°
5
I--
4
E
3
'r-
~g 2 hi
I
-I -2_2
L
0
I
I
I
I
2 3 4 5 6 Energy ~dget El" (mm/doy)
I
I
I
7
8
9
I0
Fig. 8. Scatter plot showing comparison between equilibrium evaporation rate and evaporation rate computed with the energy budget method.
COMPARISON OF METHODS A AND B The measurement periods of the two methods overlap during 1980. The recharge calculated by both methods can be compared during this time interval. The micrometeorological m e t h o d (B) calculates the recharge as the difference between precipitation and evaporation and thus, gives the vertical flux at the surface of the soft. Method A measures the flux at the 90-120 cm depth. Thus, in order to compare both methods, fluxes should be considered at the same depth. To do this we added the change in storage above the average flux plane at 105 cm to the recharge calculated with m e t h o d B (Table 6). Table 6 shows that with the exception of January and December the observed fluxes with both methods show the same trend. During January and December (of the year of measurement) a portion of the precipitation fell in the form of snow. Snow measurements with rain gauges are notoriously inaccurate. Moreover, the snow blew off the recharge plot in most cases and did not contribute to recharge calculated with m e t h o d A and gives, therefore, a lower estimate than m e t h o d B. Also, during the
165 10'
8
6 O
4 b-tAJ C O
2
"O
"~ ~,0
,., ,.:
0
•
,
-2
.4 4
I -2
i 0
i 2
Energy Budget
l 4
I 6
I B
I0
ET (mrn/doy)
Fig. 9. S c a t t e r p l o t s h o w i n g c o m p a r i s o n b e t w e e n e v a p o r a t i o n r a t e c o m p u t e d advection--aridity and with the energy budget method.
with the
TABLE 4 Relationship between applicable methods
energy budget evaporation E(mm
per day), and the routinely
Period
N o . Pts.
Regressiona
Correlation
overall
68
E -= 0 . 7 1 9 E p + 0 . 7 1 4 E -= 1 . 1 5 4 E e ± 0 . 8 2 5 E = 1.020Eaa ± 1.771
r -- 0 . 9 4 3 r = 0.938 r = 0.876
winter
24
E -= 0 . 6 2 6 E p + 0 . 8 8 4 E = 1.682E e ± 0.803 E -= 0 . 0 4 5 E a a -+ 1 . 4 9 3
r = 0.771 r = 0.772 r = 0.484
spring
24
E -= 0 . 7 0 6 E p ± 0 , 6 3 1 E = 1.241E e ± 0.916 E = 1.343Eaa ± 1.897
r = 0.839 r = 0.778 r = 0.605
summer
20
E :- 0 . 7 3 5 E p ± 0 . 5 5 8 E -- 1 , 1 0 2 E e + 0 . 5 2 3 E = 1 . 0 2 3 E a a -+ 1 . 1 1 7
r -- 0 . 8 6 5 r = 0.893 r = 0.739
a E p : P e n m a n ' s ( 1 9 4 8 ) e q u a t i o n ; E e ---- e q u i l i b r i u m e v a p o r a t i o n ( S l a t y e r a n d M c I l r o y , 1 9 6 1 ) ; a n d Eaa = a d v e c t i o n - - a r i d i t y m e t h o d ( B r u t s a e r t a n d S t r i c k e r , 1 9 7 9 ) .
166 TABLE 5 Evaporation and recharge computed with the closure method Period
Days of data
Averagea precip, (cm)
Observed precip. (cm)
E (cm)
R (cm)
12/79 1/80 2/80 3/80 4/80 5/80 6/80 7/80
5 31 29 31 30 31 30 16
1.73 8.86 8.46 10.59 8.86 8.69 6.88 4.34
0.00 4.20 2.69 17.62 13.94 4.86 6.79 1.75
0.02 --1.37 --0.23 2.36 4.69 6.56 8.42 5.64
--0.02 5.57 2.92 15.26 9.25 --1.71 --1.63 --3.89
10/80 11/80 12/80 1/81 2/81 3/81 4/81 5/81 6/81 7/81
2 30 31 31 28 31 30 31 30 17
0.51 11.43 10.72 8.86 8.46 10.59 8.86 8.69 6.88 4.61
0.06 9.18 1.30 3.12 13.85 1.97 10.91 7.70 10.77 1.47
0.12 1.04 --2.22 0.15 2.07 4.23 7.85 10.92 13.27 8.55
--0.07 8.14 3.52 2.98 11.77 --2.25 3.06 --3.22 --2.49 --7.08
a Values from NOAA Climatological Data -- 1980. Annual summary. Average precipitation for 12/79, 7/80, 10/80 and 7/81 is adjusted to the number of days,
w i n t e r m o n t h s m e a s u r e m e n t o f solar r a d i a t i o n (and, t h e r e f o r e , e v a p o r a t i o n ) has t h e g r e a t e s t d e g r e e o f u n c e r t a i n t y . During t h e r e m a i n d e r o f 1 9 8 0 b o t h m e t h o d s agree r a t h e r well (97% c o r r e l a t i o n c o e f f i c i e n t ) . This is especially t r u e w h e n t h e great n u m b e r o f possible m e a s u r e m e n t errors is t a k e n into a c c o u n t . Unlike s i m u l a t i o n m o d e l s w h e r e t h e r e c h a r g e is a l w a y s a f r a c t i o n o f t h e p r e c i p i t a t i o n b o t h m e a s u r i n g m e t h o d s are n o t r e s t r a i n e d b y t h e rainfall. T h e m o n t h w i t h t h e highest r e c h a r g e s h o w s t h e least a m o u n t o f error. D u r i n g t h e s e m o n t h s t h e m o i s t u r e c o n t e n t and h y d r a u l i c c o n d u c t i v i t y w e r e in t h e s a m e r a n g e as t h e i n s t a n t a n e o u s p r o f i l e m e t h o d a n d t h e r e f o r e c o u l d be e x p e c t e d to be m o s t a c c u r a t e . T a b l e 6 s h o w s t h a t t h e r e c h a r g e p r e d i c t e d b y m e t h o d B is generally higher t h a n b y m e t h o d A. A n e x p l a n a t i o n f o r this p h e n o m e n o n m i g h t be f o u n d in t h e n o n l i n e a r i t y o f b o t h t h e e v a p o r a t i o n and r e c h a r g e e s t i m a t e s . N o n linearity t e n d s t o u n d e r e s t i m a t e t h e a r i t h m e t i c m e a n . T h u s , f o r m e t h o d B t h e e v a p o r a t i o n is u n d e r e s t i m a t e d t h e r e b y o v e r e s t i m a t i n g t h e recharge. F o r m e t h o d A t h e r e c h a r g e itself is u n d e r e s t i m a t e d . F r o m t h e s e c o m p a r i s o n s , it seems t h a t b o t h m e t h o d s give a r e a s o n a b l e i n d i c a t i o n o f e x p e c t e d recharge d u r i n g t h e y e a r e x c e p t in t h e winter.
5.6 2.9 15.3 9.3 --1.7 --1.6 8.1 3.5
~- 4 --0.7 --1.4 + 1.1 + 5.7 +4.6 --6.5 + 3.3
C h a n g e in m o i s t u r e c o n t e n t a b o v e flux p l a n e at 105 c m (cm) a 9.6 2.2 13.9 10.4 4.0 3.0 1.6 6.8
F l u x at 105 c m by method B (cm per month)
3.1 0.4 13.7 11.4 2.4 0.9 1.6 2.4
F l u x a t 105 c m by method A (cm per month)
4.2 2.7 17.6 13.9 4.9 6.8 9.2 1.3
Precipitation (cm per month)
a A positive sign m e a n s t h a t t h e profile was drier a t t h e e n d o f t h e m o n t h t h a n t h e b e g i n n i n g ; a negative sign is t h e reverse.
January February March April May June November December
F l u x at surface by method B ( c m per m o n t h )
Recharge estimates by methods A and B
TABLE 6
168
However, when negative evaporation is set to zero in m e t h o d B, agreement between the two methods during the winter is closer. More research is needed in the correctness of both methods during the winter.
CONCLUDING REMARKS
One of the objectives of this research was to validate the presently used recharge estimate on Long Island of 50% of the annual precipitation. The relatively short duration of the experiment (three years) prevents us from making definitive statements. However, our measurements are the only available direct determination of recharge on Long Island. These data may provide long-term estimates when used by an appropriate computer simulation model. The measurements suggest that the general estimate of 50% of annual precipitation is a long-term average at best. The vertical flux past the l m depth was strongly dependent on both time of the year and precipitation amount. In late fall, winter and early spring a high percentage of the precipitation became recharge. During the summer months there was a small net upward movement of water past the l m depth. Precipitation during these months did not contribute to the annual recharge. Although it is difficult to draw general conclusions from our measurements, it seems that in order to estimate recharge special attention should be given to precipitation during the winter months. Therefore, in our view a better estimate might be that 75--90% of the precipitation from October 15 to May 15 becomes recharge while there is practically no recharge during the remaining part of the year. The exact value depends on crop cover used. The two methods used for estimating recharge were labour intensive and required experienced technicians. Currently, one m e t h o d cannot be recommended above the other. Both methods give a good estimate during the year except for the winter. The closure m e t h o d using micrometeorological data gives a slightly higher estimate than the direct measurement m e t h o d based on Darcy's law. The difference can probably be attributed to the non-linearity of the measurement and the need to use arithmetic averages. More research is needed to determine why both methods deviated during the winter. REFERENCES Arya, L.M., Farrel, D.A. and Blake, G.R., 1975. A field study of soil water depletion patterns in presence of growing soybean roots: I. determination of hydraulic properties of the soil. Soil Sci. Soc. Am., Proc., 39: 424--430. Bouchet, R.J., 1963. Evapotranspiration r4elle et potentielle, signification climatique. General Assembly Berkeley, Int. Assoc. Sci. Hydrol., Gentbrugge, Publ. 6 2 : 1 3 4 - - 1 4 2 . Bresler, E., Russo, D. and Miller, R.D., 1978. Rapid estimate of unsaturated hydraulic conductivity function. Soil Sci. Soc. Am., J. 42: 170--172. Brooks, R.H. and Corey, A.T., 1964. Hydraulic properties of porous media. Hydrol. Paper, No. 3, Colorado State Univ., Ft. Collins, Colo.
169 Brutsaert, W., 1967. Some methods of calculating unsaturated permeability. Trans. Am. Soc. Agric. Eng., 19: 400--404. Brutsaert, W., 1982. Evaporation into the Atmosphere: Theory, History and Applications. Reidel, Dordrecht. Brutsaert, W. and Stricker, H., 1979. An advection--aridity approach to estimate actual regional evaporation. Water Resour. Res., 15: 443-- 450. Cohen, P., Franke, O.L. and Foxworthy, B.L., 1968. An atlas of Long Island's water resources. U.S. Geol. Surv. and New York State Water Resources Commission. New York Water Resour. Comm., Bull. 62,117 pp. Cooper, J.D., 1980. Measurement of moisture fluxes in unsaturated soil in Thelford Forest. Inst. Hydrol., Rep. No. 66, Wallingford, Oxon. Denmead, O.T. and McIlroy, I.C., 1970. Measurement of non-potential evaporation from wheat. Agric. Meteoroh, 7 : 285--302. Dyer, A.J. 1974. A review of flux--profile relationships. Boundary-Layer Meteorol., 1: 363--372. Hebb, E.A. and Wheeler, W.B., 1978. Bromocil in Lakeland soil groundwater. J. Environ. Qual., 7 : 598--601. Jackson, R.D., Idso, S.B. and Reginato, R.J. 1976. Calculation of evaporation rates during transition from energy-limiting to soil-limiting phases using albedo data. Water Resour. Res., 12: 23--26. Josephson, J., 1976. Quality assurance for groundwater. Environ. Sci. Technol., 10: 226--227. La Fleur, K.S., 1976. Movement of carbaryl through Congaree soil into groundwater. J. Environ. Quah, 5: 91--92. McKim, H.L., Berg, R.L., McGraw, R.W., Atkins, R.T. and Ingersoll, J., 1976. Development of remote reading tensiometer/transducer system for use in sub-freezing temperatures. Proc. Sec. Conf. on Soil Water Problems in Cold Regions, Edmonton, Alta. McLendon, S.C., 1971. Water resources of Nassau and Suffolk counties. Water and Sewage Works, 118: 34--36. Obukhov, A.M., 1946. Turbulence in an atmosphere with non-uniform temperature. Engl. transl., Boundary-Layer Meteorol., 1971: 2, 7--29. Ogata, G. and Richards, L.A., 1957. Water content changes following irrigation of bare field soil that is protected from soil water changes. Soil Sci. Soc. Am. Proc., 21: 355--356. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. Lond., A193: 120--145. Priestley, C.H.B. and Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev., 100: 81--92. Rehm, B.W., Moran, S.R. and Graenewold, G.H., 1982. Natural groundwater recharge in an upland area of central North Dakota, U.S.A.J. Hydrol., 59: 293--314. Slatyer, R.O. and McIlroy, I.C., 1961. Practical Microclimatology. CSIRO, Melbourne, 310 pp. Stricker, H. and Brutsaert, W., 1978. Actual evapotranspiration over a summer period in the Hupsel catchment. J. Hydrol., 39: 139--157. Veneman, P.L.M., 1974. Measurements of soil moisture retention characteristics. In: J. Bouma, F.G. Baker and P.L.M. Veneman (Editors). Measurement of Water Movement in Soil Pedons Above the Water Table. Inf. Cir., No. 27, Univ. Wisconsin, Madison, Wisc. Watson, K.K., 1966. An instantaneous profile method for determining hydraulic conductivity of unsaturated porous materials. Water Resour. Res., 2 : 7 0 9 - - 7 1 5 . Wilson, R.G. and Rouse, W.R., 1972. Moisture and temperature limits of the equilibrium evapotranspiration model. J. Appl. Meteoroh, 11: 436--442.
145
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
[5]
MEASUREMENT OF GROUNDWATER RECHARGE ON EASTERN LONG ISLAND, NEW YORK, U.S.A.
TAMMO S. STEENHUIS 1 , CRAIG D. JACKSON 1 , SAMUEL K.J. KUNG 1 and WILFRIED B R U T S A E R T 2
1Department o f Agricultural Engineering, Cornell University, Riley Robb Hall, Ithaca, N Y 14853-031 7 (U.S.A.) 2 School o f Civil and Environmental Engineering, Cornell University, Ithaca, N Y 14853-0317 (U.S.A.) (Received February 10, 1984; revised and accepted January 15, 1985)
ABSTRACT Steenhuis, T.S., Jackson, C.D., Kung, S.K.J. and Brutsaert, W., 1985. Measurement of groundwater recharge on eastern Long Island, New York, U . S . A . J . Hydrol., 79: 145--169. Two methods were tested for their suitability to provide improved estimates of recharge in the region of eastern Long Island. The two methods tested consist, first, of measuring recharge with a direct application of Darcy's law in the vadose zone and, second, of calculating recharge by closure of the hydrologic budget equation with evaporation computed from micrometeorologic data. The recharge figure, now in general use, of 50% of the annual precipitation is a longterm average at best. Our measurements of recharge, which were performed during a three-year period, showed that the vertical flux past the 1 m depth was strongly dependent on both the time of the year and the precipitation amount. In late fall, winter and early spring a high percentage of the precipitation became recharge. During the summer months there was a small net upward movement of water past the 1 m depth. Precipitation during these months did not contribute to the annual recharge. It may be concluded from our measurements that in order to estimate recharge, special attention should be given to precipitation during the winter months. A better estimate for annual recharge than the current 50% of annual precipitation might be to take approximately 75--90% of the precipitation from October 15 until May 15. The two methods used for estimating recharge were labour intensive and required experienced technicians. Currently, one method cannot be recommended above the other. Both methods give a good estimate during the year except for the winter. The closure method using micrometeorological data gives a slightly higher estimate than the direct measurement method based on Darcy's law.
INTRODUCTION
G r o u n d w a t e r is a very i m por t a nt source of fresh water t h r o u g h o u t the world. In the United States of America, 20 of the 100 largest cities use
146
groundwater as their sole supply of potable water and an additional 13 depend on groundwater to augment surface water supplies. Underground water resources also supply domestic demand for 95% of the rural population in the U.S.A. On eastern Long Island, New York, where this study was performed, groundwater is the sole source of potable water. Because of this dependence on groundwater, water management officials and the public are becoming increasingly concerned about contamination of underground water resources. These concerns have a strong foundation. There have been many d o c u m e n t e d cases of groundwater contamination by agricultural and other chemicals (Josephson, 1976; La Fleur, 1976; Hebb and Wheeler, 1978). On Long Island, New York, tests of potable water wells have revealed a considerable multiple source pollution problem. As a result, between 10 and 20% of the wells tested have been closed. McLendon (1971) summarized many of the region's water problems, including the presence of high concentrations of nitrate, synthetic detergents, and even mercury. Recent investigations have also discovered carbamate pesticides in varying amounts. Most of this pollution has been detected in the upper 2 0 - - 3 0 m , a layer consisting of glacial deposits that lies predominately above sea level and is in direct contact with the ground surface through the vadose zone. It is obvious that to prevent further contamination of groundwater resources, it will be necessary to employ the best possible management models and operation techniques. Among the more important improvements needed in the water management scheme is an accurate statement of the hydrologic balance. A major uncertainty in the balance is the natural rate of replenishment of the groundwater reservoir. This replenishment, or recharge, is that water which percolates deep into the soft, travelling through the unsaturated zone to the water table. Recharge is a complex physical phenomenon.. Some of the factors that affect recharge include intensity, duration, and seasonal distribution of precipitation, topography, vegetative cover, soil texture, layering of the deposits, animal burrows, and other determinants of permeability. At present, estimates of recharge over an area may be obtained from the following hydrologic budget equation: R
= P--E--RE--AS
(1)
where, for a certain time period, expressed as a height of water; R = recharge or deep percolation; P = precipitation; E - - a c t u a l evapotranspiration; R F = runoff, both surface and subsurface; and AS = change in soil moisture storage. The measurement of the variables in eqn. (1)is often subject to substantial error. Although the most accurately measured variable is rainfall, standard rain gages are now recognized to be subject to a b o u t 10% error. Direct runoff on Long Island has been estimated to be only 2% of precipitation, b u t this figure is the result of only one study (Cohen et al., 1968). Soil
147
moisture storage is very difficult to quantify due to problems inherent in the soil moisture measurement techniques and due to soil variability. Evapotranspiration can be estimated from climatological measurements, but results are usually understood to be accurate only over long periods of time. It is easy to see that estimates of recharge are not reliable. The present "rule of t h u m b " of annual recharge in Suffolk County is 50% of the annual precipitation (Cohen et al., 1968). The objective of this research study was to test the suitability of two separate methods for providing improved calculations of recharge in the region of eastern Long Island, New York. The two methods tested consist, first, of measuring recharge with a direct application of Darcy's equation, and second, of calculating recharge from the hydrologic budget equation (1) with evapotranspiration c o m p u t e d from micrometeorologic data. By encompassing two separate methodologies in this objective an independent check was provided for validation of recharge results.
METHODS OF ESTIMATING RECHARGE
Two methods were used to obtain the recharge term. The first method (A) is based on measurement of moisture flux in the unsaturated soil above the aquifer. The second m e t h o d (B) calculates recharge from the water balance equation. M e t h o d A : Vertical soil w a t e r s e e p a g e
Two methods are currently in use to measure the vertical flux directly in the vadose zone -- the zero flux plane method and the Darcian flux method. These t w o methods are based on a simultaneous measurement of the matric potential (with tensiometers) and moisture content (with a neutron probe). The zero plane flux m e t h o d (Arya et al., 1975; Cooper, 1980) is based on locating the point of zero hydraulic gradient in the soil profile and then summing up the changes in water content above this point. The m e t h o d has the disadvantage that during periods of high recharge and severe drought the hydraulic gradient is positive or negative but n o t zero and the method cannot be used under these conditions. Thus this m e t h o d is inadequate for the purpose of this study. The flux in the Darcian method is calculated as the product of the unsaturated hydraulic conductivity and the hydraulic gradient. The advantage of the Darcian m e t h o d is that it can be used throughout the year provided that the unsaturated hydraulic conductivity can be measured with great enough precision. Thus a successful application depends on the success of determining hydraulic conductivity. The unsaturated hydraulic conductivity is related to matric potential (or water pressure) and moisture content. The relationship between conductivity
148 and pressure, k(~), is not unique, but depends on the history of soil wetting and drying. This p h e n o m e n o n is referred to as hysteresis. For situations where hysteresis is negligible, the following relationship has been suggested by Brooks and Corey (1964): k(~)
= ks
(2)
where: ks = saturated hydraulic conductivity; ~ = matrie potential or water pressure, ~h = a constant, also referred to as air entry pressure; and b = a constant. This equation has been found to fit data well (e.g., Bresler et al., 19'/8). There is no evidence in the literature that the relationship between conductivity and moisture content, k ( O ) , exhibits significant hysteresis (e.g., Jackson et al., 1976) so that it is safe to assume that conductivity is a unique function of moisture eontent. In a review of previous studies (Brutsaert, 1967) it was found that the following relationship has been used widely: k(0) = k s ( ~ ) n
(3)
where: n = constant which depends on pore size distribution; co = effective saturation (dimensionless), defined by: -
0 --0~ 0s --0~
(4)
where 0 = soil moisture content; 0r = a constant, also referred to as residual moisture content; and 0s = saturated moisture content. The hydraulic conductivity can also be expressed as a function of count ratio of the neutron probe (an instrument which measures moisture content) by simply substituting the neutron count ratio-moisture content relationship: =
~x(CR--CRr)
(5)
into eqn. (3) viz.: k(O)
=
ksm (CR -- CRr) n
(6)
where C R -- n e u t r o n count ratio; C R ~ = residual neutron count ratio (neutron count ratio when there is no moisture in the soil); and m = a n. The count ratio can be measured very precisely over a relatively large volume of soil with a neutron probe. In an experiment in the laboratory the m a x i m u m deviation between ten measurements of moisture content was less than 0.002 cm 3 cm -3. Therefore, the hydraulic conductivity based on count ratio satisfies the condition of great precision necessary to use Darcy's law to calculate fluxes in the soft. Field determinations of unsaturated hydraulic conductivity can be made non
149 evaporation. Water pressure and moisture content are measured simultaneously at several depths during the water redistribution phase. Water flux at any point may be calculated as the rate of change in water content in the profile above it. Unsaturated conductivity can be obtained as the quotient of flux and hydraulic pressure gradient viz.: /e(0) =
- - - ~ dz
(7)
0
where t = time, H = hydraulic potential or the sum of the matric and gravity potential and z = depth coordinate. The instrumentation used for the instantaneous profile m e t h o d is also used for calculating the recharge by Darcy's flux on the same site. Laboratory measurements were used to check the unsaturated conductivity obtained from the instantaneous profile method. For these measurements, the crust m e t h o d (Veneman, 1974) was selected. The crust m e t h o d requires undisturbed field cores, and it yields conductivity values in the range in which most water movement occurs naturally. Method B: Hydrologic budget The second m e t h o d for calculating recharge at the experimental site utilizes the hydraulic budget equation (1) with recharge as the only u n k n o w n quantity. This m e t h o d is widely applied but, because of the nature of routinely available data, does not usually provide very accurate results. In this research study, however, accurate micrometeorological data with high temporal resolution were recorded and used to obtain an accurate description of precipitation (P) and evapotranspiration (E). With precipitation and evapotranspiration known, recharge was calculated with the assumption that runoff ( R F ) and the change in soil moisture storage (AS) were negligible in the highly permeable and porous soil at the Long Island experimental site. Evapotranspiration was c o m p u t e d by means of relatively simple, routine methods that were tested and calibrated against results obtained from a detailed energy budget analysis. Three separate methods were tested against the energy budget results, including Penman's (1948) equation for potential evaporation, the equilibrium evaporation concept introduced by Slatyer and McIlroy (1961), and the advection--aridity m e t h o d introduced by Brutsaert and Stricker (1979). The energy budget m e t h o d which served as a reference was similar to the one employed by Stricker and Brutsaert (1978). For a thin layer at the earth's surface the amount of energy gained must be equal to the amount of energy lost. This conservative property provides the energy budget equation, which is a good approximation and can be written as follows: LeE = Rn - - H - - G
(8)
150 where: Le = latent heat of vaporization; E = rate of evaporation; Rn = net radiation transfer rate between the ground surface and the atmosphere; H = sensible heat transfer into the atmosphere; and G = heat transfer rate into the ground. The net radiation term, Rn, is the dominant term on the right hand side (RHS) of eqn. (8) and is easily measured. The ground heat flux, G, is difficult to measure accurately and is usually quite small. In addition, ground heat flux exhibits a strong diurnal pattern that tends to cancel itself over a 24 h period. For these reasons it was assumed that G = 0. The remaining term on the RHS of eqn. (8) is the sensible heat flux, H, which can be large in proportion to Rn. In order to quantify the sensible heat flux the assumption of the uniform atmospheric boundary layer was used. This means that horizontal gradients and vertical velocities are assumed to be negligible when compared to the vertical gradients and horizontal velocities. Hence atmospheric fluxes of m o m e n t u m , and sensible and latent heat occur only in the vertical direction and horizontal advection is not present. With these simplifying assumptions and on the basis of the similarity hypothesis of Monin and Obukhov it is possible to derive relationships between the vertical gradients of wind speed and air temperature near the ground surface and the respective vertical fluxes of m o m e n t u m and sensible heat. These Monin-Obukhov relationships are well-known and have been presented in detail elsewhere (e.g., Brutsaert, 1982; eqns. (4.34) and (4.35)). The stability functions for m o m e n t u m and sensible heat ~sm and ~sh used in this study were taken to be those suggested by Dyer (1974). These were also given by Brutsaert (1982) as eqns. (4.50) and (4.51) for unstable conditions, and (4.56) and (4.57) for stable conditions. Because of the implicit relationship between U., E, H, Rn and L in the energy budget [eqn. (8)] and in the Monin-Obukhov formulation, this was done by iteration, as explained elsewhere (Brutsaert, 1982, p. 198). The routine methods tested against the energy budget results include Penman's (1948) equation for potential evaporation. The wind function f ( U ) in this equation has been given various forms in the past, but in this study the form originally proposed by Penman (1948) was used. The equilibrium evaporation concept introduced by Slatyer and McIlroy (1961) was also tested in this study. Equilibrium evaporation has been found to be a reasonably good measure of actual evaporation (i.e., Denmead and McIlroy, 1970; Wilson and Rouse, 1972) under certain conditions. The third routinely applicable m e t h o d is the so-called advection--aridity m e t h o d introduced by Brutsaert and Stricker (1979). This method results from a combination of concepts already used in several other methods (Penman, 1948; Bouchet, 1963; Priestley and Taylor, 1972) and it has the advantage that, at least conceptually, it is able to respond to varying conditions of surface moisture availability. For the constant in this equation a consensus value of a = 1.27 (e.g., Brutsaert, 1982, p. 220) was used in this study.
151 EXPERIMENTALPROCEDURE The data for this research were recorded on the grounds of Cornell University's Horticultural Research Farm, located 4 km north of Riverhead, N.Y. and 1 km south of the Long Island Sound. The farm lies on a glacial out-wash plain with soils typical of the region. Soil at the site is a Haven sandy loam. The soil profile has a sharp distinction between the top 30 cm and the rest of the profile. The plow layer has a higher percentage of the clay and silt than the subsoil (Table 1) and is compacted by traffic. The soil below 30 cm is a yellowish-brown gravelly sand with loamy reddish-brown spots. The sand has a high saturated conductivity of over 10 m per day. Below the 150 cm depth, clay lenses are found consisting of clay and sand. Clay lenses are not continuous and water at low tension may pass between the lenses. The lens structure within 1 0 m from the recharge measurement sites is sketched in Fig. 1. Red streaks of ferric material as well as iron modules were f o u n d throughout the profile. The instrumentation for determining recharge with Method A was arranged in two circular plots, each 3 m in diameter, and designated A and B. In plot A water-mercury tensiometers with millibar-calibrated scales were installed in duplicate at each of depths 30.5, 61, 91, and 1 2 2 c m (1ft. intervals). Holes were dug by auger and a slurry of a texture finer than the original soil was poured around each tensiometer shaft. A moisture gage access hole was augered in the center of plot A and a 1.5 m section of steel tubing was snugged in. Moisture readings were taken with a Troxler TM 1257 Depth Moisture Gage, a neutron-emitting device. Auger samples from nearby soil were taken for a comparison of neutron gage and gravimetric moisture determinations. Plot B was instrumented with triplicate tensiometers at depths of 30.5, 61, 91, 122 cm and duplicates at 152, 193 and 213 cm. During the winter tensiometers were filled with a one-to-one mixture of ethylene glycol (antifreeze) and water to avoid equipment damage. Glycol in tensiometers was first used by McKim et al. (1976). A 2.5 m aluminum access tube for the neutron probe moisture gauge was also installed. Unsaturated conductivity measurements were made in situ for plots A and B with the instantaneous profile method. For plot A the unsaturated hydraulic conductivities were made during May and June of 1978. Initially during the draining phase hourly readings were taken of moisture content and soil water pressure for 12 h. Readings continued at 1--4 day intervals for one month. At the end of the measurement period, plot A was dug up to obtain soil samples for gravimetric soil moisture determination in order to calibrate the neutron probe. During August 1979 an adapted instantaneous profile m e t h o d was performed on plot B. The soil in the plot was not disturbed to obtain gravimetric samples and the plot was not covered with a plastic sheet. Readings of water pressure and moisture content were stopped after four days before evaporation became significant.
TABLE 1
0--10 10--20 20--30 45--75 75--105
(cm)
Depth
250--147
147--75
2000--75
8 9 8 15 16
9 11 11 13 14
27 32 30 50 53
7 7 7 7 6
6 6 6 2 2
57 65 62 87 91
8 10 8 3 3
50--25
589--250
2000--850
850--589
Silt
Sand
Fractions (pm)
T e x t u r a l analysis o f t h e Haven s a n d y l o a m at t h e r e c h a r g e m e a s u r e m e n t site
11 9 9 4 3
25--5
18 15 18 5 4
<5
Clay
Cgn
153
i~
PLOWLAYER ~i~i~ ii~ !i~i~ li~ li!~ i~i!~i~i!~i~i~i~ !i!~ i~ !~iii~i~i!~i~i~i~ii~ !i!i~i!~ i!]~i~ i~ i!i~ !i!ii:
SAND
-lO0
/CLAY LENSES
..... I--
:
"-':
. . . :.: .
"
: ~; ~; ~~!~>i!i~!ii:i!!ii~¸~:'~ ~'~' ':
"~
,
.
•
•
.:
:
:
.
"
:
.....
~ ;:. :..::
Fig. 1. Sketch of soil profile at recharge site.
Unsaturated hydraulic conductivity was also determined from laboratory m e a s u r e m e n t s u s i n g t h e c r u s t m e t h o d . T h i s m e t h o d , p e r f o r m e d in d u p l i c a t e , u t i l i z e d c o r e s a m p l e s 1 0 c m in d i a m e t e r a n d 3 0 c m l o n g t a k e n b y p u s h i n g
154 metal cylinders into the soft. Cores were taken at depths 0--30, 30--60 and 60--90 cm. F o r the crust m e t h o d , a ceramic disk under different constant water pressures was placed on t op of the soft. The column was placed on a soil base to facilitate draining. Tensiometers located vertically at 5 cm intervals measured water pressure. Measurements were taken when the flux through the soil became constant. The undisturbed cores were then fully saturated to determine the saturated conductivity of each core. The data needed for c om put i ng recharge by means of m e t h o d B utilized micrometeorological measurements recorded at the experimental site. T hey were recorded at one-hour intervals, by means of an automatic data acquisition system. Maintenance was p e r f o r m e d on a regular basis and as needed. A calibration of the instruments was p e r f o r m e d prior to the initiation o f the project and again after data acquisition was terminated. Precipitation measurements were made with three tipping bucket gauges. One gauge was located within a pit slightly deeper than the gauge, one gauge was surrounded by a wind shield, and one gauge was located in the open w i t h o u t shielding. The wind shield used is identical to those in standard use by U.S. National Weather Service stations. Wind speeds were measured at 1 and 3 m by lightweight cup anemometers to the nearest 0 . 1 5 m h -1 with a threshold of 0 . 5 m h -1. The non-linear o u t p u t o f each a n e m o m e t e r was linearized by sensor-matched electronic translators. The o u t p u t recorded at each ho ur interval is the total wind run for the preceding hour. Wind direction was measured by a flat vaned sensor and match ed translator. Air t e m p e r a t u r e was measured to the nearest 0.2°C at l m above ground level with a thermistor m o u n t e d in an aspirated radiation shield. O u t p u t from this thermistor was linearized with a m at ched translator. The temperature difference between 1 and 3 m , was measured to the nearest 0.01°C with a thermistor m o u n t e d at 3 m in an aspirated radiation shield and c o n n e c t e d in a bridge circuit to the thermistor at 1 m. O u t p u t from the bridge circuit was linearized by the same translator used for the air temperature at 1 m. Tem pe r at ur e and t e m p e r a t u r e difference o u t p u t for each hour were provided by single instantaneous samples. Since the time constants for the thermistors are short, a hollow aluminum cylinder was inserted over each thermistor to dampen the response time of the measurement. For c o m p u t a t i o n a l purposes, the mean temperatures representing each h o u r were th en obtained by averaging the recordings taken at the beginning and end o f each hour. Net radiation flux was measured using an aspirated Fritschen-type net radiometer m o u n t e d at I m. O u t p u t from the net radiometer was integrated over the h o u r with an integrating translator. Dew p o in t temperatures to the nearest 0.5°C at 1.68 and 2 . 6 8 m above the soil surface were measured using self-heated lithium chloride dew cells m o u n t e d in aspirated radiation shields. O u t p u t from each dew cell was linearized with m a t c he d translators and recorded from single instantaneous
155 samples taken at hourly intervals. For c o m p u t a t i o n a l purposes the dew point for each h o u r was averaged from recordings taken at the beginning and end of each hour. Total atmospheric p r e s s u r e was measured using a stacked diaphragm sensor and matched translator.
RECHARGE CALCULATION -- RESULTS
Method A The d eter min a t i on of the unsaturated hydraulic conductivity is an integral part o f the recharge measurements. Initially the analysis of the results of the two instantaneous profile m e t h o d s for plots A and B was complicated by the fact that residual neut r on c o u n t ratios (CRr) varied with dept h and between b o t h plots. Residual n e u t r o n c o u n t ratio is the background reading at zero moisture content. Thus absolute moisture contents are n o t know n but differences in moisture c o n t e n t in time at the same depth still can be calculated. Fluxes and resulting unsaturated hydraulic conductivities can be calculated as usual but the unsaturated hydraulic conductivities could n o t be expressed directly as a f unc t i on of moisture content. Instead, to obtain this expression and the residual n e u t r o n c o u n t ratios the k(O ) vs. c o u n t ratio (CR) relationship for the 30.5--61, 61--91 and the 9 1 - - 1 2 2 c m depths for b o th plots were regressed according to eqn. (6) with various trial values of residual n e u t r o n c o u n t ratios. The set of residual n e u t r o n c o u n t ratios was selected such th a t the k(O ) vs. (CR -- CRr) was identical for the three depths in b o th plots and when substituted in eqn. (5) the moisture contents determined with the n e u t r o n probe agreed with the observed gravimetric moisture contents. The selected set of residual c o u n t ratios is shown in Table 2. Values o f unsaturated hydraulic conductivity calculated with the crust m e t h o d (overlapping the instantaneous profile m e t h o d in the 1 5 - - 8 0 m b range) agreed well with the measurements in situ as exemplified in Fig. 2 for the 60--90 cm depth. The unsaturated hydraulic conductivity values for the 30--120 cm depth according to the relationships expressed in eqns. (2), (3) and (6) were f o u n d to be: k(~P) = 7.7 x 106(~t) -2"7
r 2 -- 75%
(9)
k(O)
r 2 = 97%
(10) (11)
= 5 . 0 x 106 x ( 0 - - 0 . 0 0 5 ) s'22
k(CR) = 1 . 4 x 1 0 3 ( C R - C R r ) s'22
r 2 = 97%
where the conductivity k is expressed in cm per day; the water pressure in cm and the moisture c o n t e n t in cm 3 cm -3 . The unsaturated hydraulic conductivity as a function of the matric potential has, as expect ed due to hysteresis, the lowest correlation coefficient. It is of interest to not e t hat
156 TABLE 2 R e s i d u a l c o u n t r a t i o s ( C R r ) for p l o t s A a n d B Depth
plot A
plot B
0.185 0.053 0.109
0.196 0.174 0.053
(cm) 60 90 120
15
o x JL
LABORATORY (CRUST METHOD) FIELD (INST. METHOD 1978) FIELD (INST. METHOD 1979)
12 @ >I--.
-I -
@
9
,,.,, Z 0 u
6
n,,,,
>,',r"
3
@
lip
°o
I
i
ao
J'~A I
.L
** % 2 . . . . . . . ~0
MATRIC
SUCTION
:
60
do
i
,;o
(MBARS)
Fig. 2. U n s a t u r a t e d c o n d u c t i v i t y as a f u n c t i o n o f m a t r i c s u c t i o n for a H a v e n loam at t h e 6 0 - - 9 0 cm d e p t h .
the value of the exponent in eqn. (9) approximates very closely the value of 2.6 proposed by Bresler et al. (1978) as a universal value. With the unsaturated hydraulic conductivity function known, the recharge to the aquifer was calculated from measurements of the matric potential and neutron count ratio. These measurements were made bi-weekly over the periods June through October 1978; March through June 1979; January 1980 through June 1980 and during November and December of 1980. Analysis of the instantaneous profile data showed that the values of hydraulic gradient varied linearly with time after a rainfall while the -
-
157 conductivity varied linearly with the square root of time. Therefore, the computation of recharge volume in the period between two measurements employed a geometric mean of conductivities and a straight average of the hydraulic gradients with time, viz.:
R =
(kt, kt2)ln
.1-2 "
~z~H tl +
-~z t2 ( t 2 - - t l )
(12)
where, R -- quantity of water that flowed past a certain depth over a time interval ( t 2 - - t l ); tl - - t i m e of first measurement; t2 = time of second measurement. Triplicate measurements of soil water pressure at the same depth were often different during prolonged dry periods. Differences of up to 3 0 m b were noted at these times. In comparison, differences of 0 - - 4 m b were observed during periods of downward flux. Tensiometer readings obviously in error were omitted. Averages of the remaining measurements were used in calculations. For days when measurements were available, the instantaneous recharge rate was calculated for the 90--120 cm depth. At this depth most rapid fluctuations of the flux rate at the surface were dampened out and therefore gave a better estimate with the bi-weekly measurement schedule. The hydraulic conductivity, k, was based on neutron probe count ratio, CR, when those data were available and on water pressure, 4 , whenever CR data were not available. The k (CR) relationship [eqn. (11)] is preferred over the k ( ~ ) relationship [eqn. (9)] because the former has a higher correlation coefficient and is hysteresis-free. Cumulative vertical flux at the 90--120 cm depth is shown with cumulative rainfall in Figs. 3--6. Figure 3 shows that there was, from June through the end of September 1978, equal up and downward movement at 90 cm resulting in zero net recharge. In the period of March 10 through June 1, 1979, a significant portion of the rainfall became recharge (Fig. 4). The higher recharge than cumulative rainfall in the beginning of the period was caused by drainage of the profile above the 120 cm depth of rainfall which fell before the data collection period started. The water movement below the 120 cm depth during the 75 days in spring amounted to 50--60% of the total rainfall. The plot was only sparsely vegetated during this period. An abundant, actively transpiring crop would be expected to allow a smaller proportion of the rainfall to become recharge. During January and February of 1980 the precipitation was below normal. Part of the precipitation which fell in the form of snow was removed from the site by wind. During February there was a net upwards m o v e m e n t of water to the frost front (Fig. 5). March and April 1980 were high recharge months. Starting in May the recharge leveled off. This was earlier than in 1979 resulting from lower precipitation in May 1980 than May 1979. Although not shown, there was a small upward flux during
158
5C
4O
=o 3O g
n-
.~
I_S
g IO o
u
C-
/
20
Rechorge
~
0
I
June
I
July
I
August
__Y I
September
I
October
Fig. 3. P r e c i p i t a t i o n a n d r e c h a r g e f r o m J u n e t h r o u g h O c t o b e r 1978.
25 Precipitotion 2O
Rechorge
~
~0
.&
~ 5
u o
I,/
Morch
I
I
April
I
May
Fig. 4. P r e c i p i t a t i o n a n d r e c h a r g e d u r i n g spring 1 9 7 9 .
159
600
Precipitolion ~ f o
S
40.C
g o
'&
Q.
20.0
3
E u
0
I Jonuory
I Februory
I Morch
I April
I Moy
June
Fig. 5. Precipitation and recharge from January through June 1980.
I ~ 15.0 ~' ~25 Precipitotion~ I0.0 i
I
7.5i 50
Rechorge
._o 2,5 u
I November
December
Fig. 6. Precipitation and recharge during November and December 1980.
July and August 1980 and a small downward flux during September and October. The soil was rather dry during the beginning of November. A portion of the rainfall during November was stored in the profile above 90 cm rather than being lost as vertical percolation (Fig. 6). The recharge pattern was similar to that observed by Rehm et al. (1982) in the prairie region in North Dakota. However, the quantity of recharge on Long Island is at least five times as high.
160 Method B
The hydrologic budget m e t h o d for determining recharge was applied to the experimental data for the period from December 27, 1979 through July 17, 1981. The data record for this period contains a period of 3½ months, July 16, 1980 through October 30, 1980 for which data were not recorded due to a malfunction in the automatic data recorder. Several other portions of the record did not contain all data parameters necessary for applying the energy budget analysis. The energy budget m e t h o d was also not applicable when the wind was incident between 10 ° (magnetic) and 235 °. This restriction is due to physical obstructions upwind in this general direction that could cause violation of the assumption requiring a large uniform surface. A perusal of all days for which data were available was performed to select days for which the energy budget analysis could be expected to produce valid evaporation results. This selection process provided 68 days to which the energy budget was applied. The routine methods, Penman's equation, the equilibrium evaporation equation and the advection-aridity method equation, were also applied to the 68 days. All four methods computed evaporation on an hourly basis, from which daily values were obtained. Results of the four evaporation methods are presented in Table 3. Figures 7--9 compare the results of the three routine methods with the energy budget results. These figures show that for eastern Long Island none of the routine methods are perfect predictors of evaporation. At low rates of evaporation the equilibrium evaporation and the Penman equation seem to provide the best results. At high evaporation rates the equilibrium method and the advection--aridity m e t h o d both appear to provide fairly good results. It can be seen from Table 4 that all three equations have reasonably good correlations with energy budget evaporation throughout the year. There is, however, a large degree of scatter during the winter and spring, as evidenced b y the high standard deviations. During the summer the standard deviations drop considerably. The advection--aridity method, with a = 1.27, does not appear to be a good predictor of evaporation during the winter and spring, but during the summer its performance was seen to improve substantially. The advection--aridity method gives the best overall prediction of the evaporation for the entire period. Still, Table 4 shows that on a seasonal basis, Ep and Ee can be used with the regression equations to give more reliable predictions of E than Eaa. To c o m p u t e a complete recharge time series for the experimental site, the routine methods which had the smallest standard deviation and the highest correlation, were applied to each day in the data base. Thus, for the winter months the regression relationship: E = 1.682Ee
(13)
161
I0 9 8 7
6
4
:5 2
0 -I -2 -2
!
!
I
!
!
i
|
I
|
I
[
-I
0
I
2
3
4
5
6
"/'
8
9
Energy Budget
I0
ET (mmldoy)
Fig. 7. Scatter plot showing comparison between evaporation rate computed with Penman equation and with the energy budget method.
was used to determine daily evaporation from hourly data, and similarly for the spring months: E = 0.706Ep
(14)
and for the summer months: E = 1.102Ee
(15)
These regression relationships were derived from data taken over a wide range of soil moisture conditions and should provide reasonable values for the study period. It should be noted, however, that precipitation during the study period was somewhat below normal. Therefore these regression relationships cannot be considered general and their use should be limited to the study period as well as to the area of study. Since the predictive ability of the routine methods seems to vary with the rate of evaporation, regressions were performed with the data separated into winter, spring and summer data. The results of the recharge calculations are presented as m o n t h l y sums in Table 5 along with measured and average m o n t h l y precipitation values. The recharge pattern is the same as for m e t h o d
162 TABLE 3 Computed values for energy budget (Eb) , potential evaporation (Ep), equilibrium evaporation (Ee) and evaporation based on advection--aridity method (Eaa) in nm per day Date
Eb
1/19/80 1/20/80 1/21/80
--0.470 0.167 1.742
3/4/80 3/6/80 3/9/80 3/16/80 3/19/80
Ep
Ee
Eaa
0.806 1.598 1.975
--0.467 --0.689 --0.531
--1.992 --3.349 --3.325
0.096 1.046 1.060 0.187 0.865
1.679 2.020 1.992 2.137 2.385
0.397 0.755 0.641 0.683 0.917
--0.671 --0.101 --0.365 --0.403 --0.056
1/4/81 1/5/81 I/7/81 1/19/81 1/23]81 1/24/81 1/26/81 1/27/81
--1.778 --1.525 --1.865 1.509 1.798 1.416 1.317 2.081
0.347 0.525 --0.355 1.219 1.661 1.612 1.188 2.030
--0.426 --0.495 --0.975 0.450 0.737 0.605 0.328 0.959
--1.430 --1.781 --2.121 --0.074 0.212 --0.076 --0.355 0.407
2/17/81 2/18/81
1.607 1.668
1.510 1.532
0.689 0.819
0.241 0.548
3/9/81 3/10/81 3/11/81 3/12/81 3/13/81 3/14]81 3/15/81 3/20/81 3/21/81 3/24/81 3/29/81 3/30/81 3/31/81
0.708 1.279 0.808 1.947 1.827 1.838 2.151 2.002 1.802 2.834 3.802 3.159 4.487
1.175 2.169 1.542 2.383 2.588 3.551 3.423 2.309 2.097 3.069 4.808 2.931 5.836
0.436 1.012 0.726 1.216 0.940 0.828 0.945 1.268 1.018 1.744 2.105 1.356 2.860
--0.067 0.403 0.302 0.706 --0.201 -- 1.447 --1.023 0.912 0.490 1.360 0.539 0.514 1.427
4]6]81 4/7/81 4/10/81 4/15/81 4/18181 4/19/81 4/20/81 4/21181 4122/81 4/25/81 4/26/81
2.982 4.162 4.830 3.595 4.392 3.543 2.145 3.361 2.572 2.553 3.857
3.407 4.869 5.828 4.855 5.669 5.057 3.658 5.189 4.396 3.066 4.760
1.598 2.261 2.725 2.182 2.851 2.706 1.606 2.332 2.401 1.616 3.040
0.652 0.873 1.092 0.687 1.573 1.817 0.421 0.735 1.703 1.039 2.962
5/13/81 5/17/81
3.889 4.625
6.497 6.325
3.866 4.107
3.324 4.108
(Continued)
163 TABLE 3 (Continued} Date
Eb
Ep
Ee
Eaa
5/23181 5/24/81 5/25/81 5/26/81
4.133 5.207 3.877 3.926
6.023 7.164 8.013 5.999
4.167 4.586 4.596 3.943
4.561 4.485 3.660 4.016
6/4/81 6/5181 6/6/81 617/81 6/9/81 6/10/81 6/11]81 6/17/81 6/26/81
4.894 5.628 3.578 5.828 4.781 2.669 5.710 5.395 6.300
5.719 6.867 5.316 7.779 6.955 4.206 7.293 7.193 7.738
4.167 5.158 3.338 5.038 3.449 1.960 4.908 5.118 5.049
4.864 6.234 3.164 5.018 1.805 0.771 5.172 5.806 5.086
7/5/81 7]6/81 7/7/81 7]8]81 7]9]81 7/11/81 7/13/81 7/14/81 7/15/81 7/16181 7/17/81
3.718 5.978 6.187 6.384 6.456 6.549 3.779 5.153 5.123 5.090 4.110
4.094 6.919 8.008 9.073 9.193 8.034 6.132 8.284 7.331 7.691 6.463
2.814 5.629 5.626 5.711 5.735 5.457 4.052 4.949 5.020 5.162 4.756
3.054 7.379 6.282 5.433 5.374 5.826 4.161 4.287 5.420 5.421 5.616
A, high during w i n t e r a n d negative during t h e s u m m e r . T h e negative evapor a t i o n values o b s e r v e d during s o m e w i n t e r m o n t h s are p r o b a b l y t h e result o f local m o i s t u r e a d v e c t i o n . L o n g Island is s u r r o u n d e d b y w a t e r a n d d u r i n g w i n t e r relatively w a r m m o i s t o c e a n winds b l o w i n g across t h e c o l d e r land surface m a y cause c o n d e n s a t i o n a n d dew. In a d d i t i o n , t h e local advective s i t u a t i o n resulting f r o m t h e p r o x i m i t y o f t h e s h o r e (1 k m ) , m a y on o c c a s i o n h a v e i n v a l i d a t e d t h e a s s u m p t i o n o f a large u n i f o r m surface. An e v a p o r a t i o n t i m e series was c o m p u t e d using t h e regressions d e f i n e d a b o v e . T h e t i m e series was t h e n s u m m e d o n a m o n t h l y basis a n d used to d e t e r m i n e m o n t h l y r e c h a r g e values using eqn. (1). F o r s o m e d a y s t h e d a t a r e c o r d was i n c o m p l e t e a n d t h e e v a p o r a t i o n t i m e series d i s c o n t i n u o u s . T o c o m p l e t e t h e t i m e series an average daffy evapor a t i o n r a t e was c o m p u t e d f o r each m o n t h f r o m t h e d a t a available f o r t h a t m o n t h . This average daily e v a p o r a t i o n was t h e n a p p l i e d t o t h e d a y s w i t h i n t h a t m o n t h f o r w h i c h d a t a w e r e u n a v a i l a b l e a n d t h e r e c h a r g e t i m e series was c o m p l e t e d . T h e m a x i m u m n u m b e r o f d a y s in o n e m o n t h t o w h i c h this t e c h n i q u e was a p p l i e d was f o u r during J u n e o f 1 9 8 1 . I t s h o u l d b e n o t e d t h a t t h e p r e c i p i t a t i o n r e c o r d was c o m p l e t e .
164 I0 9 8
7 O "O
6 I
E E
t
SI
°
5
I--
4
E
3
'r-
~g 2 hi
I
-I -2_2
L
0
I
I
I
I
2 3 4 5 6 Energy ~dget El" (mm/doy)
I
I
I
7
8
9
I0
Fig. 8. Scatter plot showing comparison between equilibrium evaporation rate and evaporation rate computed with the energy budget method.
COMPARISON OF METHODS A AND B The measurement periods of the two methods overlap during 1980. The recharge calculated by both methods can be compared during this time interval. The micrometeorological m e t h o d (B) calculates the recharge as the difference between precipitation and evaporation and thus, gives the vertical flux at the surface of the soft. Method A measures the flux at the 90-120 cm depth. Thus, in order to compare both methods, fluxes should be considered at the same depth. To do this we added the change in storage above the average flux plane at 105 cm to the recharge calculated with m e t h o d B (Table 6). Table 6 shows that with the exception of January and December the observed fluxes with both methods show the same trend. During January and December (of the year of measurement) a portion of the precipitation fell in the form of snow. Snow measurements with rain gauges are notoriously inaccurate. Moreover, the snow blew off the recharge plot in most cases and did not contribute to recharge calculated with m e t h o d A and gives, therefore, a lower estimate than m e t h o d B. Also, during the
165 10'
8
6 O
4 b-tAJ C O
2
"O
"~ ~,0
,., ,.:
0
•
,
-2
.4 4
I -2
i 0
i 2
Energy Budget
l 4
I 6
I B
I0
ET (mrn/doy)
Fig. 9. S c a t t e r p l o t s h o w i n g c o m p a r i s o n b e t w e e n e v a p o r a t i o n r a t e c o m p u t e d advection--aridity and with the energy budget method.
with the
TABLE 4 Relationship between applicable methods
energy budget evaporation E(mm
per day), and the routinely
Period
N o . Pts.
Regressiona
Correlation
overall
68
E -= 0 . 7 1 9 E p + 0 . 7 1 4 E -= 1 . 1 5 4 E e ± 0 . 8 2 5 E = 1.020Eaa ± 1.771
r -- 0 . 9 4 3 r = 0.938 r = 0.876
winter
24
E -= 0 . 6 2 6 E p + 0 . 8 8 4 E = 1.682E e ± 0.803 E -= 0 . 0 4 5 E a a -+ 1 . 4 9 3
r = 0.771 r = 0.772 r = 0.484
spring
24
E -= 0 . 7 0 6 E p ± 0 , 6 3 1 E = 1.241E e ± 0.916 E = 1.343Eaa ± 1.897
r = 0.839 r = 0.778 r = 0.605
summer
20
E :- 0 . 7 3 5 E p ± 0 . 5 5 8 E -- 1 , 1 0 2 E e + 0 . 5 2 3 E = 1 . 0 2 3 E a a -+ 1 . 1 1 7
r -- 0 . 8 6 5 r = 0.893 r = 0.739
a E p : P e n m a n ' s ( 1 9 4 8 ) e q u a t i o n ; E e ---- e q u i l i b r i u m e v a p o r a t i o n ( S l a t y e r a n d M c I l r o y , 1 9 6 1 ) ; a n d Eaa = a d v e c t i o n - - a r i d i t y m e t h o d ( B r u t s a e r t a n d S t r i c k e r , 1 9 7 9 ) .
166 TABLE 5 Evaporation and recharge computed with the closure method Period
Days of data
Averagea precip, (cm)
Observed precip. (cm)
E (cm)
R (cm)
12/79 1/80 2/80 3/80 4/80 5/80 6/80 7/80
5 31 29 31 30 31 30 16
1.73 8.86 8.46 10.59 8.86 8.69 6.88 4.34
0.00 4.20 2.69 17.62 13.94 4.86 6.79 1.75
0.02 --1.37 --0.23 2.36 4.69 6.56 8.42 5.64
--0.02 5.57 2.92 15.26 9.25 --1.71 --1.63 --3.89
10/80 11/80 12/80 1/81 2/81 3/81 4/81 5/81 6/81 7/81
2 30 31 31 28 31 30 31 30 17
0.51 11.43 10.72 8.86 8.46 10.59 8.86 8.69 6.88 4.61
0.06 9.18 1.30 3.12 13.85 1.97 10.91 7.70 10.77 1.47
0.12 1.04 --2.22 0.15 2.07 4.23 7.85 10.92 13.27 8.55
--0.07 8.14 3.52 2.98 11.77 --2.25 3.06 --3.22 --2.49 --7.08
a Values from NOAA Climatological Data -- 1980. Annual summary. Average precipitation for 12/79, 7/80, 10/80 and 7/81 is adjusted to the number of days,
w i n t e r m o n t h s m e a s u r e m e n t o f solar r a d i a t i o n (and, t h e r e f o r e , e v a p o r a t i o n ) has t h e g r e a t e s t d e g r e e o f u n c e r t a i n t y . During t h e r e m a i n d e r o f 1 9 8 0 b o t h m e t h o d s agree r a t h e r well (97% c o r r e l a t i o n c o e f f i c i e n t ) . This is especially t r u e w h e n t h e great n u m b e r o f possible m e a s u r e m e n t errors is t a k e n into a c c o u n t . Unlike s i m u l a t i o n m o d e l s w h e r e t h e r e c h a r g e is a l w a y s a f r a c t i o n o f t h e p r e c i p i t a t i o n b o t h m e a s u r i n g m e t h o d s are n o t r e s t r a i n e d b y t h e rainfall. T h e m o n t h w i t h t h e highest r e c h a r g e s h o w s t h e least a m o u n t o f error. D u r i n g t h e s e m o n t h s t h e m o i s t u r e c o n t e n t and h y d r a u l i c c o n d u c t i v i t y w e r e in t h e s a m e r a n g e as t h e i n s t a n t a n e o u s p r o f i l e m e t h o d a n d t h e r e f o r e c o u l d be e x p e c t e d to be m o s t a c c u r a t e . T a b l e 6 s h o w s t h a t t h e r e c h a r g e p r e d i c t e d b y m e t h o d B is generally higher t h a n b y m e t h o d A. A n e x p l a n a t i o n f o r this p h e n o m e n o n m i g h t be f o u n d in t h e n o n l i n e a r i t y o f b o t h t h e e v a p o r a t i o n and r e c h a r g e e s t i m a t e s . N o n linearity t e n d s t o u n d e r e s t i m a t e t h e a r i t h m e t i c m e a n . T h u s , f o r m e t h o d B t h e e v a p o r a t i o n is u n d e r e s t i m a t e d t h e r e b y o v e r e s t i m a t i n g t h e recharge. F o r m e t h o d A t h e r e c h a r g e itself is u n d e r e s t i m a t e d . F r o m t h e s e c o m p a r i s o n s , it seems t h a t b o t h m e t h o d s give a r e a s o n a b l e i n d i c a t i o n o f e x p e c t e d recharge d u r i n g t h e y e a r e x c e p t in t h e winter.
5.6 2.9 15.3 9.3 --1.7 --1.6 8.1 3.5
~- 4 --0.7 --1.4 + 1.1 + 5.7 +4.6 --6.5 + 3.3
C h a n g e in m o i s t u r e c o n t e n t a b o v e flux p l a n e at 105 c m (cm) a 9.6 2.2 13.9 10.4 4.0 3.0 1.6 6.8
F l u x at 105 c m by method B (cm per month)
3.1 0.4 13.7 11.4 2.4 0.9 1.6 2.4
F l u x a t 105 c m by method A (cm per month)
4.2 2.7 17.6 13.9 4.9 6.8 9.2 1.3
Precipitation (cm per month)
a A positive sign m e a n s t h a t t h e profile was drier a t t h e e n d o f t h e m o n t h t h a n t h e b e g i n n i n g ; a negative sign is t h e reverse.
January February March April May June November December
F l u x at surface by method B ( c m per m o n t h )
Recharge estimates by methods A and B
TABLE 6
168
However, when negative evaporation is set to zero in m e t h o d B, agreement between the two methods during the winter is closer. More research is needed in the correctness of both methods during the winter.
CONCLUDING REMARKS
One of the objectives of this research was to validate the presently used recharge estimate on Long Island of 50% of the annual precipitation. The relatively short duration of the experiment (three years) prevents us from making definitive statements. However, our measurements are the only available direct determination of recharge on Long Island. These data may provide long-term estimates when used by an appropriate computer simulation model. The measurements suggest that the general estimate of 50% of annual precipitation is a long-term average at best. The vertical flux past the l m depth was strongly dependent on both time of the year and precipitation amount. In late fall, winter and early spring a high percentage of the precipitation became recharge. During the summer months there was a small net upward movement of water past the l m depth. Precipitation during these months did not contribute to the annual recharge. Although it is difficult to draw general conclusions from our measurements, it seems that in order to estimate recharge special attention should be given to precipitation during the winter months. Therefore, in our view a better estimate might be that 75--90% of the precipitation from October 15 to May 15 becomes recharge while there is practically no recharge during the remaining part of the year. The exact value depends on crop cover used. The two methods used for estimating recharge were labour intensive and required experienced technicians. Currently, one m e t h o d cannot be recommended above the other. Both methods give a good estimate during the year except for the winter. The closure m e t h o d using micrometeorological data gives a slightly higher estimate than the direct measurement m e t h o d based on Darcy's law. The difference can probably be attributed to the non-linearity of the measurement and the need to use arithmetic averages. More research is needed to determine why both methods deviated during the winter. REFERENCES Arya, L.M., Farrel, D.A. and Blake, G.R., 1975. A field study of soil water depletion patterns in presence of growing soybean roots: I. determination of hydraulic properties of the soil. Soil Sci. Soc. Am., Proc., 39: 424--430. Bouchet, R.J., 1963. Evapotranspiration r4elle et potentielle, signification climatique. General Assembly Berkeley, Int. Assoc. Sci. Hydrol., Gentbrugge, Publ. 6 2 : 1 3 4 - - 1 4 2 . Bresler, E., Russo, D. and Miller, R.D., 1978. Rapid estimate of unsaturated hydraulic conductivity function. Soil Sci. Soc. Am., J. 42: 170--172. Brooks, R.H. and Corey, A.T., 1964. Hydraulic properties of porous media. Hydrol. Paper, No. 3, Colorado State Univ., Ft. Collins, Colo.
169 Brutsaert, W., 1967. Some methods of calculating unsaturated permeability. Trans. Am. Soc. Agric. Eng., 19: 400--404. Brutsaert, W., 1982. Evaporation into the Atmosphere: Theory, History and Applications. Reidel, Dordrecht. Brutsaert, W. and Stricker, H., 1979. An advection--aridity approach to estimate actual regional evaporation. Water Resour. Res., 15: 443-- 450. Cohen, P., Franke, O.L. and Foxworthy, B.L., 1968. An atlas of Long Island's water resources. U.S. Geol. Surv. and New York State Water Resources Commission. New York Water Resour. Comm., Bull. 62,117 pp. Cooper, J.D., 1980. Measurement of moisture fluxes in unsaturated soil in Thelford Forest. Inst. Hydrol., Rep. No. 66, Wallingford, Oxon. Denmead, O.T. and McIlroy, I.C., 1970. Measurement of non-potential evaporation from wheat. Agric. Meteoroh, 7 : 285--302. Dyer, A.J. 1974. A review of flux--profile relationships. Boundary-Layer Meteorol., 1: 363--372. Hebb, E.A. and Wheeler, W.B., 1978. Bromocil in Lakeland soil groundwater. J. Environ. Qual., 7 : 598--601. Jackson, R.D., Idso, S.B. and Reginato, R.J. 1976. Calculation of evaporation rates during transition from energy-limiting to soil-limiting phases using albedo data. Water Resour. Res., 12: 23--26. Josephson, J., 1976. Quality assurance for groundwater. Environ. Sci. Technol., 10: 226--227. La Fleur, K.S., 1976. Movement of carbaryl through Congaree soil into groundwater. J. Environ. Quah, 5: 91--92. McKim, H.L., Berg, R.L., McGraw, R.W., Atkins, R.T. and Ingersoll, J., 1976. Development of remote reading tensiometer/transducer system for use in sub-freezing temperatures. Proc. Sec. Conf. on Soil Water Problems in Cold Regions, Edmonton, Alta. McLendon, S.C., 1971. Water resources of Nassau and Suffolk counties. Water and Sewage Works, 118: 34--36. Obukhov, A.M., 1946. Turbulence in an atmosphere with non-uniform temperature. Engl. transl., Boundary-Layer Meteorol., 1971: 2, 7--29. Ogata, G. and Richards, L.A., 1957. Water content changes following irrigation of bare field soil that is protected from soil water changes. Soil Sci. Soc. Am. Proc., 21: 355--356. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. Lond., A193: 120--145. Priestley, C.H.B. and Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev., 100: 81--92. Rehm, B.W., Moran, S.R. and Graenewold, G.H., 1982. Natural groundwater recharge in an upland area of central North Dakota, U.S.A.J. Hydrol., 59: 293--314. Slatyer, R.O. and McIlroy, I.C., 1961. Practical Microclimatology. CSIRO, Melbourne, 310 pp. Stricker, H. and Brutsaert, W., 1978. Actual evapotranspiration over a summer period in the Hupsel catchment. J. Hydrol., 39: 139--157. Veneman, P.L.M., 1974. Measurements of soil moisture retention characteristics. In: J. Bouma, F.G. Baker and P.L.M. Veneman (Editors). Measurement of Water Movement in Soil Pedons Above the Water Table. Inf. Cir., No. 27, Univ. Wisconsin, Madison, Wisc. Watson, K.K., 1966. An instantaneous profile method for determining hydraulic conductivity of unsaturated porous materials. Water Resour. Res., 2 : 7 0 9 - - 7 1 5 . Wilson, R.G. and Rouse, W.R., 1972. Moisture and temperature limits of the equilibrium evapotranspiration model. J. Appl. Meteoroh, 11: 436--442.