subsurface hydrology and phosphorus transport in the Kissimmee River Basin, Florida

ECOLOGICAL ENGINEERING ELSEVIER

Ecological Engineering 5 (1995) 301-330

Surface/subsurface hydrology and phosphorus transport in the Kissimmee River Basin, Florida * K e n n e t h L. C a m p b e l l a,., J o h n C. C a p e c e b, T e r r y K. T r e m w e l ~ a Agricultural Engineering Department, Unit'ersity of Florida, Gainest'ille, FL 32611-0570, USA b Southwest Florida Research and Education Center, Uniuersity of Florida, lmmokalee, Florida, USA CApplied Technology and Management, Inc., Gainest'ille, Florida, USA

Abstract

A comprehensive research program was conducted to enhance understanding of the hydrologic and phosphorus movement relationships of flat, sandy, high-water-table soils pervasive in the Kissimmee River Basin Region of Florida which drains into Lake Okeechobee. This region generates high phosphorus loads into the lake relative to other basins, causing environmental concerns. Field instrumentation and monitoring wells were installed at four experimental pasture sites in the Kissimmee River Basin to conduct research concerning water and phosphorus movement in fiat, sandy, high-water-table soils. The focus was to obtain hydrologic/water quality data sufficient to determine surface/subsurface flow partitioning, and thus phosphorus flowpaths from the land phase to stream channels. A field-scale model was developed to simulate water and phosphorus movement from individual fields. This model, named FHANTM, is based on DRAINMOD with modifications to include simulation of phosphorus movement and routing of overland flow. The field monitoring data were used to calibrate and verify the modified model. This model was developed for use in predicting the effects on phosphorus movement to streams from application of management practices on the dairies. Keywords: Hydrology; Hydrologic modeling; Runoff; Phosphorus; Tracers; Flatwoods soils

1. I n t r o d u c t i o n

Available data and scientific knowledge has indicated that excess phosphorus (P) inputs into Lake Okeechobee were the controlling factor in the degradation

Paper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the South Florida Water Management District and the University of Florida, Institute of Food and Agricultural Sciences. " Corresponding author. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0925-8574(95)00029-1

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process which was occurring in the lake (DER, t986). Previous studies and data available at that time indicated that the Kissimmee River Basin Region (KRBR), i.e. the Lower Kissimmee River Basin, Taylor Creek/Nubbin Slough Basin, and other similar nearby basins to a lesser extent, were the primary source of the excess P being discharged into Lake Okeechobee (SFWMD, 1989). While the general source of the P was known, there was a lack of knowledge about the specific mechanisms and processes involved. The effects of the physical and chemical characteristics of the soils on P movement to the lake were not well understood. The hydrology of these flatwoods (see next section for definition) soils as it affects water and P movement through the landscape to the lake was not well quantified. Previous research information obtained in the KRBR had shed some light on the problem; however, much was still to be learned concerning the behavior of P in these soils, the flowpaths of water and dissolved constituents through and over the soil, and the quantities, storages, and mass flows of P within the KRBR resulting from agricultural and other activities. Capece et al. (1987,1988) analyzed hydrologic data collected from KRBR watersheds and developed improved rainfall-runoff relationships. Heatwole et al. (1987a, 1987b,1988) subsequently incorporated these findings into and developed the CREAMS-WT and BASIN models. This software package and database allows evaluation of BMP water-quality impacts upon the region. In brief, modifications to the hydrologic component of CREAMS restrict deep seepage out of the surficial water table and introduce a recession curve model to handle water-table dynamics below the root zone (Heatwole et al., 1987b). Within the root zone an evapotran° spiration (ET)-precipitation accounting procedure budgets soil moisture. The high phosphorus buffering capacity of the CREAMS soil phosphorus model was also drastically reduced to better reflect flatwoods soil conditions (Heatwole et al., 1988). This was accomplished by increasing the depth of the active layer within which soluble P is assumed to be available for extraction into runoff and for leaching into the root zone. This has the same effect as reducing the P extraction coefficients. Outputs from the modified CREAMS model serve as inputs to the BASIN model. BASIN takes the CREAMS-WT generated "edge-of-stream" nutrient loads and introduces factors to reflect nutrient loads, BMPs, and stream and wetlands attenuation, as well as non-agricultural background levels, to arrive at estimates of water-quality impacts at the basin outlet (Heatwole et al., 1987a). These efforts answered some questions regarding phosphorus sources and transport, but also identified other important questions. Among the additional needed research topics identified was that of the region's unique runoff flowpaths and the related phosphorus transport mechanisms. Specifically identified was the need for clarification of appropriate hydrologic parameters relating watershed characteristics, land use, and management practices to the runoff parameters used by CREAMS-WT. The need for more detailed information regarding hydrologic events and associated P transport led to development of a new model which is described in a later section. The major goal of this research was the development of a methodology enabling quantification of hydraulic phosphorus transport in the Kissimmee River Basin

K.L. Campbell et al. /Ecological Engineering 5 (1995) 301-330

303

Region in response to changes in land management. Two major objectives were identified and can be summarized as: (1) further characterization and quantification of hydrologic, hydraulic, and chemical processes within the land phase affecting phosphorus movement in the Basin through extensive field experiments and data collection, and (2) development and delivery of a mathematical model and data methodology sufficient to quantify significant constituent processes on a field scale as well as estimate impacts of management alternatives upon the system's phosphorus dynamics.

2. Experimental methods and pasture site descriptions The dominant soil order in the Kissimmee River Basin Region is Spodosols, commonly referred to as fiatwoods. Approximately one-third of Florida is classified as having flatwoods soils. These soils are characterized by amorphous materials (organic matter, aluminum and iron oxides) in a subsurface horizon. The specific suborder in Florida is Aquods, referring to areas which are seasonally saturated with water, gently rolling range or woodland. The general topographic classification of flatwoods in Florida includes the Gulf Coast and Atlantic Coast Flatwoods (thermic zone) and the Southern Florida flatwoods (hypothermic zone). Data collection sites for this study lie within the Lower Kissimmee River and Taylor Creek/Nubbin Slough Basins. The predominant soil associations in both basins are Myakka-Immokalee-Waveland and Wabasso-Felda-Pompano. Despite the high hydraulic conductivities of these soils (> 16 cm/h), drainage is poor unless augmented by extensive ditching. Soil Conservation Service (SCS) hydrologic classification is A / D or B / D , the exact class determined by the effectiveness of drainage improvements at lowering the water table. Natural vegetation consists primarily of wet prairie grasslands interspersed with strands of pine-palmetto woodlands. In the depressional areas, wetlands species predominate. Land use in the two basins is dominated by improved and unimproved pasture. Transformation from a natural marsh and slough system to agricultural lands has been realized through drainage improvements, i.e. ditching. Without extensive ditching and diking, the extremely small watershed slopes ( < 0.5%) make delineation of watershed boundaries a difficult task. In addressing objective 1, four research sites were selected and established to provide detailed monitoring data. The approximate location of each site is shown in Fig. 1. Maps describing topographic features and locations of instrumentation systems for each research pasture are shown in Figs. 2-5. An extensive hydrologic data collection system was installed on these four beef and dairy pasture sites. Field personnel collected surface and well water samples and lab personnel analyzed the samples for nutrients according to established Q A / Q C procedures (Campbell et al., 1991). Three specific Spodosols described with each site vary in the depth to the top of the spodic layer, which varies from approximately 0.5 to 3 m. All data were obtained under natural rainfall conditions. Field ditches were blocked to mimic undrained, natural conditions. Two sites were in high density

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Fig. 1. Locationsof research sites near Lake Okeechobee,Florida(Capece, 1994). dairy pasture. One of these sites (Dry Lake #1) has a medium-depth spodic layer. The other site (Larson #6) has a deep spodic layer. Two sites (Williamson Cattle Ranch and W.F. Rucks) are in low density beef pasture. Williamson has a spodic layer at medium depth. W.F. Rucks has a shallow spodic layer. Surface water flows from the three sites with shallow or medium-depth spodic layers were measured using flumes. Samples were collected automatically at the flumes and then analyzed for substrate concentrations in the laboratory (Campbell et al., 1990). This resulted in discharge hydrographs with chemical concentrations correlated with flowrates by a flow-proportional discrete sampling scheme. Samples were taken at six volume increments in sets of four samples, such that the increments are in a geometric progression from 0.05 cm to 1.6 cm of runoff. For example, the first four samples were taken at 0.05 cm increments, the second four at 0.10 cm increments, etc. In this way, the 24 sample bottles available in the automatic sampler sample adequately to describe chemically all events from small to large. The smallest adequately sampled event is 0.1 cm of runoff. The largest adequately sampled event in this scheme is 13 cm of runoff. This 13-cm runoff event corresponds to the 5-year, 24-h event (Fig. 4; Capece et al., 1987), while assuming a pre-storm water-table depth of 0.4 m. The first sample was taken upon

K.L. Campbell et al. / Ecological Engineering 5 (1995) 301-330

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detection of a runoff event, sampling the substrate content in the first trickle of runoff. Subsurface water was monitored and sampled in banks of wells initially distributed in a modified " I " pattern as shown in Fig. 6. The network consists of a tracer application compound, a primary well transect, and two orthogonal well transects. Each well station consists of two to four wells screened at different depths. Field personnel collected water-table depth data from wells screened

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either above or below the spodic layer throughout each of the sites. These and other wells were used in tracer experiments as described in Campbell et al. (1990). The 150 or more wells at each site were monitored for piezometric heads to obtain information on vertical and horizontal hydraulic gradients at various depths in the soil. The depth to water level was recorded in all wells containing water at the time of measurement. The elevations were measured and averaged on approximately a weekly basis. These values were later compared with model outputs for calibration and verification.

K.L. Campbell et aL /Ecological Engineering 5 (1995) 301-330

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During the 17 months from July 1990 to D e c e m b e r 1991 a total of more than 5000 ground-water samples were taken from the 550 wells located on the four pasture sites. More than 4000 of these samples were analyzed for presence of the salt tracers to determine the vertical and lateral movement of the tracers. In addition, more than a year after application of the salt tracers, deep soil cores were obtained from two locations on each monitoring site and the pore water was analyzed for bromide content. Weather data were recorded as half-hour averages near the center of each plot by an automatic datalogger with electronic sensors. Sensors, placed two meters above the ground surface, continuously monitored temperature, humidity, wind

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speed, solar radiation, and rainfall. Rainfall depths were recorded every minute during events using a tipping bucket rain gage. All equipment was maintained on a weekly basis. Collected data were analyzed and manipulated in a spreadsheet program set up to tabulate and provide statistical comparisons• The measured weather data were collected for the purpose of having an accurate, on-site record of rainfall and factors necessary to estimate evapotranspiration (ET). Rainfall and ET data provided necessary inputs to hydrologic simulation models. The method of Jones et al. (1984) was followed in calculating daily Penman ET. The following sub-sections provide a brief physical description of the layout of

K.L. Campbell et al. / Ecological Engineering 5 (1995) 301-330

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each research site. Additional details regarding site characteristics are included in Campbell et al. (1988) and Capece (1994).

2.1, Larson Dairy #6 The Larson Dairy #6 site is a moderately well drained Pomello (sandy, siliceous, hyperthermic Arenic Haplohumods) fine sand knoll (USDA-SCS, 1971). The area selected for intensive study lies approximately 300 m from the milking barn and directly adjacent to the waste treatment lagoons. Mosquito Creek forms one edge of the site (Fig. 2). No flow measurement flume was installed on this site because there was no expected overland flow. The pasture area is composed of a highly permeable sand with significant surfade slope. Observations indicated that all rainfall rapidly infiltrates and re-emerges as a seepage zone on the area near the creek. Under moderate to heavy rainfall conditions the creek boundary effectively merges with the pasture seepage zone into one continuous water body

3 l0

K.L. Campbell et al./Ecological Engineering 5 (I995) 301-330

making runoff measurements impractical. Livestock density at this site was high (18 dairy c o w s / h a ) during the study. A total of 71 well stations were installed on this site. Each well station was composed of two or three wells of different depths. The primary transect is offset from the center row of wells in the compound rather than being aligned with the center well stations. The standard network layout was modified because the curvature of the hill suggests that a ground-water streamline might also curve from the east-southeast direction to the east-northeast direction as it moves down the hill. The primary transect offset provides a better chance to detect any tracer movement along a ground-water streamline. A cattle shade structure is located to the north of the primary transect. Unfortunately, the location of this shade structure obstructs the most obvious (and straight) streamline path. Thus, the network established represents a compromise.

2.2. Dry Lake Dairy #1 The Dry Lake Dairy #1 site lies in an Immokalee (sandy, siliceous, hyperthermic Arenic Haplaquods) fine sand area and is poorly drained. The pasture selected for intensive study is adjacent to a drainage ditch and slough (Fig. 3). The flume station was located approximately 200 m from the weather station. Two supplemental wells were constructed at the weather station for continuous water-table monitoring. Water-table depth in one well near the flume also was continuously monitored by the flume datalogger. A low, broad berm isolated the research area from the rest of the pasture. The berm varied in height from 45 to 60 cm above ground surface and was approximately 5 m across at its base. The total area contained within this berm was 5.9 ha. A shallow collection ditch paralleled this berm for approximately one third of its distance nearest the flume station. This V-shaped collection ditch was grassed and was less than 30 cm deep and approximately 3 m wide. The newly constructed collection ditch discharged into a trapezoidal flow measurement flume with a maximum capacity of 7.1 cfs (200 l/s). A total of 69 well stations were established on this site. Each well station was composed of two or three wells of different depths. A recently filled ditch was located to the west of the primary well transect. Unfortunately, like the shade structure at Larson, the location of this ditch obstructed the most obvious streamline path. Thus, the location of the Dry Lake well network represents a compromise. Also like the Larson site, livestock density at Dry Lake was high (15 dairy cows/ha). The Dry Lake site was a difficult location in which to conduct research due to extreme wetness and high cattle density. Because the site is on Immokalee soil with surface depressions and little slope, the pasture becomes flooded during the rainy season. The spodic horizon of an Immokalee fine sand typically occurs at approximately 90 cm depth and averages 50 cm in thickness. The upper zone of the organic horizon is black and weakly-cemented, while the lower zone is a mottled, dark reddish brown and is even more weakly cemented than the upper layer.

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During the wet season, the water table stands near or at the surface for short periods and recedes to below 120 cm during the dry season. 2.3. Williamson Cattle Ranch The Williamson Ranch site is a low-intensity (1 beef cow/ha) cattle grazing area in the Taylor Creek Basin. Soil survey maps indicate lmmokalee (sandy, siliceous, hyperthermic Arenic Haplaquods) and Myakka (sandy, siliceous, hyperthermic Aeric Haplaquods) soils in this area. The site has soils similar to the W.F. Rucks and Dry Lake dairy sites. Unlike the other three sites, trees and palmettos are dispersed through the Williamson Ranch site. A general description of Immokalee soils is given under the Dry Lake Dairy section. The A and E horizons for Myakka are 50-75 cm thick. The organic horizon is typically about 15 cm thick, dark reddish-brown in color, strongly acid and weakly cemented. This layer is often highly irregular with many tongues and pockets of white sand. However on this site, the organic pan is unusually uniform and dense, far more dense than at any of the other sites. Immediately below the organic pan is a dark brown layer about 25 cm thick. Normal water-table depth is 75 cm, varying between 0 and 40 cm during the wet season and dropping below 120 cm during the dry season. The Williamson weather station was located approximately 30 m south of the tracer application compound west corner (Fig. 4). The flume station was located approximately 180 m from the weather station. Three supplemental wells were located at the west corner of the tracer application compound. Two of these wells were instrumented for continuous water-table monitoring by the datalogger located at the weather station. Two supplemental wells were also constructed at the flume station. One was instrumented for continuous water-table monitoring. A small berm isolated the research area from the rest of the pasture. The berm varied in height from 30 to 60 cm above ground surface and was approximately 1 m across at its base. The total area contained within this berm was 2.8 ha. A shallow collection ditch paralleled this berm for approximately one half of its length nearest the flume station. This collection ditch was less than 30 cm deep and approximately 1 m wide. The shallow collection ditch discharged into the trapezoidal flow measurement flume with a maximum capacity of 1.8 cfs (50 l/s). The flume discharged into a newly constructed shallow ditch draining into a small wetland which, in turn, drained into a large ditch leading to Taylor Creek. A total of 63 well stations were installed on this site. Each well station was composed of two or three wells of different depths. The Williamson primary transect and network follows the most probable surface and ground-water streamline. Three additional well stations were located on the Williamson site. These stations were used primarily for collecting water quality samples. 2.4. W.F. Rucks Dairy The W.F. Rucks Dairy site is a low-intensity improved pasture. While Myakka (sandy, siliceous, hyperthermic Aeric Haplaquods) is the dominant soil series, soil

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maps indicate that the northeast corner of the surveyed area falls into an Immokalee zone. A detailed site investigation to ground-truth a ground penetrating radar survey actually found six soil series on the site: Smyrna, Myakka. Immokalee, Punta, Basinger and St. Johns, with Myakka and Smyrna dominating. Myakka and Smyrna fine sands are poorly drained with a well-developed organic pan. The solum thickness for the two soils is typically less than 1 m for Smyrna and more than 1 m for Myakka. The W.F. Rucks weather station was located approximately 50 m west of the tracer application compound and the flume station was located approximately 250 m from the weather station (Fig. 5). One well near the weather station and one well at the flume station were instrumented for continuous water-table monitoring. Two drainage ditches formed the north and south boundaries of the research site. Each of these ditches had spoil berms on their north and south sides. Originally these berms contained several breaches. These breaches were filled. The continuous berms were approximately 30 cm above ground surface and approximately 2 m across at their base. The total area contained within this berm was 3.9 ha. A shallow collection ditch 120 m in length was constructed at the east edge of the site. This collection ditch was less than 30 cm deep and approximately 1 m wide. This ditch had an accompanying berm on its eastern edge. This berm was less than 30 cm high and 1 m wide. The shallow collection ditch discharged into the trapezoidal flow measurement flume with a maximum capacity of 1.8 cfs (50 I/s). The flume, in turn, discharged into an existing 2 m deep perimeter ditch. Originally the two drainage ditches which form the north and south edges of the site also flowed into this perimeter ditch. These drainage ditches were, however, blocked at four locations along their length and no longer have an outlet into the perimeter ditch. However, two very shallow ditches running the length of the W.F. Rucks site still are capable of carrying flow. Both these ditches empty into the flow collection ditch leading to the flume. A total of 51 well stations were established on this site. Each well station was composed of two, three or four wells of different depths. Unlike the other three research site tracer application compounds, the well lines of this compound had four, not five, well stations each. Two shallow drainage ditches formed convenient boundaries for the tracer application compound, thus making this compound more narrow than the others. The W.F. Rucks primary transect and network follows the most probable surface and ground-water streamline. Like the Williamson site, livestock density at W.F. Rucks (0.6 beef cows/ha) was very much lower than at Dry Lake and Larson.

3. Results and discussion

Weather, runoff and ground water quality measurements were obtained at the four above-described experimental sites in the Kissimmee River Basin to provide information concerning water and phosphorus movement in fiat, sandy, high-

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water-table soils. A major purpose for these measurements was to support the development of tools useful in determining the effectiveness of alternative management practices in minimizing the movement of phosphorus into streams and Lake Okeechobee. The surface water measurement system well documented the water and nutrient discharges from the areas. Figs. 7 and 8 show examples of large and small runoff events with their associated phosphorus concentrations and calculated loads. Note the phosphorus concentrations and loadings superimposed on the figures at the actual sample spacing during the event (the vertical lines indicate the actual sample collection time). The loadings are on a per sample basis, representing the P load carried by the runoff volume occurring during the time increment from the midpoint of the previous sample interval to the midpoint of the following sample

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K L. Campbell et al. / Ecological Engineering 5 (1995) 301-330

Table 1 Summary of annual weather and surface runoff data collected on each site (Tremwel, 1992) Location and years

100 ( E T + RO - R) R (%)

P load in runoff (kg/ha)

R rain (cm)

ET (cm)

RO runoff (cm)

88.66 117.47 133.00 339.13

91.37 100.30 92.18 283.85

5.15 25.71 44.29 75.14

+ 8.87 +7.27 +2.61 +5.86

3.94 31.08 46.16 81.19

78.58 97.46 101.80 277.84

74.50 89.12 86.81 250.43

None None None None

- 5.19 - 8.56 - 14.73 - 9.87

None None None None

1989 (April-Dec.) 1990 1991 Total

93.85 90.60 128.59 313.04

83.95 93.61 88.84 266.40

8.68 7.03 37.59 53.30

- 1.30 + 11.08 - 1.68 +2.13

0.66 0.33 1.66 2.66

W'dliamson 1989 (April-Dec.) 1990 1991 Total

96.54 100.34 112.07 308.95

69.73 75.48 83.96 229.17

0.19 18.10 0.00 18.29

- 27.57 -6.74 - 25.08 - 19.94

0.01 1.56 0.00 1.57

Dry Lake # l 1989 (April-Dec.) 1990 1991 Total

Larson #6 1989 (April-Dec.) 1990 1991 Total

W.F. Rucks

interval. Since the samples were taken based on a geometrically increasing sample volume, the later samples have more weighting and the last sample is weighted largest to account for the entire increment up to the end of the hydrograph. Summary data for annual rainfall, ET, observed runoff, and P loads calculated from sample concentrations and observed runoff are in Table 1. The summary data confirm that 1989 and 1990 were drought years when compared to the long-term average rainfall of approximately 125 cm. The data for 1989 began on April 1, which is before the wet season but January through March can still bring appreciable rain in Florida. The data also confirm that in 1991 the eastern two sites (Larson #6 and Williamson Cattle Ranch) were still in drought, while the western sites (Dry Lake #1 and W.F. Rucks) received average rainfall. The wetter weather in the west resulted in a decrease in ET in 1991 compared to 1990, presumably from increased cloudiness and lower soil temperatures resulting from more rain. The installed network of wells provided an effective means for collecting data on water-table gradients, flow vectors, water quality, and tracer movement on these sites. The sandy soils yield well-behaved water tables and are well suited for drainage investigations. Example ground-water levels recorded along the well transects at each site are presented in Figs. 9-12. Water-table profiles along the

K.L. Campbell et al. / Ecological Engineering 5 (l 995) 301-330

315

Larson Daiw #6 Water Table Along P r i m o w Transect 48 . J 46

A B" C D E --

~44

F

G

H

--

/

Ground Sur~--e

~ 42 ~ 38

-_ 36

..~ 34 ~ 3o

i

To~ of Sl~0dic Hod,zon

28

-50

50

150

250

350

450

550

650

750

850

7~o

~.~

Water Table Along G-Row Transect O3 4( 4~ 4~ m

G1

G2

G3

G4

G5 G6 G7 G8 G9

GIO

4C

cu

3d 3~ [] -SO

_J

ft

,-n

48 4~ 44 42 40. 38~ 36; 34~ 32:

Waste Treatment Lagoon

sb

1~o

~.~

~o

~

~,o

~

Water Table Along K-Row Transect

K1._____-~

sb

~

1~o

K,

~

~

~o

K~...__P

~o

r~o

C~o

.K.9

r~o

eso

Distance Along Transect, feet Fig. 9. Example water-table profile at Larson Dairy # 6 (Capece, 1994).

primary transects exhibit the expected curvature as the transect approaches the ditch or stream. Gradients along transects orthogonal to the primary transect are generally negligible, with certain exceptions owing to boundary conditions. During high water-table periods the Williamson Cattle Ranch site exhibited a consistent head differential between wells screened above the spodic horizon and those screened below (Fig. 10). This differential decreases and eventually disappears with increasing distance along the primary transect. This observation is consistent with soil observations which document a well defined spodic horizon in the higher elevation which gradually becomes more irregular and permeable at lower elevations on this site. The fact that both water-tables register above the spodic horizon suggests that the spodic horizon is causing air entrapment and a corresponding rise in below-spodic well levels.

K.L. Campbell et al. /Ecological Engineering 5 (1995) 301-330

316

Williamson Cattle Ranch Water Table Along Primary Transect 3837"

36-

~~

Above-Spocllc Well

A3 n_'~. IO"

Below-Spoclic We~l

17

[-]

GrounO Surface

/

Well Station 1.0

35-

33- 32-

XTopof SpOcliC

Horizon

28 -so

~o

ss0

Water Table Along G-Row Transect G1

G2

Ga

G4

G5

G6

i37

0 o

:~ 2 8 / UJ -50

~ ~f 35 "~ 341

~

rn

28|

.so

Table

Water J1

b

,12

~b

....

J3

Along J-Row J4

J5

JS

16o 1~o 26o z~o ~o

Transect Jr

~

,~o 4~o s6o

55O

Distance Along Transect, feet Above,-,Spoclic Wells

~

Below-Spodic

Wells

[ I

Fig. 10. Example water-table profile at Williamson Cattle Ranch (Capece, 1994).

Total phosphorus data from wells at each of the four sites are presented in Figs. 13-16. These data are averages from two or more wells at each depth at different locations within the site. The data reflect generally higher P concentrations at the very shallow well depth at all sites. They also reflect the difference in land use intensity. Both the Dry Lake data (Fig. 15) and Larson data (Fig. 13) show very high concentrations (3-50 mg P / l ) of total phosphorus in the more shallow ground water. As water moves into the deeper horizons higher in organic content, phosphorus concentration drops dramatically at Dry Lake becoming less than 1 mg P/I in the 3- and 6-m wells. At Larson, however, little improvement in water quality occurs below the spodic horizon on the upslope portion of the pasture. As mentioned above, the soils on this site consist of very deep, coarse sands with a barely detectable spodic horizon. A significant improvement does occur in the downslope area where the spodic becomes well defined and total phosphorus concentrations decreased to less than 2.5 mg P/I early in the study, however concentrations increased in this well to about 7 mg P/I later in the study. These

K.L. Campbell et al. / Ecological Engineering 5 (I 995) 301-330

317

Dry Lake DQiry #1 W a t e r T a b l e A l o n g Primary Transect 38._1 03 >

Above-Spo
Well Station I.D.

~

36,

1 ~ Ground Surface

3534-

J

m

g, rn

3,3-

Below-Spo¢llc Well

To~ Of Spod|c Hodzon

32' -50 _.J cO

~

s~o

~

7~o

~o

Water Table Along G-Row Transect 38/

37-

G1

G2

G. ~_. ~_= p_= G7

G8

G9

(310

Gll

3635"

e-- 340 33"

~ . . ,

/

¢ I

32.~

5b

-4

1~o Water Table Along K-Row Transect K8

Kt

~

K5 K3 Ka ~

K6 ~

~

321

DistanceAlongTransect,feet Fig.

1l.

Example water-table profile at Dry Lake Dairy # 1 (Capece,

1994).

data combined with the water-table observations depicted in Fig. 9 suggest that in the upslope area, ground water of poor quality is ponded and isolated from the ground water of higher quality near the creek. The very limited shallow water-quality data collected at the Williamson site show total phosphorus concentrations generally less than 2 mg P/1. Observations show concentrations less than 0.4 mg P/I at 3 m below the ground surface. Data from the W.F. Rucks site are similar to the observations from Williamson with near surface ground water total phosphorus concentrations in the 0.05 to 1 mg P/1 range. Extreme variability in the data make any trend with depth impossible to detect at W.F. Rucks. The occurrence and integrity of the soil spodic horizon is a major factor influencing drainage characteristics and ground-water quality on these sandy soils. The information obtained from these monitoring systems is extremely

318

K.L. Campbell et al. / Ecological Engineering 5 (I 995) 30/-330 W. F. Rucks Dairy Water Table Along Pfimow Transect

"1I

-"q

,~ +,+

O

~^ ~

..........

~, E3F3 ~ ' H4 I1

:-+~

_

I~ 31

'~

/ Well St;at]on I.D.

.+

+

+,

....

TOp of SpO¢lic HortZofl

+~so}

l+-.~,°w.,

-so

so

+~o

z+,o

K.~

4.+0

~

...

e.+o

7.+0

85O

Water Table Along H-Row Transect

64-

¢)

§

¢tl 6261c

O 60m

-SO

Water Table Along K-Row Transect

#, 64 =E

>~ 63-

K1 ~

KK2 _ s

Ke

62-

E°

b

0

6160-

n1 59+ +50 D i s t a n c e A l o n g Transect, f e e t

Fig. 12. Example water-table profile at W.F. Rucks Dairy (Capece, 1994). Larson Dairy #6, 1989-1991 Total Phosphorus in Ground Water 100

+10

........

O

C 0 ¢..) 0.1 NOV-89 Fe~90 Ma;-90 Aug'-90 Oct'-90 Jan'-91 Apt'-91

Jul:91

0ct'-91 Jan-92

Sampling Date J -411-2m Well (F1).-~- 5 m W e l l (F1)41=- 3 m W e l l ( N 1 ) l

Fig+ 13. Ground water total phosphorus concentration at Larson Dairy #6 (Capece, 1994).

K.L. Campbell et al. / Ecological Engineering 5 (1995) 301-330

319

Williamson Cattle Ranch, 1989-1991 Total Phosphorus in Ground Water

10 A .J Q. P~

el

g

1

~= ¢) 0.1 (J O

0.01 Aug-89

Dec-89 I~

Apr-90

Aug-90 Dec-90 Sampling Date

lm Well (G2) ~

Apt-91

Aug-91

Dec-91

3m Well (G2) -<>- 6m Well (G2) I

Fig. 14. Ground water total phosphorus concentration at Williamson Cattle Ranch (Capece, 1994).

valuable for calibration and verification of computer simulation models of these hydrologic systems. Subsurface water movement studies were conducted on the four pasture sites using salt and dye tracers (Capece, 1994). The Rhodamine dye tracer experiment proved to be inconclusive due to poor tracer performance resulting from dye adsorption to soil particles. Salt tracer (chloride and bromide) experiments were initiated in July of 1990 with greater success. Chloride salt solution was applied to the surface of the tracer application zone and used to monitor local percolation. Due to high background levels in shallow ground water, chloride could not be tracked far beyond its local application zone. The bromide salt tracer was injected

100

,0 t-"

¢-

Dry Lake Dairy #1, 1989-1991 Total Phosphorusin Ground Water

...................

......

-7

...........

.

. . . . . .

1

0.1

O

0.01 . . . . . . . . . . . . . . . Oct-89 Jan-90 May-90 Aug-90 Nov-90 Mar-91 Jun-gl Sep-gl Dec-g1

Sampling Date

14- lm Well (K2) -a,- 3m Well (K2) -o- 6m Well (K2) I Fig. 15. Ground water total phosphorus concentration at Dry Lake Dairy #1. (Capece, 1.994).

320

K.L. Campbell et al. / Ecological Engineering 5 (1095) 301-330 W.F. Ruc,ks Dairy, 1989-1991 Total Phosphorus in Ground Water

1

n

E

o.1

0

c

O tO

0.01

c.) 0.001 Aug-89 Nov-89 Feb-90 May-g0Aug-90 Nov-90 Feb-91 May-91 Aug-91 Nov-91 •

=

-

,

-

,

-

,

•

.

•

,

•

,

-

g

•

•

•

Sampling Date I - I - lm Well (M 1)-ilk- 2m Well (M1).o- 6m Well

(M1)I

Fig. 16. Ground water total phosphorus concentration at W.F. Rucks Dairs' (Capece, ].994).

into the soil profile via the shallow wells just above the spodic horizon, 0.5 to I m below the ground surface. Bromide proved to be effective as a subsurface watermovement tracer. As expected, the hilly Larson site showed the highest rate of tracer movement. Within 30 days of application, the surface-applied chloride had reached the ground-water table and was appearing in both the shallow (2 m) and medium (4 m) wells. A year after application, soil cores showed the bromide concentrations at the application point to be highest between 4 and 6 m below the surface with maximum concentration occurring at 4.5 m. In moving from the C12 to F1 locations, the bromide plume does not appear to have moved deeper in the profile; maximum tracer concentration remained at 4.5 m. Fig. 17 shows the lateral progression of the tracer plume. The plume appears to have arrived at the D row 100

## i#

.~

0.1

IIX

i

"

0

.-"

F

i

:\

m O.Ol

q

•

/

;

100

nii

i ii : "

200 300 400 Days After Injection at Row "C"

I~k--O ....

m~ Erow ,,.O= F . . . .

500

I - Grow J

Fig. ].7. Bromide tracer movement at [.arson Dairy # 6 (Capece, 1994).

K.L. Campbell et aL / Ecological Engmeering 5 ~1995) 301-330

321

(7.5 m from the application point) approximately 80 days after injection. Tracer appearance at the E row (7.5 m from D row) followed approximately 100 days later. The plume was apparent at the F well station (15 m from the E row) 140 days after that. It required only 40 additional days to reach the G row of wells (15 m from the F station). Over the entire 17 months, average plume velocity was 0.5 cm/h. At the end of the data collection project (December, 1991) bromide concentration at all sampling points (D, E, F, and G rows) dramatically increased to approximately 20 mg/l level. This may have been the result of relatively high water-table conditions, which prevailed from September to November of that year at the Larson site, causing additional tracer movement. The Williamson site exhibited the second highest lateral advective transport rate. Tracer appearance at the first row of monitoring wells (D row) was extremely variable. Some wells showed evidence of both chloride and bromide tracers 4 months after application while others showed no tracer until 7 months after application. However, this variability was damped by the time the plume had traversed an additional 7.5 m to reach the E row 380 days after application. On average, the plume moved at a rate of 0.16 c m / h across the Williamson site during the study period. At the Dry Lake Dairy site vertical movement of the bromide tracer from the shallow injection wells to the deeper monitoring wells was slow, requiring over 2 months travel time. Lateral movement of the plume to the D row wells (7.5 m from the application point) occurred approximately 225 days after injection. Inconclusive appearance of bromide in one of the E row wells was recorded approximately 430 days after application. While an exact calculation is difficult with such slow movement and few positive tracer appearances, approximate plume velocity is 0.09 cm/h. Subsurface lateral movement of bromide at the Rucks pasture was the slowest of the four sites. Bromide appeared in the E row wells (7.5 m from application) 420 days after application. Approximate velocity of the bromide plume at Rucks was 0.08 cm/h. Table 2 summarizes selected attributes of each investigation site including the average plume velocity determined by the bromide tracer study. The data collected were very sparse during the first two-thirds of the project period due to abnormally dry weather conditions which resulted in very little runoff, low water-table levels, and little tracer movement. Only the last year of the project had more normal weather conditions with corresponding high water-table levels and more frequent runoff events at the experimental sites. Fortunately this last wetter year provided enough runoff data that the field-scale model could be calibrated, verified and tested using the complete period of record for the various testing phases. Some problems were encountered in model calibration and verification due to the inherent differences in model response from the dry calibration data period to the wetter verification data period. Inspection of these results suggests several generalizations relating surface/subsurface hydrology to potential for phosphorus transport. It is apparent that topography and drainage boundary conditions play a dominant role in phosphorus transport on these pastures, emphasizing the importance of site specific factors in

322

K.L. Campbell et al. / Ecological Eng,neering 5 (1995) 30l -330

Table 2 Summary of selected attributes for each investigation site (Capece, 1994) Characteristic

Attribute by site Larson

Ground slope, % Surface runoff, cm/yr Surface runoff P, kg h a - t yr- i Water-table gradient, %

Williamson

W.F. Rucks

1.7 0 0

0.42 6 0.5

0.14 18 0.9

Dry Lake 0.13 25 27

1.4

0.4

0.2

0.2

Plume velocity,cm/h Subsurface flow, cm/yr Shallow GW TP, mg/I Deeper GW TP, mg/l Subsurface flow P, kg ha- t yr- t

0.5 25 30 10 25

0. t6 17 0.3 0.I 0.2

0.08 - 2 0.2 0.1 0

0.09 -2 6 0.2 0

Land use intensity, cows/ha Total P discharge, kg ha- t yr- ~

18 25

1

0.7

0.6 0.9

15 27

phosphorus movement. At all sites, the very shallow (1 m) ground water generally contained higher levels of phosphorus than deeper ground water (3-6 m). The magnitude of the phosphorus concentration appears proportional to land use intensity. Thus at sites which exhibit low relief, slow ground-water flow, and a tendency to generate surface runoff (Rucks and Dry Lake), surface drainage is likely to be a significant transport mechanism for phosphorus (see Table 2). At Dry Lake, shallow (1 m) ground-water concentration of phosphorus is on the order of 2-10 mg P / I . This, combined with a tendency to flood and discharge surface runoff, provides a high potential for phosphorus discharges. A similar hydrologic potential for phosphorus discharge exists at Rucks. However lower intensity land use offsets this hydrologic factor. The less intense land use is reflected in lower shallow ground water total phosphorus content which seldom exceeds 0.5 mg P / I . At those sites with greater relief, more rapid ground-water flow, and little, if any, tendency to generate surface runoff (Larson and Williamson), any off-site discharge of phosphorus will likely be generated by subsurface drainage to ditches and nearby natural water bodies. While exhibiting a potential for significant subsurface phosphorus discharge, off-site impacts are minimized as a result of the reduction in phosphorus concentration which occurs as the ground water moves through the soil profile. At WiUiamson, shallow (0.75-1 m) concentrations of 0.2-2 mg P / I are reduced to generally less than 0.2 mg P / I upon reaching deeper depths (2.5-3.5 m). Below that depth, a clay horizon both restricts ground-water flow and provides a sink for phosphorus. Despite a very dense spodic horizon at the higher elevations of the site which creates a perched water table and standing water at times, rarely did the site generate surface runoff. This characteristic is attributable to the disappearance

K.L. Campbell et al. /Ecological Engineering 5 (1995) 301-330

323

of a well-defined spodic and the perched water table at lower elevations of the site. The water table is also influenced by relatively low drainage boundary conditions induced by nearby ponds and deep drainage ditches. At Larson, the effects are even more dramatic. In the higher elevations of the site, shallow ground water (2 m) shows in excess of 40 mg P/I. This concentration decreases to less than 20 mg P / I at greater depths (4 m). Yet downslope near the receiving water body (Mosquito Creek) the ground water exhibits 1 to 7 mg P/I. Table 2 summarizes hydrologic and phosphorus transport characteristics of the four research sites based on data measured during the period April 1989 to December 1991. Subsurface flow and phosphorus mass transport were estimated from water balance and other hydrologic and soil parameters measured and combined with ground water phosphorus concentrations over the period of record. In estimating surface runoff potential, surface slope appears to be a good indicator. The extremely fiat sites ( < 0.3% slope) generated more surface runoff than the sites with slightly greater slopes ( > 0.3%). This suggests that the real factor which controls surface runoff is subsurface storage. Surface runoff is less on those sites where significant slope induces significant subsurface flow and thus greater soil profile storage potential. Topography and boundary conditions dictate subsurface flow which in turn dictates surface runoff. The magnitude of total phosphorus discharge from a site is closely related to land use intensity while the specific hydrologic fiowpath (surface or subsurface) of the exported phosphorus is a function of the topographic features. Given the dramatic differences in water/phosphorus flowpaths, no uniform water-management strategy will reduce phosphorus surface and subsurface discharge from these flatwoods pasture sites. Thus, water-quality management strategies should be tailored to site specific conditions to be both effective and cost efficient. Data from these four pastures suggest that on extremely flat sites phosphorus control strategies should emphasize surface flow control or collection/treatment. For sites with slightly greater slopes, subsurface flow is the primary phosphorus flowpath and local surface water collection/treatment is less likely to yield water-quality benefits. It is important to recognize that, for flatwoods watersheds, surface runoff is a term defined more by measurement point reference rather than by absolute flowpath. While the bottom of runoff measurement flumes may be at the groundsurface elevation or slightly below this elevation ( < 45 cm), the water reaching these flumes may have, at some point in traversing the pasture, experienced contact with shallow portions of the soil profile.

4. Model development and testing The concluding segment of this field investigation and modeling effort was the development of a hydrologic/water quality simulation model to quantify significant constituent processes on a field scale as well as estimate impacts of management alternatives upon the system's phosphorus dynamics. Infiltration and runoff in Florida's sandy soils are storage-limited functions of soil porosity. In all practical

32-t

K.L. Campbell et aL /Ecological Engineering 5 (1995) 301-330

cases, the soil profile must be saturated before runoff begins. DRAINMOD was chosen as a platform for modeling these soils due to its excellent soil water storage and movement components. The immediate objectives of the Field Hydrologic And Nutrient Transport Model (FHANTM) modeling effort were to: (1) simulate surface and subsurface water discharge from Spodosols, (2) simulate constituent (especially phosphorus) loss from Spodosols, (3) simulate the effects of boundary conditions such as edge-of-field ditch water levels and edge-of-field subsurface water pressure gradients combined with deep barriers, (4) verify the FHANTM model on a data set independent from the data used in calibration, and (5) determine the sensitivity of FHANTM to key input parameters identified during calibration. Sensitivity was to be determined for surface and subsurface water flow, and for surface and subsurface P movement. Details regarding the FHANTM model development and use of the model can be found in the FHANTM User's Manual (Campbell and Tremwel, 1992b) and in Tremwel (1992). The DRAINMOD model developed to simulate water-table management (Skaggs, 1980) proved to be a useful platform due to its focus on continuous storage calculations. FHANTM was developed from DRAINMOD for continuous simulation of the phreatic zone moisture balance, thereby allowing the prediction of runoff volumes, peaks, and timing. The following functions were added to DRAINMOD: (1) an algorithm for overland flow routing, (2) a dynamic deep seepage boundary, and (3) algorithms to describe the fate of soluble phosphorus input, mass balance, and transport (Figs. 18 and 19). Seepage volumes are predicted in accordance with water-table fluctuations. FHANTM also simulates P (or any conservative solute) concentration in the phreatic zone and P loads in runoff. Solute transport is based on water-movement simulation, which makes physical sense. The results are an improved ability to model P movement when compared to previous field-scale hydrologic models used in this setting. FHANTM was calibrated and verified on the four pasture sites described in an earlier section. The sites were calibrated in the following order: Dry Lake #1, Williamson, Rucks, and Larson #6. Each site was calibrated and verified before going on to the next site. Model calibration was based on personal judgement of the simulated representation of water-table depth, runoff rate and volume, and surface and subsurface P concentrations compared with observed values for the first half of the record. Calibration was performed for these output variables sequentially in the order listed above. During calibration, relevant input parameters were varied within physically reasonable limits until the output variables were judged to be acceptable. The resulting input data sets were then used for independent verification simulations. The observed water-table data used for calibration is a field average of water-table depth for the wells containing water on the day depicted. The calibration period on all sites for all simulations was April 1, 1989 to August 31, 1990. The verification period was September i, 1990 to December 31, 1991. This allowed 17 months for calibration and 16 months for verification. After calibration was complete, simulation of the entire 33-month period ensued. The simulations were continuous and uninterrupted from the April 1, 1989 starting date.

K.L. Campbell et al. /Ecological Engineering 5 (I 995) 30l-330

325

FHANTMChanges to DRAINMOD Water Table M a n a g e m e n t In DRAINMOD

:

STOR <- STMAX

O=~70~. $1MAX funalf,llO=cO*

In FHAN/~f :S T O e > S T M A X is O K

qP

Runoff

Z~ILTRATION

DeepHe~DfEPH ~ C ~

DEEP SEEPAGE

~>

Fig. 18. FHANTM modifications to DRAINMOD for water handling and water balances (Tremwel, 1992).

FHANTM Additions to DRAINMOD for Substrate Modeling

iI

RUNOFF>

*-I ZNFZLTRATION

PINF

LATERAL FLOW>

/ / / / DEEP SEEPAGE

POEEP

Fig. 19. FHANTM additions to DRAINMOD for P handling and P pool balances (Tremwel, 1992).

326

ICL. Campbell et al. / Ecological Engineering 5 (1995) 301-330

Table 3 Summary of annual observed and simulated data for each site (Tremwel, 1992) Location and years

Obs. RO (cm)

Sim. RO (cm)

Differ. in RO

Obs. P load in RO (kg/ha)

Sim. P load in RO (kg/ha)

Differ. in P load

5.15 25.71 44.29 75.14

2.90 21.20 32.29 56.39

-43.7% - 17.5% -27.1% -25.0%

3.94 31.08 46.16 81.19

5.33 28.21 34.87 68.41

35.3% - 9.2% -24.5% - 15.7%

8.68 7.03 37.59 53.30

14.61 6.38 35.10 56.09

68.3% - 9.2% -6.6% 5.2%

0.66 0.33 1.66 2.66

1.79 1.45 3.69 6.93

171.2% 339.4% 122.3% 160.5%

0.19 18.10 0.00 18.29

0.25 11.89 5.27 17.41

31.6% -34.3% NA -4.8%

0.01 1.56 0.00 1.57

0.03 1.41 0.68 2.11

200.0% -9.6% NA 34.49%

0.19 18.10 0.00 18.29

0.98 7.25 0.00 8.23

415.8% - 59.9% 0.0% - 55.0%

0.01 1.56 0.00 1.57

0.10 0.88 0.00 0.98

900.0% - 43.6% 0.0% 37.6%

Dry Lake #1

1989 (April-Dec.) 1990 1991 Total W.F. Rucks

1989 (April-Dec.) 1990 1991 Total Verified

Williamson 1989 (April-Dec.) 1990 1991 Total Recalibrated

Williamson 1989 (April-Dec.) 1990 1991 Total

U n f o r t u n a t e l y , d e s p i t e the s h o r t n e s s of t h e p e r i o d o f r e c o r d for c a l i b r a t i o n a n d v e r i f i c a t i o n p u r p o s e s , the c a l i b r a t i o n p e r i o d was very dry, while the verification p e r i o d was w e t t e r t h a n a v e r a g e in the w e s t e r n p a r t of O k e e c h o b e e C o u n t y w h e r e b o t h the D r y L a k e a n d t h e R u c k s sites a r e l o c a t e d . This i n c r e a s e d the difficulty of c a l i b r a t i n g for w e t t e r p e r i o d s b e c a u s e t h e c a l i b r a t i o n p e r i o d did not include b o t h dry a n d wet p e r i o d s . H o w e v e r , t h e lack of a w e t p e r i o d for c a l i b r a t i o n allows for a g o o d e v a l u a t i o n o f t h e r o b u s t n e s s o f the m o d e l in s i m u l a t i n g the wet verification p e r i o d . P e r f o r m a n c e d u r i n g t h e v e r i f i c a t i o n d a t a p e r i o d d e m o n s t r a t e d the a c c u r a c y o f F H A N T M in c o n t i n u o u s m o d e l i n g . T a b l e 3 s u m m a r i z e s the results of the c a l i b r a t i o n a n d v e r i f i c a t i o n p r o c e d u r e on all r e s e a r c h p a s t u r e sites with surface runoff. E x a m p l e c u m u l a t i v e m a s s curves o f s i m u l a t e d a n d o b s e r v e d surface r u n o f f a n d r u n o f f P a r e shown in Figs. 20 a n d 21. Sensitivity a n a l y s e s i n d i c a t e d t h a t the i n p u t p a r a m e t e r s which F H A N T M o u t p u t is m o s t sensitive to a r e the o n e s which r e p r e s e n t physical p r o p e r t i e s i m p o r t a n t in a s t o r a g e - l i m i t e d system. Sensitivity analysis was c o n d u c t e d by varying c h o s e n calib r a t e d i n p u t s by + 5 0 % , + 10%, + 1%, - 1 0 % , a n d - 5 0 % , t h e n c o m p a r i n g four

K.L. Campbell et aL /Ecological Engineering 5 (1995) 301-330

FHANTM

Runoff

Dry Lake #1,

327

Simulofion

Apr '89 fo Dec '91

80i

7oJ E

civ,

6o i 50-~ 40~

>~ 3o4 2o~ 0

'°l o

A

J

~

o

j

1989

i

I

J- -

;

o

j

1990

I

FHANTM

.~

J

o

1991

Observed

Fig. 20. Cumulative mass curves of runoff from F H A N T M simulation and observations on Dry Lake #1 (Tremwel, 1992).

cumulative outputs. Model runoff prediction is most affected by ET and porosity. Surface storage and overland flow factors have little effect on runoff quantities, but calibration experience showed these two variables to have a great influence on

FHANTM S i m u l o f i o n

of P

Dry Lake #1, Apr '89 to Dec '91 ,-,90

~.8o 70 60. C

50 ._~ 40

/

30 20 10

o

A

J

'~, 0 1989

A~6 1990

FHANTId

I

Iggl

Observed

Fig. 21. Cumulative mass curves of P in runoff from F H A N T M simulation and observations on Dry Lake #1 (Tremwel, 1992).

328

ICL. Campbell et al. /Ecological Engmeering 5 (1995) 301-330

runoff timing. Seepage simulation is most affected by horizontal permeability. nearby canal water elevation relative to the field, and porosity, in order of most to least. Vertical permeability has little effect on simulated seepage. Simulated P in runoff is most affected by manure additions, rain leaching effects, stored surface water solubility effects, and least by P concentration in rain. Simulated P in seepage is most affected by relative plant P yield and manure additions. There is little effect on simulated P seepage from initial P concentration in phreatic water, P concentration in rain, and rain leaching effects. More details of the calibration, verification, sensitivity, and uncertainty analysis of FHANTM can be found in Campbell and Tremwel (1992a) and Tremwel (1992).

5. Conclusion

This research study, spanning nearly 5 years, has involved a huge amount of experimental field data collection and analyses as well as significant efforts in mathematical model development, modification, and testing. Field instrumentation and monitoring wells were installed at three experimental sites on dairies and one site on a beef ranch in the Kissimmee River Basin to conduct research concerning water and phosphorus movement in fiat, sandy, high-water-table soils. Results of these studies suggest that topography and drainage boundary conditions play a dominant role in phosphorus transport in this region, however the magnitude of total phosphorus discharge is dictated by intensity of land use as it affects phosphorus loading to the land. Given the dramatic differences in water/phosphorus flowpaths, no uniform water-management strategy, will reduce phosphorus surface and subsurface discharge from these flatwoods pasture sites. Thus, waterquality management strategies should be tailored to site specific conditions to be both effective and cost efficient. Data from these four pastures suggest that on extremely fiat sites phosphorus control strategies should emphasize surface flow control or collection/treatment. For sites with slightly greater slopes, subsurface flow is the primary phosphorus flowpath and local surface water collection/treatment is less likely to yield water-quality benefits. Another result of this study was the development of a hydrologic model useful in determining the effectiveness of alternative management practices in minimizing the movement of phosphorus into streams and Lake Okeechobee. A field-scale model was developed to simulate water and phosphorus movement from individual fields. This model, named FHANTM, is based on DRAINMOD with modifications to include simulation of phosphorus movement and routing of overland flow. The field monitoring data were used to calibrate and verify the modified model. This model was developed for use in predicting the effects on phosphorus movement to streams from application of management practices on the dairies. This provides the capability to quantify water and phosphorus transport, both in magnitude and flowpath (surface or subsurface), from other sites in the KRBR in addition to the specific research pastures used for this study.

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Acknowledgements C o n t r i b u t i o n f r o m the I n s ti tu te o f F o o d a n d A g r i c u l t u r a l Sciences, U n i v e r s i t y o f Florida, as a p a r t of S o u t h e r n R e g i o n P r o j e c t S-249 of the U S D A - C S R S with s u p p o r t f r o m S o u t h F l o r i d a W a t e r M a n a g e m e n t District. ( U n i v e r s i t y of F l o r i d a A g r i c u l t u r a l E x p e r i m e n t Station, J o u r n a l Series No. R-04454.)

References Campbell, K.U and T.K. Tremwel, 1992a. Biogeochemical behavior and transport of phosphorus in the Lake Okeechobee Basin: FHANTM sensitivity and uncertainty analysis. Deliverable 2.4.5. South Florida Water Management District, West Palm Beach, Florida, 42 pp. Campbell, K.L. and T.K. Tremwel, 1992b. Biogeochemical behavior and transport of phosphorus in the Lake Okeechobee Basin: FHANTM users manual. Deliverable 2.4.4. South Florida Water Management District, West Palm Beach, Florida, 65 pp. Campbell, K.L.. J.C. Capece and K.C. Stone, 1988. Biogeochemical behavior and transport of phosphorus in the Lake Okeechobee Basin: Site characteristics report. Deliverable 2.2.2A. South Florida Water Management District, West Palm Beach, Florida. 109 pp. Campbell, K.L., J.C. Capece and T.K. Tremwel, 1990. Biogeochemical behavior and transport of phosphorus in the Lake Okeechobee Basin: Documentation of physical research site plans and installation procedures. Deliverable 2.2.3, Subtask 2. South Florida Water Management District, West Palm Beach, Florida, 105 pp. Campbell, K.L., J.C. Capece and T.K. Tremwel, 1991. Biogeochemical behavior and transport of phosphorus in the Lake Okeechobee Basin: Comprehensive quality assurance plan. Deliverable 2.2.2C. South Florida Water Management District. West Palm Beach, Florida, 104 pp. Capece, J.C., 1994. Hydrology and contaminant transport on flatwoods watersheds. Ph.D. Dissertation, University of Florida, Gainesville, Florida, 225 pp. Capece, J.C., K.L. Campbell, L.B. Baldwin and K.D. Konyha, 1987. Estimating runoff volumes from flat, high-water-table watersheds. Trans. ASAE, 30: 1397-1402. Capece, J.C., K.L. Campbell and L.B. Baldwin, 1988. Estimating runoff peak rates from flat, highwater-table watersheds. Trans. ASAE, 31: 74-81. DER, 1986. Lake Okeechobee technical committee final report. Florida Department of Environmental Regulation, November 1986. Heatwole, C.D., A.B. Bottcher and K.U Campbell, 1987a. Basin scale water quality model for Coastal Plain flatwoodL Trans. ASAE, 30: 1023-1030. Heatwole, C.D., K.L. Campbell and A.B. Bottcher, 1987b. Modified CREAMS hydrology model for Coastal Plain flatwoods. Trans. ASAE, 30: 1014-1022. Heatwole, C.D., K.L. Campbell and A.B. Bottcher, 1988. Modified CREAMS nutrient model for Coastal Plain watersheds. Trans. ASAE, 31: 154-160. Jones, J.W., UH. Allen, S.F. Shih, J.S. Rogers, L.C. Hammond, A.G. Smajstrla and J.D. Martsolf, 1984. Estimated and measured evapotranspiration for Florida climate, crops, and soils. Technical Bulletin No. 840. Agricultural Experiment Stations, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida, 65 pp. SFWMD, 1989. Interim surface water improvement and management (SWIM) plan for Lake Okeechobee. South Florida Water Management District, West Palm Beach, Florida. Skaggs, R.W., 1980. DRAINMOD reference report: Methods for design and evaluation of drainagewater management systems for soils with high water tables. USDA-SCS, South National Technical Center, Fort Worth, Texas, 329 pp.

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