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The water budget of irrigated pasture land near murray bridge, South Australia
Agricultural Meteorology -
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
T H E W A T E R B U D G E T OF I R R I G A T E D PASTURE L A N D N E A R M U R R A Y BRIDGE, SOUTH A U S T R A L I A J. W . HOLMES A N D C. L. W A T S O N
C.S.LR.O., Division of Soils, Adelaide, S.A.; Department osCAgriculture, Adelaide, S.A. (Australia)
(Received November 4, 1966)
SUMMARY
The components of the water budget of irrigated pasture land were measured at a locality about 35°S latitude. The techniques of measurement yielded a standard error of about 1 0 ~ of the total inventory, lncome, comprising rain and irrigation, was 2,100 ram/year. Expenditure was 2,060 mm, comprised of 1,140 mm evaporation and 920 mm drainage. The evaporation, measured with the help of small lysimeters, was strongly correlated with measured net radiation. Three formulae for estimating evaporation, based upon combination principles (MCILROY and ANGUS, 1964) were tried. By suitable choice of the coefficient for introducing the effect of the drying power of the air into the formula it is possible to obtain a close estimate of evaporation. But the correlation is no better than the correlation between evaporation and net radiation.
INTRODUCTION
There are several aspects to the concept of efficiency in irrigation practice, one of which is to consider volumes of water employed. Evaporation, which term will be used to include evaporation from the soil and transpiration from plants, is an unavoidable expenditure. Frequently there is a further expenditure in leaching the soil, which cannot be foregone if the profile is to be kept safe against the accumulation of salts. Deep seepage, or underground drainage, which goes to a building up of the country water table is undesirable but sometimes cannot be avoided. Other expenditure goes to channel losses and escape of water to a surface drainage system. The measurement of water efficiency on an irrigation district basis is often inaccurate because of inadequate control of these above-mentioned elements. Rather favourable conditions for experiment are offered by reclaimed swamps of the lower River Murray, which now are devoted almost exclusively to the growing of pasture. The water budget there may be described by the equation: P+I=E+SD+
UD
(1) Agr. Meteorol.,
4 (1967) 177-188
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J.w.
HOLMES AND ( . 1.. WATSON
where P is the rainfall, I is the amount of irrigation, E is the evaporation, SD is the surface drainage and where UD, the seepage past the root-zone, is retained as a~, element of the balance, but will be shown to be negligible. There have been no long-term observations of the water budget, in which all components have been measured, tk~r irrigation areas of southeastern Australia. Irrigation system losses have been assessed by engineers of the various water supply authorities, and the results appear in annual reports. (See, for example, the Annual
Report of the State Rivers and Water Supply Commission, Victoria, .[br 195671957). BARTELS (1965) measured the seepage loss of water past the root-zone of irrigated pasture at Werribee, Victoria. He estimated evaporation from his pasture from measured tank evaporation. The atmospheric processes controlling evaporation are reasonably well understood (e.g., DEACON et al., 1958), but evaporation itself is largely beyond human control. Measurements of water loss from evaporimeter tanks are widespread. HOUNAM (1961) compiled such data to produce maps of tank evaporation for the Australian continent. Evaporation from lake or land surfaces is more difficult to measure. Using large, weighable lysimeters MClLROY and ANGUS (1964) measured the evaporation from irrigated grassland and also from water-filled lysimeters during 1959-1961 at Aspendale, near Melbourne. Their site was 200 km south of the main irrigation areas. The site of the experiment described below was on Long Flat Irrigation Area. Murray Bridge, at 35 10' S. Most of the area of about one million irrigated hectares served by water of the Murray River and its tributaries lies closer than 200 km north or south of the 35°S parallel. Evaporation measured at Long Flat should be generally relevant to those districts, if there are no complicating orographical effects. TAYLORand POOLE11931 ) outlined the development of irrigation on the reclaimed swamps of the lower Murray River. of which the 136 hectares of Long Flat form a part. The shape of the latter is a rather narrow strip, 0.65 km wide. bordering the river for 2.6 km. The soil is a heavy clay which cracks markedly when dry, and is derived from alluvium, deposited upon the river flood-plain. The measurement of the water budget began in August 1962 and continued until March 1965. There were fifteen irrigation farmers operating their normal watering schedule, without any special demands being made by this experiment upon the frequency or amount. The area was administered by the Lands Department, South Australia, who were responsible for the original lay-out. The irrigated area comprised 20 lots of graded land. each 100 m wide, extending from the river levee bank to a main drain 0.65 km approximately from the river. Watering was done by flooding, with border check-banks between lots. There were seven sluice gates serving the whole area, under supervision of the pump-master. The purpose of the experiment was to measure the components of the water balance given in eq. (1). Long Flat Irrigation Area was chosen because it was as representative as possible of all the irrigated pasture land of the nearby region, Agr. Meteorol.,4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
179
totalling about 6,000 hectares. As will be seen, the main effort of the experiment had to go into the task of measuring evaporation from the pasture, which is the component least amenable to measurement.
METHODS OF MEASUREMENT
Irrigation amount The amount of irrigation applied was obtained by gauging the flow through the sluice gates. A new, inclined flap-gate, designed for this experiment by EDWARDS and CULVER (1967), was able to measure flows between 0.2 and 1.4 m3/sec, with a standard deviation of about 3 ~ . The signal of this water meter was an electric current proportional to instantaneous flow. It was integrated by and read out from a modified kWh meter to give water volume. Each of the seven sluice-gates was equipped with such a water meter, the records for each irrigation being taken by the pump-master. The maintenance of these water meters proved to be a problem, because the site of the experiment was 100 km away from the work-place of personnel engaged on the project. During the course of the experiment, there were 552 individual waterings through sluice-gates, of which 497 were measured by water meters and 55 were estimated from the duration of opening of the gates assuming the usually observed rate of flow. The sluice-gates were soundly designed and constructed. Despite the river head of about 5 ft., the leakage past the shut gates was negligible compared with the amount of irrigation applied.
Rainfall The rainfall was measured in standard 20 cm diameter raingauges. During the course of the experiment gauges at three different locations were used from time to time. Gauge 1 was located centrally in the irrigation area, gauge 2 was placed in the lysimeter field to be described below and gauge 3 was located 3 km distant, at Murray Bridge, under supervision of the Bureau of Meteorology. Gauges 1 or 2 were used for adjusting the change in weight of the lysimeters in the measurement of evaporation. Gauge 3 was used in the estimate of rain falling upon the whole irrigation area, because there were no breaks in its recording. For a total catch of 200 mm during the course of part of the experiment, gauge 3 exceeded gauge 2 by 8 ~ in its measurement of rainfall. The rainfall was usually a small component of the water budget.
Evaporation The evaporation was measured by repeated weighing of eight small lysimeters. Agr. Meteorol., 4 (1967) 177-188
180
J, w . HOLMES AND ( , 1, WA~IS() x,
The containers used were polythene buckets, nominal capacity 91. The diameter across the top was 25.9 cm giving an effective evaporating area of 530 cmL A sod of the pasture was shaped and fitted snugly into each bucket, which was nested inside a second bucket, the two together being placed in the hole left by removal of the sod. The inner bucket was periodically removed and weighed on a portable platform balance. it is necessary to justify the use of such small lysimeters and to assign an experimental error to the measured evaporation. The depth for the root zone of the pasture was 25 cm in the buckets compared to perhaps 50 cm in the soil at large. Alter a general field watering the water table rose to a few cm below soil surface and then descended to about 90 cm before the next watering, which generally was 21 days later. The buckets were drained, after the water had left the field, by tipping on their sides for 5 min. Theywere watered additionallyto the field irrigation, to their maximum weight, every 7 days. The amount of water evaporated from the buckets by transpiration of the pasture was about 14°~ of the volume of the whole soil, during summer. This water content change corresponded to a change in soil water suction from an assumed 0.01 bar at full capacity to 0.2 bar when ready for reweighing and watering after a 7-day period of dry weather. The conversion from water content to suction was done by using water content-suction curves determined with suction plate and pressure membrane apparatus. We conclude that the soil in the lysimeters was adequately supplied with water. Nor was there any evidence of unfavourable water-logging, possibly because the sod was established in a top-soil clay with extremely good crumb structure. Tensiometers installed in the soil, at large, showed that the mean integrated soil water suction was about 0.3 bar at 10 cm, 0.2 bar at 20 cm and 0.! bar at 40 cm during the interval between field irrigations. We conclude that the field also was adequately supplied with water for transpiration. Further support for this conclusion is afforded by the radiometer records, to be discussed below, which showed no change in net radiation absorbed by the pasture surface, after a watering, whenever fine cloudless days enabled such comparisons to be made. The pasture grew well in the sods in the buckets. The species composition and general vigour of the plants made them indistinguishable from the pasture at large. Deliberately, the buckets had no markers for their location. They were sometimes quite hard to find in the sward. Despite the good growth, it was deemed prudent to renew the sods in the buckets in September 1963 and October 1964. The pasture comprised white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.) and paspalum (Paspalum dilatatum Pore.). The ryegrass flushed in October. The paspalum grew vigorously only during the summer. A herd of dairy cows grazed the pasture throughout the course of the experiment. The average height of the pasture was about l0 cm. The variability in change of weight of the buckets was large. Occasionally a pat of dung, or urine, would spoil the record of a bucket. But allowing for such misadventure, the variation was much too large for error in weighing and it remains Agr. Meteorol., 4
(:1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
181
a puzzle. The standard deviation was usually about 12 ~ of the loss in weight. Such variability, in the drying of soil under pasture, has also been observed in another experiment (HOLMESand COLVILLE,1964). The bucket lysimeters were removed from the field during 4 months of the year, from mid-May to mid-September, when rainfall was so large that there was danger of the buckets overflowing. For 4 months of each year, therefore, the evaporation was estimated by extrapolation from the regression of evaporation on net radiation, found to hold during the remaining 8 months of the year.
Seepage The amount of seepage from the river was estimated from measurements of hydraulic conductivity and the flow net of equipotential lines and streamlines. The latter was investigated with piezometer tubes installed from the levee bank to the drainage channel. The former was measured by the two-well and Kirkham tube methods (CH1LDSet al., 1957), and roughly estimated from the rate of rise in the piezometer-tubes. These investigations, using an analysis of the flow-net, showed that the seepage from the river into the irrigated pasture, by flow through and beneath the levee bank, amounted to less than 0.1 mm/day. This is about the magnitude of the standard deviation of the irrigation amount, and is neglected in subsequent calculations.
Surface drainage The surface water remaining after irrigation, together with some subsurface drainage to shallow ditches, was collected by the main drain and pumped up into the river. The duty-cycle of the pump was recorded continuously. The pump itself was calibrated and operated by the Engineering and Water Supply Department, South Australia. The standard deviation in pumping was about 5 ~, estimated from the error in the calibration and fluctuations of river head.
Net radiation and climatic data The net radiation absorbed by the pasture surface, was measured with a radiometer of a type developed by FUNK (1962) and designated "an improved polytheneshielded net radiometer". It was exposed continuously above the pasture, the signal being recorded on a 5 mV potentiometric recorder. Electrical resistance thermometers were exposed in a Stevenson screen, erected over the pasture. The signal from these was also recorded continuously, giving a graph of air temperature and the depression of the wet-bulb temperature. The run of the wind was measured during the experiment by a cup-anemometer, placed at a height of 2 m. The site was operated unattended but visited once in 7 days for the removal
Agr. Meteorol., 4 (1967) 177-188
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J. W . H O L M E S A N D C. I . W A T S O N
of the chart, weighing of the bucket lysimeters and general maintenance. Any fault in operation therefore tended to have a mean life of 31/2 days before being observed. Three different net radiometers were exposed at one time or another during the experiment. Each was calibrated to a quoted accuracy of 21/2 ~o. One radiometer was recalibrated after 19 months exposure, when its sensitivity for incoming day-time radiation had increased by 2 }~,. On another occasion two radiometers were exposed side by side, for one day, when the net radiation derived from their signals was found to differ by 10 0/o. The standard deviation of the radiation on a monthly total is taken to be 3 %.
RESULTS
The water budget of irrigated pasture Each measured item of the water budget, on a total-for-one-month basis is shown in Table I. In addition, the monthly records were grouped into summer and winter, the totals of which are shown separately. The sum (P ÷ I) should equal (E ÷ SD) and the agreement actually found is acceptable. The standard deviation of the difference between (P + I) and (E + SD) for any one summer month is 24 mm. For the total of 6 summer months it is therefore approximately 60 mm. The discrepancy in each of the three summer periods of observation is therefore within experimental error. There does appear to be a trend, during the 1963/1964 year, for (P-~- 1) to exceed (E ÷ SD) culminating in a difference for the year of ÷ 184 mm, which is larger than the expected experimental error. However, the discrepancy is not too serious, being less than 10 ~o of the total water budget. The average annual evaporation for the 2 years of observations was 1,143 mm. For the 6 summer months (three observations) it was 884 mm. This measured evaporation is likely to have been the maximum possible evaporation, because the pasture was adequately watered.
Evaporation and weather data The water budget shown in Table I includes evaporation measured for 30 months. ostensibly without any gaps in the record. Fig.1 shows the experimental results of measured evaporation through the 30 months of observation. The smooth curve through the points was drawn by eye through the mean monthly values plotted at the 15th day. When the bucket lysimeters were inoperative, the evaporation was estimated from measured net radiation. Even so, there were 200 days out of a total of 900, for which evaporation had to be obtained by interpolation, because of some fault. in the radiation and lysimeter record, simultaneously. Fig.2 shows the plot of evapo-
Agr. Meteorol., 4 (1967) 177-188
183
THE WATER BUDGET OF IRRIGATED PASTURE LAND TABLE I THE WATER BUDGET OF IRRIGATED PASTURE LAND NEAR MURRAY
BRIDGE,
SOUTH AUSTRALIA;
ALL
COMPONENTS IN MM1
Rainfall (P)
Irrigation (I)
P + 1
Evaporation (E)
Drainage (SO)
E + SO
1962 Oct. Nov. Dec.
73 14 40
103 254 201
176 268 241
93 133 166
104 99 85
197 232 251
Jan. 42 Feb. 2 March 1 T o t a l 6 m o n t h s 172 April 54 May 84 June 63 July 56 Aug. 43 Sep. 23 T o t a l 6 m o n t h s 323 Oct. 49 Nov. 4 Dec. 1
240 197 211 1206 135 0 0 0 0 128 263 186 214 314
282 199 212 1378 189 84 63 56 43 151 586 235 218 315
176 130 115 813 64 28 31 31 57 112 323 139 157 163
105 84 103 580 104 69 56 20 18 50 317 89 111 113
281 214 218 1393 168 97 87 51 75 162 640 228 268 276
264 277 240 1495 86 156 0 0 115 95 452 128 183 234
272 293 245 1578 127 175 38 51 148 145 684 165 250 256
184 135 108 886 50 38 22 27 56 82 275 123 129 152
94 97 108 612 53 77 17 17 73 68 305 71 97 137
278 232 216 1498 103 115 39 44 129 150 580 194 226 289
244 294 235 1318
245 294 237 1447
161 147 120 832
96 108 121 630
257 255 241 1462
1963
1964 Jan. 8 Feb. 16 March 5 T o t a l 6 m o n t h s 83 April 41 May 19 June 38 July 51 Aug. 33 Sep. 50 T o t a l 6 m o n t h s 232 Oct. 37 Nov. 67 Dec. 22
1965 Jan. 1 Feb. 0 March 2 T o t a l 6 m o n t h s 129
1 The period October-March is regarded as summer; April-September is regarded as winter.
Agr. MeteoroL, 4 (1967") 17%188
184
J . w . HOLMESAND (, 1.. WATSON
671
×. . .×. . .x• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .o . . . . 1962 . . . . . . . . . . 11963 . . . . . . . . . . . . . .]r,rigot~on ...... season x
~.•
gO
.'o "
~3
°°~×
o
i
Jan
i
Feb
i
Mar
x
,~,
x
oI
x
!963 i1964 Ir,r,igation season
x
xO x
i
Apt"
o
i __ Jun
[vlQy
•
o
~ . ~ - ~x
o o
,-"
i
Jul
i°
o
x~o
"I
Aug
~
Sap
I
Oct
Nov
I
Dec
Fig.1. Measured evaporation from grass, for the years 1962-1965. Each plotted point is the mean daily evaporation for a period of about 7 days. ration against net radiation absorbed at the pasture surface, in equivalent units. The period of observation represented by each point was about 7 days. Several formulae qere employed to calculate evaporation from weather d a t a . These were the equation of PENMAN (1963): E1 =
A
7
•H -
A+7
A=
" Ea
(2)
7
in which the wind function Ea has the recommended form:
(3)
Ea = 0.35 (1 -F u/lO0) (ea - - ea)
~8
E
." " J
Ld
•
.
5 } a
EG= 0 5 4 +
E c o
0.75 H
2
oe l >o IM
c
I
I
2
I
3
I
;
I
o
i
7
Equivalent net radiation received, H (m m#doy)
Fig.2. Evaporation measured by weighable bucket tysimeters as a function of net radiation.
Agr. Meteorol., 4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
185
Secondly, the Penman equation in which an attempt was made to take account of diffusive resistance in the stomata of the evaporating leaves and the length of daylight during which the stomata were assumed to remain open (PENMAN, 1953): A y E2 = A + ~/SD" H + A + ~,/SD " E~
(4)
Thirdly, the equation of Mcllroy found to be applicable in the Aspendale environment (McILROY and ANGUS, 1964): A E3 . . . . H + h O' A-:- 7
(5)
In eq. (2) to (5), the symbols are consistent with those defined by Penman. Additional quantities are D' the wet bulb depression in °C at screen height and a wind function, h. It was soon found that the magnitude of h was different from that found in the Aspendale environment. The best agreement with measured evaporation, Eel, was given by using: h = 0.045 (1 ÷ u/60)
(6)
for the period October 1962-December 1963, and: h = 0.052 (1 + u/60)
(7)
for the period January 1964-March 1965. The regressions Of EG upon H, Eh E2 and E8 were found to be: EG = 0.54 + 0.75H
(8)
with correlation coefficient ----0.80; EG = 0.36 q- 0.81E1
(9)
with correlation coefficient = 0.82; EG = 0.32 q- 1.05E2
(10)
with correlation coefficient = 0.81; and: EG = 0.26 + 0.93E3
(11)
with correlation coefficient 0.84. A full list of symbols and their definitions is given in the Appendix at the end of the paper.
DISCUSSION
The water budget shown in Table I reveals that it is the custom to use too much Agr. Meteorol., 4 (1967)
177-188
186
J.w.
HOLMES AND ( . t
WATSON
water for irrigation, on Long Flat Irrigation Area. The amount of water pumped back to the river was nearly half of the irrigation applied. The usual practice is to use a water crest depth of about 20 cm, when flooding, in order to cover high spots on the land. More economical t~se of water would certainly require that the land surface be smoothed and perhaps re-graded. The soil does not require leaching to keep soluble salts out of the root zone. In fact, the water table is lower than river level, there are no sub-surface drains and there can be no sub-soil leaching, no matter how much irrigation is applied. Of the components of the water budget, evaporation is the most difficult measure. We should consider how reliable is the estimate given here, and how generally applicable are these results, to irrigation areas of southeastern Australia. In presenting Table I, it was remarked that the balance of the items was satisfactory. This does not remove the possibility of a cancelling of errors. The evaporation from the grass-covered lysimeters at Aspendale, at latitude 38 ° 02' S, totalled 1,296 mm/year, for the 3 years (1959-1961) of observations there. The evaporation at Long Flat totalled 1,143 mm/year. One would, however, expect the evaporation at Long Flat to be more than that at Aspendale, not only because of its lower latitude but because the location of our experiment was in a more arid region (annual rainfalls about 250 mm and 700 ram). There are probably two reasons for the discrepancy between the results. Firstly, MCILRo¥ and ANGUS (1964) watered their lysimeters very frequently in an attempt to obtain potential evaporation. Although they stated that there was no measurable change in evaporation rate from before to after sprinkling, it seems unlikely that evaporation rate is not affected by general wetness of foliage and turgor of the leaves in the advective situation which they postulate for Aspendale. The intention of the Long Flat experiment was to measure the evaporation for the whole irrigation area as reliably as possible. The two experiments are not strictly comparable on the score of water availability to the plants. That there was no soil water stress upon the pasture of the Long Flat experiment, significant enough to affect evaporation, may be inferred from comparison of the net radiation measured before and after watering. For example, the pasture was irrigated on 28th January, 1964. It happened that the weather was fine and the sky cloudless on the three days 27th, 28th and 29th January. The totals of day-time net radiations were the equivalents of 8.48, 10.73 and 8.51 mm of evaporation, respectively. On six other occasions when such comparisons were possible, the records showed similar results, that there was no significant difference in the radiation totals. Note that the net radiation absorbed by the flood water during irrigation was about 1.25 times the net radiation absorbed by the pasture. The second reason for the discrepancy may be that the general weather conditions are different. The annual net radiation and evaporation totalled 1,360 and 1,143 mm at Long Flat and 1,084 and 1,296 mm at Aspendale, respectively. At the first named site there was an annual export of sensible heat, amounting to 16 % of the net Agr. MeteoroL,4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
187
r a d i a t i o n . A t the second there was an a n n u a l i m p o r t o f sensible heat to m a i n t a i n the o b s e r v e d e v a p o r a t i o n , a m o u n t i n g to 20 ~o o f the net radiation. A l t h o u g h , in the i n t r o d u c t i o n , we stated that the e x p e r i m e n t was intended to o b t a i n a m e a s u r e m e n t o f e v a p o r a t i o n generally relevant to the m a i n irrigation areas o f southeastern Australia, the result o f c o m p a r i s o n with A s p e n d a l e suggests c a u t i o n in accepting this application. P e r h a p s the L o n g F l a t site was affected by the M o u n t Lofty Range, which has a height o f a b o u t 500 m, 30 k m to the west. E x p a n s i o n of the air mass in the prevailing westerly winds, in the lee o f the range could introduce a c o m p l i c a t i o n to the energy balance. P e r h a p s the l o c a t i o n o f the A s p e n d a l e site in the general e n v i r o n m e n t o f a very large city w o u l d encourage atypical local advection. A s a w o r k i n g estimate, it w o u l d be r e a s o n a b l e to t a k e c o n t i n e n t a l p o t e n t i a l evaporation, a b o u t the 35°S parallel, as being 1,200 m m ± less t h a n 10 ~ .
ACKNOWLEDGEMENTS Messrs J. W. H a r v e y a n d M. W. Hughes, C.S.I.R.O., m a d e m a n y o f the observations a n d calculations for e v a p o r a t i o n a n d climatic data. W e are grateful to Mr. P. Judd, D e p a r t m e n t o f Agriculture, for m a n y helpful discussions. The w o r k was s u p p o r t e d by a financial g r a n t f r o m the D a i r y I n d u s t r y Research Committee.
REFERENCES
BARTELS,L. F., 1965. Estimation of soil drainage losses following irrigation. Australian J. Exptl. Agr. Animal Husbandry, 5 : 59-64. CHILDS, E. C., COLLIS-GEORGE,N. and HOLMES,J. W., 1957. Permeability measurements in the field as an assessment of anisotropy and structure development. J. Soil Sci., 8 : 27~ ' l . DEACON,E. L., PR1ESTLEY,C. H. B. and SW~NBANK,W. C., 1958. Evaporation and the water balance. UNESCO AridZone Res., 10 : 9-34. EDWARDS,J. A. and CULVER,R., 1967. A flap-type integrating flow meter. Agr. Eng., in press. FVNK, J. P., 1962. A net radiometer designed for optimum sensitivity and a ribbon thermopile used in a miniaturised version. J. Geophys. Res., 67 : 2753-2760. HOLMES, J. W. and COLVILLE,J. S., 1964. The use of the neutron moisture meter and lysimeters for water balance studies. Trans. Intern. Congr. Soil Sci., 8th, Bucharest, 1964, in press. HOUNAM, C. E., 1961. Evaporation irt Australia. Commonwealth Australia, Bur. Meteorol., Bull., 44 : 88 pp. MClLROY, I. C. and AN~tJS, D. E., 1964. Grass, water and soil evaporation at Aspendale. Agr. Meteorol., 1 : 201-224. PENMAN, H. L., 1953. The physical bases of irrigation control. Intern. Hort. Congr., 13th, 1953, pp.913-924. PENMAN, H. L., 1963. Vegetation and hydrology. Commonwealth Agr. Bur. (Gt. Brit.), Tech. Commun., 53 : 124 pp. TAYLOR,J. K. and POOLE,H. G., 1931. A soil survey of the swamps of the lower Murray River, S.A. C.S.LR.O. Bull., 51 : 42 pp.
Agr. Meteorol., 4 (1967) 177-188
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J.W. HOLMES AND C. L. WATSON
APPENDIX
List of symbols and their definitions D D' E EG E1 E2 E3
a day-length fraction, defined by PENMAN(1953), dimensionless. the depression of the wet-bulb temperature, in the Stevenson screen, °C. evaporation, mm/day. evaporation measured by the use of the bucket lysimeters, mm/day. evaporation computed by using the formula of PENMAN(1963) (eq.2), mm/day. evaporation computed by using the formula of PENMAN(1953)(eq.4), mm/day. evaporation computed by using the formula of MclLRoY (MclLRoY and ANGUS, 1964) (eq.5), ram/day. Ea evaporation computed by the Dalton type formula (eq.3), mm/day. ea saturation vapour pressure of water at mean air temperature, mm Hg. ed saturation vapour pressure of water at mean dew-point temperature (i.e., the actual vapour pressure in the air), mm Hg. H the energy absorbed at the grass surface from radiation of all wavelengths, expressed as the amount of water which could be evaporated by utilising all that energy, ram/day. h a function of the wind, defined by MCILROY and ANGUS(1964), the product of which with D' gives the evaporation estimated by a Dalton-type equation, mm/day/°C. I irrigation, mm/day. P precipitation, mm/day. S a stomatal factor, defined by PENMAN(1953). The product SD gives a coefficient by which the Dalton-type equation is multiplied to make some allowance for the effect of stomatal resistance to vapour flow and stomatal closing during night-time, upon water vapour diffusion into the atmosphere from the evaporating leaves. S is dimensionless. SD surface drainage, used only in eq. (1), mm/day. UD underground drainage, used only in eq. (1), ram/day. u mean wind speed at a height of 2 m, miles/day. 7 a conversion factor, which is the constant of the wet and dry bulb psychrometer, mm Hg/°F. It is taken to have the value 0.27. A the slope of the saturation vapour pressure curve at mean air temperature, mm Hg/°F.
Agr. Meteorol.,4 (1967) 177-188
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
T H E W A T E R B U D G E T OF I R R I G A T E D PASTURE L A N D N E A R M U R R A Y BRIDGE, SOUTH A U S T R A L I A J. W . HOLMES A N D C. L. W A T S O N
C.S.LR.O., Division of Soils, Adelaide, S.A.; Department osCAgriculture, Adelaide, S.A. (Australia)
(Received November 4, 1966)
SUMMARY
The components of the water budget of irrigated pasture land were measured at a locality about 35°S latitude. The techniques of measurement yielded a standard error of about 1 0 ~ of the total inventory, lncome, comprising rain and irrigation, was 2,100 ram/year. Expenditure was 2,060 mm, comprised of 1,140 mm evaporation and 920 mm drainage. The evaporation, measured with the help of small lysimeters, was strongly correlated with measured net radiation. Three formulae for estimating evaporation, based upon combination principles (MCILROY and ANGUS, 1964) were tried. By suitable choice of the coefficient for introducing the effect of the drying power of the air into the formula it is possible to obtain a close estimate of evaporation. But the correlation is no better than the correlation between evaporation and net radiation.
INTRODUCTION
There are several aspects to the concept of efficiency in irrigation practice, one of which is to consider volumes of water employed. Evaporation, which term will be used to include evaporation from the soil and transpiration from plants, is an unavoidable expenditure. Frequently there is a further expenditure in leaching the soil, which cannot be foregone if the profile is to be kept safe against the accumulation of salts. Deep seepage, or underground drainage, which goes to a building up of the country water table is undesirable but sometimes cannot be avoided. Other expenditure goes to channel losses and escape of water to a surface drainage system. The measurement of water efficiency on an irrigation district basis is often inaccurate because of inadequate control of these above-mentioned elements. Rather favourable conditions for experiment are offered by reclaimed swamps of the lower River Murray, which now are devoted almost exclusively to the growing of pasture. The water budget there may be described by the equation: P+I=E+SD+
UD
(1) Agr. Meteorol.,
4 (1967) 177-188
178
J.w.
HOLMES AND ( . 1.. WATSON
where P is the rainfall, I is the amount of irrigation, E is the evaporation, SD is the surface drainage and where UD, the seepage past the root-zone, is retained as a~, element of the balance, but will be shown to be negligible. There have been no long-term observations of the water budget, in which all components have been measured, tk~r irrigation areas of southeastern Australia. Irrigation system losses have been assessed by engineers of the various water supply authorities, and the results appear in annual reports. (See, for example, the Annual
Report of the State Rivers and Water Supply Commission, Victoria, .[br 195671957). BARTELS (1965) measured the seepage loss of water past the root-zone of irrigated pasture at Werribee, Victoria. He estimated evaporation from his pasture from measured tank evaporation. The atmospheric processes controlling evaporation are reasonably well understood (e.g., DEACON et al., 1958), but evaporation itself is largely beyond human control. Measurements of water loss from evaporimeter tanks are widespread. HOUNAM (1961) compiled such data to produce maps of tank evaporation for the Australian continent. Evaporation from lake or land surfaces is more difficult to measure. Using large, weighable lysimeters MClLROY and ANGUS (1964) measured the evaporation from irrigated grassland and also from water-filled lysimeters during 1959-1961 at Aspendale, near Melbourne. Their site was 200 km south of the main irrigation areas. The site of the experiment described below was on Long Flat Irrigation Area. Murray Bridge, at 35 10' S. Most of the area of about one million irrigated hectares served by water of the Murray River and its tributaries lies closer than 200 km north or south of the 35°S parallel. Evaporation measured at Long Flat should be generally relevant to those districts, if there are no complicating orographical effects. TAYLORand POOLE11931 ) outlined the development of irrigation on the reclaimed swamps of the lower Murray River. of which the 136 hectares of Long Flat form a part. The shape of the latter is a rather narrow strip, 0.65 km wide. bordering the river for 2.6 km. The soil is a heavy clay which cracks markedly when dry, and is derived from alluvium, deposited upon the river flood-plain. The measurement of the water budget began in August 1962 and continued until March 1965. There were fifteen irrigation farmers operating their normal watering schedule, without any special demands being made by this experiment upon the frequency or amount. The area was administered by the Lands Department, South Australia, who were responsible for the original lay-out. The irrigated area comprised 20 lots of graded land. each 100 m wide, extending from the river levee bank to a main drain 0.65 km approximately from the river. Watering was done by flooding, with border check-banks between lots. There were seven sluice gates serving the whole area, under supervision of the pump-master. The purpose of the experiment was to measure the components of the water balance given in eq. (1). Long Flat Irrigation Area was chosen because it was as representative as possible of all the irrigated pasture land of the nearby region, Agr. Meteorol.,4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
179
totalling about 6,000 hectares. As will be seen, the main effort of the experiment had to go into the task of measuring evaporation from the pasture, which is the component least amenable to measurement.
METHODS OF MEASUREMENT
Irrigation amount The amount of irrigation applied was obtained by gauging the flow through the sluice gates. A new, inclined flap-gate, designed for this experiment by EDWARDS and CULVER (1967), was able to measure flows between 0.2 and 1.4 m3/sec, with a standard deviation of about 3 ~ . The signal of this water meter was an electric current proportional to instantaneous flow. It was integrated by and read out from a modified kWh meter to give water volume. Each of the seven sluice-gates was equipped with such a water meter, the records for each irrigation being taken by the pump-master. The maintenance of these water meters proved to be a problem, because the site of the experiment was 100 km away from the work-place of personnel engaged on the project. During the course of the experiment, there were 552 individual waterings through sluice-gates, of which 497 were measured by water meters and 55 were estimated from the duration of opening of the gates assuming the usually observed rate of flow. The sluice-gates were soundly designed and constructed. Despite the river head of about 5 ft., the leakage past the shut gates was negligible compared with the amount of irrigation applied.
Rainfall The rainfall was measured in standard 20 cm diameter raingauges. During the course of the experiment gauges at three different locations were used from time to time. Gauge 1 was located centrally in the irrigation area, gauge 2 was placed in the lysimeter field to be described below and gauge 3 was located 3 km distant, at Murray Bridge, under supervision of the Bureau of Meteorology. Gauges 1 or 2 were used for adjusting the change in weight of the lysimeters in the measurement of evaporation. Gauge 3 was used in the estimate of rain falling upon the whole irrigation area, because there were no breaks in its recording. For a total catch of 200 mm during the course of part of the experiment, gauge 3 exceeded gauge 2 by 8 ~ in its measurement of rainfall. The rainfall was usually a small component of the water budget.
Evaporation The evaporation was measured by repeated weighing of eight small lysimeters. Agr. Meteorol., 4 (1967) 177-188
180
J, w . HOLMES AND ( , 1, WA~IS() x,
The containers used were polythene buckets, nominal capacity 91. The diameter across the top was 25.9 cm giving an effective evaporating area of 530 cmL A sod of the pasture was shaped and fitted snugly into each bucket, which was nested inside a second bucket, the two together being placed in the hole left by removal of the sod. The inner bucket was periodically removed and weighed on a portable platform balance. it is necessary to justify the use of such small lysimeters and to assign an experimental error to the measured evaporation. The depth for the root zone of the pasture was 25 cm in the buckets compared to perhaps 50 cm in the soil at large. Alter a general field watering the water table rose to a few cm below soil surface and then descended to about 90 cm before the next watering, which generally was 21 days later. The buckets were drained, after the water had left the field, by tipping on their sides for 5 min. Theywere watered additionallyto the field irrigation, to their maximum weight, every 7 days. The amount of water evaporated from the buckets by transpiration of the pasture was about 14°~ of the volume of the whole soil, during summer. This water content change corresponded to a change in soil water suction from an assumed 0.01 bar at full capacity to 0.2 bar when ready for reweighing and watering after a 7-day period of dry weather. The conversion from water content to suction was done by using water content-suction curves determined with suction plate and pressure membrane apparatus. We conclude that the soil in the lysimeters was adequately supplied with water. Nor was there any evidence of unfavourable water-logging, possibly because the sod was established in a top-soil clay with extremely good crumb structure. Tensiometers installed in the soil, at large, showed that the mean integrated soil water suction was about 0.3 bar at 10 cm, 0.2 bar at 20 cm and 0.! bar at 40 cm during the interval between field irrigations. We conclude that the field also was adequately supplied with water for transpiration. Further support for this conclusion is afforded by the radiometer records, to be discussed below, which showed no change in net radiation absorbed by the pasture surface, after a watering, whenever fine cloudless days enabled such comparisons to be made. The pasture grew well in the sods in the buckets. The species composition and general vigour of the plants made them indistinguishable from the pasture at large. Deliberately, the buckets had no markers for their location. They were sometimes quite hard to find in the sward. Despite the good growth, it was deemed prudent to renew the sods in the buckets in September 1963 and October 1964. The pasture comprised white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.) and paspalum (Paspalum dilatatum Pore.). The ryegrass flushed in October. The paspalum grew vigorously only during the summer. A herd of dairy cows grazed the pasture throughout the course of the experiment. The average height of the pasture was about l0 cm. The variability in change of weight of the buckets was large. Occasionally a pat of dung, or urine, would spoil the record of a bucket. But allowing for such misadventure, the variation was much too large for error in weighing and it remains Agr. Meteorol., 4
(:1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
181
a puzzle. The standard deviation was usually about 12 ~ of the loss in weight. Such variability, in the drying of soil under pasture, has also been observed in another experiment (HOLMESand COLVILLE,1964). The bucket lysimeters were removed from the field during 4 months of the year, from mid-May to mid-September, when rainfall was so large that there was danger of the buckets overflowing. For 4 months of each year, therefore, the evaporation was estimated by extrapolation from the regression of evaporation on net radiation, found to hold during the remaining 8 months of the year.
Seepage The amount of seepage from the river was estimated from measurements of hydraulic conductivity and the flow net of equipotential lines and streamlines. The latter was investigated with piezometer tubes installed from the levee bank to the drainage channel. The former was measured by the two-well and Kirkham tube methods (CH1LDSet al., 1957), and roughly estimated from the rate of rise in the piezometer-tubes. These investigations, using an analysis of the flow-net, showed that the seepage from the river into the irrigated pasture, by flow through and beneath the levee bank, amounted to less than 0.1 mm/day. This is about the magnitude of the standard deviation of the irrigation amount, and is neglected in subsequent calculations.
Surface drainage The surface water remaining after irrigation, together with some subsurface drainage to shallow ditches, was collected by the main drain and pumped up into the river. The duty-cycle of the pump was recorded continuously. The pump itself was calibrated and operated by the Engineering and Water Supply Department, South Australia. The standard deviation in pumping was about 5 ~, estimated from the error in the calibration and fluctuations of river head.
Net radiation and climatic data The net radiation absorbed by the pasture surface, was measured with a radiometer of a type developed by FUNK (1962) and designated "an improved polytheneshielded net radiometer". It was exposed continuously above the pasture, the signal being recorded on a 5 mV potentiometric recorder. Electrical resistance thermometers were exposed in a Stevenson screen, erected over the pasture. The signal from these was also recorded continuously, giving a graph of air temperature and the depression of the wet-bulb temperature. The run of the wind was measured during the experiment by a cup-anemometer, placed at a height of 2 m. The site was operated unattended but visited once in 7 days for the removal
Agr. Meteorol., 4 (1967) 177-188
182
J. W . H O L M E S A N D C. I . W A T S O N
of the chart, weighing of the bucket lysimeters and general maintenance. Any fault in operation therefore tended to have a mean life of 31/2 days before being observed. Three different net radiometers were exposed at one time or another during the experiment. Each was calibrated to a quoted accuracy of 21/2 ~o. One radiometer was recalibrated after 19 months exposure, when its sensitivity for incoming day-time radiation had increased by 2 }~,. On another occasion two radiometers were exposed side by side, for one day, when the net radiation derived from their signals was found to differ by 10 0/o. The standard deviation of the radiation on a monthly total is taken to be 3 %.
RESULTS
The water budget of irrigated pasture Each measured item of the water budget, on a total-for-one-month basis is shown in Table I. In addition, the monthly records were grouped into summer and winter, the totals of which are shown separately. The sum (P ÷ I) should equal (E ÷ SD) and the agreement actually found is acceptable. The standard deviation of the difference between (P + I) and (E + SD) for any one summer month is 24 mm. For the total of 6 summer months it is therefore approximately 60 mm. The discrepancy in each of the three summer periods of observation is therefore within experimental error. There does appear to be a trend, during the 1963/1964 year, for (P-~- 1) to exceed (E ÷ SD) culminating in a difference for the year of ÷ 184 mm, which is larger than the expected experimental error. However, the discrepancy is not too serious, being less than 10 ~o of the total water budget. The average annual evaporation for the 2 years of observations was 1,143 mm. For the 6 summer months (three observations) it was 884 mm. This measured evaporation is likely to have been the maximum possible evaporation, because the pasture was adequately watered.
Evaporation and weather data The water budget shown in Table I includes evaporation measured for 30 months. ostensibly without any gaps in the record. Fig.1 shows the experimental results of measured evaporation through the 30 months of observation. The smooth curve through the points was drawn by eye through the mean monthly values plotted at the 15th day. When the bucket lysimeters were inoperative, the evaporation was estimated from measured net radiation. Even so, there were 200 days out of a total of 900, for which evaporation had to be obtained by interpolation, because of some fault. in the radiation and lysimeter record, simultaneously. Fig.2 shows the plot of evapo-
Agr. Meteorol., 4 (1967) 177-188
183
THE WATER BUDGET OF IRRIGATED PASTURE LAND TABLE I THE WATER BUDGET OF IRRIGATED PASTURE LAND NEAR MURRAY
BRIDGE,
SOUTH AUSTRALIA;
ALL
COMPONENTS IN MM1
Rainfall (P)
Irrigation (I)
P + 1
Evaporation (E)
Drainage (SO)
E + SO
1962 Oct. Nov. Dec.
73 14 40
103 254 201
176 268 241
93 133 166
104 99 85
197 232 251
Jan. 42 Feb. 2 March 1 T o t a l 6 m o n t h s 172 April 54 May 84 June 63 July 56 Aug. 43 Sep. 23 T o t a l 6 m o n t h s 323 Oct. 49 Nov. 4 Dec. 1
240 197 211 1206 135 0 0 0 0 128 263 186 214 314
282 199 212 1378 189 84 63 56 43 151 586 235 218 315
176 130 115 813 64 28 31 31 57 112 323 139 157 163
105 84 103 580 104 69 56 20 18 50 317 89 111 113
281 214 218 1393 168 97 87 51 75 162 640 228 268 276
264 277 240 1495 86 156 0 0 115 95 452 128 183 234
272 293 245 1578 127 175 38 51 148 145 684 165 250 256
184 135 108 886 50 38 22 27 56 82 275 123 129 152
94 97 108 612 53 77 17 17 73 68 305 71 97 137
278 232 216 1498 103 115 39 44 129 150 580 194 226 289
244 294 235 1318
245 294 237 1447
161 147 120 832
96 108 121 630
257 255 241 1462
1963
1964 Jan. 8 Feb. 16 March 5 T o t a l 6 m o n t h s 83 April 41 May 19 June 38 July 51 Aug. 33 Sep. 50 T o t a l 6 m o n t h s 232 Oct. 37 Nov. 67 Dec. 22
1965 Jan. 1 Feb. 0 March 2 T o t a l 6 m o n t h s 129
1 The period October-March is regarded as summer; April-September is regarded as winter.
Agr. MeteoroL, 4 (1967") 17%188
184
J . w . HOLMESAND (, 1.. WATSON
671
×. . .×. . .x• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .o . . . . 1962 . . . . . . . . . . 11963 . . . . . . . . . . . . . .]r,rigot~on ...... season x
~.•
gO
.'o "
~3
°°~×
o
i
Jan
i
Feb
i
Mar
x
,~,
x
oI
x
!963 i1964 Ir,r,igation season
x
xO x
i
Apt"
o
i __ Jun
[vlQy
•
o
~ . ~ - ~x
o o
,-"
i
Jul
i°
o
x~o
"I
Aug
~
Sap
I
Oct
Nov
I
Dec
Fig.1. Measured evaporation from grass, for the years 1962-1965. Each plotted point is the mean daily evaporation for a period of about 7 days. ration against net radiation absorbed at the pasture surface, in equivalent units. The period of observation represented by each point was about 7 days. Several formulae qere employed to calculate evaporation from weather d a t a . These were the equation of PENMAN (1963): E1 =
A
7
•H -
A+7
A=
" Ea
(2)
7
in which the wind function Ea has the recommended form:
(3)
Ea = 0.35 (1 -F u/lO0) (ea - - ea)
~8
E
." " J
Ld
•
.
5 } a
EG= 0 5 4 +
E c o
0.75 H
2
oe l >o IM
c
I
I
2
I
3
I
;
I
o
i
7
Equivalent net radiation received, H (m m#doy)
Fig.2. Evaporation measured by weighable bucket tysimeters as a function of net radiation.
Agr. Meteorol., 4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
185
Secondly, the Penman equation in which an attempt was made to take account of diffusive resistance in the stomata of the evaporating leaves and the length of daylight during which the stomata were assumed to remain open (PENMAN, 1953): A y E2 = A + ~/SD" H + A + ~,/SD " E~
(4)
Thirdly, the equation of Mcllroy found to be applicable in the Aspendale environment (McILROY and ANGUS, 1964): A E3 . . . . H + h O' A-:- 7
(5)
In eq. (2) to (5), the symbols are consistent with those defined by Penman. Additional quantities are D' the wet bulb depression in °C at screen height and a wind function, h. It was soon found that the magnitude of h was different from that found in the Aspendale environment. The best agreement with measured evaporation, Eel, was given by using: h = 0.045 (1 ÷ u/60)
(6)
for the period October 1962-December 1963, and: h = 0.052 (1 + u/60)
(7)
for the period January 1964-March 1965. The regressions Of EG upon H, Eh E2 and E8 were found to be: EG = 0.54 + 0.75H
(8)
with correlation coefficient ----0.80; EG = 0.36 q- 0.81E1
(9)
with correlation coefficient = 0.82; EG = 0.32 q- 1.05E2
(10)
with correlation coefficient = 0.81; and: EG = 0.26 + 0.93E3
(11)
with correlation coefficient 0.84. A full list of symbols and their definitions is given in the Appendix at the end of the paper.
DISCUSSION
The water budget shown in Table I reveals that it is the custom to use too much Agr. Meteorol., 4 (1967)
177-188
186
J.w.
HOLMES AND ( . t
WATSON
water for irrigation, on Long Flat Irrigation Area. The amount of water pumped back to the river was nearly half of the irrigation applied. The usual practice is to use a water crest depth of about 20 cm, when flooding, in order to cover high spots on the land. More economical t~se of water would certainly require that the land surface be smoothed and perhaps re-graded. The soil does not require leaching to keep soluble salts out of the root zone. In fact, the water table is lower than river level, there are no sub-surface drains and there can be no sub-soil leaching, no matter how much irrigation is applied. Of the components of the water budget, evaporation is the most difficult measure. We should consider how reliable is the estimate given here, and how generally applicable are these results, to irrigation areas of southeastern Australia. In presenting Table I, it was remarked that the balance of the items was satisfactory. This does not remove the possibility of a cancelling of errors. The evaporation from the grass-covered lysimeters at Aspendale, at latitude 38 ° 02' S, totalled 1,296 mm/year, for the 3 years (1959-1961) of observations there. The evaporation at Long Flat totalled 1,143 mm/year. One would, however, expect the evaporation at Long Flat to be more than that at Aspendale, not only because of its lower latitude but because the location of our experiment was in a more arid region (annual rainfalls about 250 mm and 700 ram). There are probably two reasons for the discrepancy between the results. Firstly, MCILRo¥ and ANGUS (1964) watered their lysimeters very frequently in an attempt to obtain potential evaporation. Although they stated that there was no measurable change in evaporation rate from before to after sprinkling, it seems unlikely that evaporation rate is not affected by general wetness of foliage and turgor of the leaves in the advective situation which they postulate for Aspendale. The intention of the Long Flat experiment was to measure the evaporation for the whole irrigation area as reliably as possible. The two experiments are not strictly comparable on the score of water availability to the plants. That there was no soil water stress upon the pasture of the Long Flat experiment, significant enough to affect evaporation, may be inferred from comparison of the net radiation measured before and after watering. For example, the pasture was irrigated on 28th January, 1964. It happened that the weather was fine and the sky cloudless on the three days 27th, 28th and 29th January. The totals of day-time net radiations were the equivalents of 8.48, 10.73 and 8.51 mm of evaporation, respectively. On six other occasions when such comparisons were possible, the records showed similar results, that there was no significant difference in the radiation totals. Note that the net radiation absorbed by the flood water during irrigation was about 1.25 times the net radiation absorbed by the pasture. The second reason for the discrepancy may be that the general weather conditions are different. The annual net radiation and evaporation totalled 1,360 and 1,143 mm at Long Flat and 1,084 and 1,296 mm at Aspendale, respectively. At the first named site there was an annual export of sensible heat, amounting to 16 % of the net Agr. MeteoroL,4 (1967) 177-188
THE WATER BUDGET OF IRRIGATED PASTURE LAND
187
r a d i a t i o n . A t the second there was an a n n u a l i m p o r t o f sensible heat to m a i n t a i n the o b s e r v e d e v a p o r a t i o n , a m o u n t i n g to 20 ~o o f the net radiation. A l t h o u g h , in the i n t r o d u c t i o n , we stated that the e x p e r i m e n t was intended to o b t a i n a m e a s u r e m e n t o f e v a p o r a t i o n generally relevant to the m a i n irrigation areas o f southeastern Australia, the result o f c o m p a r i s o n with A s p e n d a l e suggests c a u t i o n in accepting this application. P e r h a p s the L o n g F l a t site was affected by the M o u n t Lofty Range, which has a height o f a b o u t 500 m, 30 k m to the west. E x p a n s i o n of the air mass in the prevailing westerly winds, in the lee o f the range could introduce a c o m p l i c a t i o n to the energy balance. P e r h a p s the l o c a t i o n o f the A s p e n d a l e site in the general e n v i r o n m e n t o f a very large city w o u l d encourage atypical local advection. A s a w o r k i n g estimate, it w o u l d be r e a s o n a b l e to t a k e c o n t i n e n t a l p o t e n t i a l evaporation, a b o u t the 35°S parallel, as being 1,200 m m ± less t h a n 10 ~ .
ACKNOWLEDGEMENTS Messrs J. W. H a r v e y a n d M. W. Hughes, C.S.I.R.O., m a d e m a n y o f the observations a n d calculations for e v a p o r a t i o n a n d climatic data. W e are grateful to Mr. P. Judd, D e p a r t m e n t o f Agriculture, for m a n y helpful discussions. The w o r k was s u p p o r t e d by a financial g r a n t f r o m the D a i r y I n d u s t r y Research Committee.
REFERENCES
BARTELS,L. F., 1965. Estimation of soil drainage losses following irrigation. Australian J. Exptl. Agr. Animal Husbandry, 5 : 59-64. CHILDS, E. C., COLLIS-GEORGE,N. and HOLMES,J. W., 1957. Permeability measurements in the field as an assessment of anisotropy and structure development. J. Soil Sci., 8 : 27~ ' l . DEACON,E. L., PR1ESTLEY,C. H. B. and SW~NBANK,W. C., 1958. Evaporation and the water balance. UNESCO AridZone Res., 10 : 9-34. EDWARDS,J. A. and CULVER,R., 1967. A flap-type integrating flow meter. Agr. Eng., in press. FVNK, J. P., 1962. A net radiometer designed for optimum sensitivity and a ribbon thermopile used in a miniaturised version. J. Geophys. Res., 67 : 2753-2760. HOLMES, J. W. and COLVILLE,J. S., 1964. The use of the neutron moisture meter and lysimeters for water balance studies. Trans. Intern. Congr. Soil Sci., 8th, Bucharest, 1964, in press. HOUNAM, C. E., 1961. Evaporation irt Australia. Commonwealth Australia, Bur. Meteorol., Bull., 44 : 88 pp. MClLROY, I. C. and AN~tJS, D. E., 1964. Grass, water and soil evaporation at Aspendale. Agr. Meteorol., 1 : 201-224. PENMAN, H. L., 1953. The physical bases of irrigation control. Intern. Hort. Congr., 13th, 1953, pp.913-924. PENMAN, H. L., 1963. Vegetation and hydrology. Commonwealth Agr. Bur. (Gt. Brit.), Tech. Commun., 53 : 124 pp. TAYLOR,J. K. and POOLE,H. G., 1931. A soil survey of the swamps of the lower Murray River, S.A. C.S.LR.O. Bull., 51 : 42 pp.
Agr. Meteorol., 4 (1967) 177-188
188
J.W. HOLMES AND C. L. WATSON
APPENDIX
List of symbols and their definitions D D' E EG E1 E2 E3
a day-length fraction, defined by PENMAN(1953), dimensionless. the depression of the wet-bulb temperature, in the Stevenson screen, °C. evaporation, mm/day. evaporation measured by the use of the bucket lysimeters, mm/day. evaporation computed by using the formula of PENMAN(1963) (eq.2), mm/day. evaporation computed by using the formula of PENMAN(1953)(eq.4), mm/day. evaporation computed by using the formula of MclLRoY (MclLRoY and ANGUS, 1964) (eq.5), ram/day. Ea evaporation computed by the Dalton type formula (eq.3), mm/day. ea saturation vapour pressure of water at mean air temperature, mm Hg. ed saturation vapour pressure of water at mean dew-point temperature (i.e., the actual vapour pressure in the air), mm Hg. H the energy absorbed at the grass surface from radiation of all wavelengths, expressed as the amount of water which could be evaporated by utilising all that energy, ram/day. h a function of the wind, defined by MCILROY and ANGUS(1964), the product of which with D' gives the evaporation estimated by a Dalton-type equation, mm/day/°C. I irrigation, mm/day. P precipitation, mm/day. S a stomatal factor, defined by PENMAN(1953). The product SD gives a coefficient by which the Dalton-type equation is multiplied to make some allowance for the effect of stomatal resistance to vapour flow and stomatal closing during night-time, upon water vapour diffusion into the atmosphere from the evaporating leaves. S is dimensionless. SD surface drainage, used only in eq. (1), mm/day. UD underground drainage, used only in eq. (1), ram/day. u mean wind speed at a height of 2 m, miles/day. 7 a conversion factor, which is the constant of the wet and dry bulb psychrometer, mm Hg/°F. It is taken to have the value 0.27. A the slope of the saturation vapour pressure curve at mean air temperature, mm Hg/°F.
Agr. Meteorol.,4 (1967) 177-188