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A comparison of six methods for estimating crop evapotranspiration

T. Swaminathan, D.J. Connor and G.M. Rimmington

School of Agriculture and Forestry
The University of Melbourne, Parkville, Victoria 3052

Water is a major factor affecting crop production in Australia and there exists considerable variability among different Methods of measuring or estimating crop evapotranspiration (ET). This paper presents an objective comparison of six different Methods which were applied to two wheat cultivars, Banks and Quarrion, for four sowing dates over two growing seasons.


Crops of wheat (cv. Banks and Quarrion) were sown on four dates in May, June, July and September in two seasons (19846 1985) on a red duplex soil (Dbl. 13) at Werribee, Victoria (37055'S, 144401E).Measurements were made of crop dry matter components, development, daily weather and soil moisture changes.Five soil monolith lysimeters were used (ETm) to measure the changes and these data were compared with pan evaporation (ETp), estimates of the energy (ETe), and aerodynamic (3) (ETvp) components of the Penman-Monteith combination function (2). Two additional forms of ETpm were used one with the Dilley and Shepherd wind function (ETds) (1) and one with the Thom and Oliver wind function (ETto) (4).


Estimates of ET using each Method are presented in Table 1 for 4 sowing dates for Quarrion grown in 1985. If ETm is used as a reference, the ranking of agreement among the Methods is: ETto>ETds>ETe>ETvp>>ETp.

Table 1 Comparison of six Methods for estimating ET with ETm

13: slope;() SE of slope; SD sowing date (day of year);
r2 Coefficient of determination for each Method versus the lysimeter data (ETm)

1. Dilley, A.C. and Shepherd, E. (1972) Potential evaporation from pasture and potatoes at Aspendale. Agric. Met. 10, 283-300.

2. Monteith, J.L. (1965). Evaporation and environment. Proc. Symp. Soc. Exp. Bot. 19, 205-234.

3. Tanner, C.B. and Sinclair, T. (1981). Transpiration efficiency of potatoes. Agron. J. 73, 59-64.

4. Thom, A.S. and Oliver, H.R. (1977). On Penman's equation for estimating regional evaporation. Quart. J. Roy. Met. Soc. 103, 3145-357.

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