Characterizing water use by irrigated wheat at Griffith, New South Wales

Abstract

Wheat is being grown increasingly in the irrigated areas of south-east Australia. Its profitability depends on high yields, which in turn, are highly dependent on accurate water management. This combination, together with the increasing need for greater water use efficiency to minimize accessions to rising water-tables, calls for effective irrigation scheduling. To achieve this, accurate estimates of crop water use and upward fluxes of water into the root zone from shallow water-tables are required. A weighing lysimeter, installed in 1984, measured hourly evaporation (Ea) from a wheat crop which enabled the accuracy of water use estimates to be assessed. Daily potential evaporation (Ep) was calculated from a combination equation previously calibrated over lucerne, while previously developed crop coefficients for wheat were used to convert Ep to estimated Ea. Daily Ea was the major component in a water balance model for irrigated wheat. The model was quite efficient (r2 = 0.911, but with a bias of -8.8%, which indicated that Ea values were generally underestimated. The underestimate was due primarily to the wind function used in the calculation of Ep, and alternative functions for both daily and hourly calculations were derived. The 1984 lysimeter data also showed that change in soil water content was accurately measured with the field-calibrated neutron probe. Comparisons of measured and estimated water use from field experiments in 1981 and 1982 indicated that upward flux from a water-table between 1 a5 and 2.1 m below the soil surface may be up to 30% of daily Ea. This upward flux will need to be taken into account if irrigation scheduling is to promote efficient use of irrigation water.