Calculating the Water Requirements of Irrigated Crops in Australia Using the Matt-Shuttleworth Approach

Abstract

Use of the one-step Matt-Shuttleworth (M-S) approach for estimating the water requirements of irrigated crops from surface resistance is investigated relative to the two-step method currently recommended by FAO based on crop coefficients. Calculations are performed using data from five climate stations in important irrigation districts in Australia. The results show that the humid conditions that are an implicit prerequisite of the FAO-56 approach rarely apply at these sites. Hence, the derivation and use of crop coefficients, which are shown to be highly weather dependent, will always be problematic. As an alternative, the surface resistance required by the M-S approach can be calculated from FAO crop coefficients if the crop height and the air temperature when the crop coefficient was calibrated are known. For certain irrigated crops in Australia, this surface resistance is not sensitive to the calibration temperature and it can therefore be defined to within 5%. For other crops where this is not the case, it is recommended that surface resistance is calculated assuming a calibration temperature of 20C. Field studies and/or retrospective re-analyses to directly derive values of surface resistance are called for, with priority given to studies of crops where the surface resistance is likely to be significantly different from 70 s m-1. In the typically arid, windy climate of Australian irrigation districts, using the M-S approach gives estimates of evapotranspiration that differ from FAO estimates in individual growth stages by -27% to +19% for well-watered sugar cane and by -36% to +14% for well-watered cotton, depending on location. However, for entire growing seasons, M-S evapotranspiration estimates are 3% to 15% higher than when using the FAO method for sugar cane and 4% to 6% higher for a (shorter) cotton crop. The difference when estimating well-watered pasture evapotranspiration is smaller (0.5% to 2.5%). Adoption of the M-S approach into irrigation practice is proposed on the grounds that this approach is consistent with present-day understanding of the evaporation process, is simple and feasible, and will facilitate future adoption of realistic representations of the effect on evapotranspiration of plant stress and of crops with partial ground cover.