Growing season evapotranspiration from duplex soils in south-western Australia

In the Mediterranean-type climate of south-western Australia, evapotranspiration (ET) is of particular importance because of the expanding threat of dryland salinity. This paper reports on studies at two sites for a total of 5 years aimed at quantifying ET from pastures on duplex (sand over clay) soils within the region. ET was assessed with the Bowen ratio energy balance technique, and compared with potential ET and ET calculated from the water balance. ET closely matched potential ET from the break of the season (May or June) until some seasonally dependent time in spring (September or October). The date of divergence of actual from potential ET was not clearly defined, but occurred gradually during a 2-week period, and was associated with the rapid depletion of soil water at the end of the season. The gradual transition was due to natural variation in the magnitude of daily potential ET during the transition period, which ranged from 1.1 to 4.5 mm. The water balance tended to overestimate ET during winter, by assuming deep drainage was zero, whereas the energy balance tended to overestimate ET very late in the season. For duplex soils in south-western Australia, a composite approach may give a cheap and accurate estimate of growing season ET, provided that rainfall is adequate to maintain surface soil moisture. This approach assumes that ET equals potential ET until the rapid decline in soil water begins, and ET equals changes in soil water storage after this time.