In the Mediterranean-type climate of south-western Australia, evapotranspir
ation (ET) is of particular importance because of the expanding threat of d
ryland 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) so
ils 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 Octob
er). The date of divergence of actual from potential ET was not clearly def
ined, but occurred gradually during a 2-week period, and was associated wit
h the rapid depletion of soil water at the end of the season. The gradual t
ransition 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 i
n the season. For duplex soils in south-western Australia, a composite appr
oach 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 b
egins, and ET equals changes in soil water storage after this time. (C) 200
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