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Why, in a globally warming climate, have observed rates of atmospheric evaporative demand (AED) declined at many sites over recent decades? The answer is that the evaporative process is primarily driven by radiative and aerodynamic components (the latter mainly influenced by wind speed and atmospheric humidity). So, although increases in air temperatures in isolation would result in increases of both pan evaporation (Epan) observations, and fully physically based model estimates of potential evapotranspiration (ETp), results show that changes in the other meteorological variables governing AED are currently exerting a greater opposing influence. Widespread declines in wind speed (denoted ‘stilling’) of ~−0.014 m s−1 a−1 have recently been reported. In studies assessing the relative contribution of the four primary meteorological variables on AED dynamics, wind speed trends are often indentified as being important. Stilling reduces AED, but its impact on actual evapotranspiration and streamflow is situation dependant. For wet catchments under steady state conditions, stilling-induced declines in actual evapotranspiration will result in an approximate complementary increase in streamflow. In contrast, for dry catchments, actual evapotranspiration dynamics more closely follow precipitation dynamics so stilling has negligible impact on streamflow. In ‘equitant’ catchments (i.e. those that straddle the energy–water limitation divide), the likely impact of stilling on the catchment water balance depends on sub-annual processes and requires that new approaches (either top–down or bottom–up) be developed to allow better predictions to be made. Copyright © 2012 Commonwealth of Australia |
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