Interannual variability of catchment water balance in Australia

Prediction of annual evapotranspiration and streamflow in Australia is considered using one-parameter models based on the water and energy limits concept of Budyko. These interannual models of catchment water balance assume that the principal variables controlling annual evapotranspiration are potential evapotranspiration and annual rainfall. Based on a dataset of unimpaired streamflow in Australia, we specifically test whether annual evapotranspiration ratios (i.e. the ratio of annual evapotranspiration to annual rainfall) follow Budyko’s curve and other related curves. We consider the effect on predictions of evapotranspiration and streamflow of the shape of the curve used. Rainfall seasonality is shown to be important in determining the shape of the optimal curve used to model annual evapotranspiration ratios. Many catchments, particularly with summer-dominant rainfall, followed a stricter energy limit than Budyko’s energy limit. That is, instead of approaching potential evapotranspiration during high-rainfall years, actual evapotranspiration approached a fixed proportion of potential evapotranspiration. We theorise that this energy limit is principally caused by variability in interstorm evapotranspiration due to high average rainfall amounts.