Estimating the impact of rainfall seasonality on mean annual water balance using a top-down approach

Seasonal variations of climate and catchment water storage affect the partitioning of rainfall into evapotranspiration and runoff. A new method was developed to estimate the seasonality effect on catchment-scale mean annual water balance using a top-down approach. The model is based on observed rainfall, potential evapotranspiration and streamflow data from 326 unregulated catchments in Australia. It assumes that catchment-scale annual evapotranspiration consists of two components: climate-controlled evapotranspiration and storage-controlled evapotranspiration. The distinction made here is to allow the effects of climate and catchment storage to be estimated separately. The climate-controlled evapotranspiration is affected by rainfall and potential evapotranspiration and can be accurately estimated by Budyko-type relationships using dryness index for different rainfall regimes. The storage-controlled evapotranspiration is influenced by seasonal catchment water storage. When rainfall and potential evapotranspiration are in phase, the effect of rainfall seasonality is to increase climate-controlled evapotranspiration. However, storage-controlled evapotranspiration tends to be smaller under this rainfall regime and exhibits the opposite behaviour of climate-controlled evapotranspiration. As a consequence, the seasonality effect on mean annual evapotranspiration cannot be adequately represented by phase difference between rainfall and potential evapotranspiration alone and the effect of water storage needs to be considered. Results show that inclusion of seasonal changes in catchment water storage significantly improves evapotranspiration predictions for catchments with winter-dominant rainfall.