The use of the aridity index to assess climate change effect on annual runoff

Available energy (often expressed in terms of potential evaporation) and precipitation largely determine annual evapotranspiration and runoff rates in a region. The ratio of annual potential evaporation to precipitation, referred to as the aridity index by Budyko, has been shown to describe the evaporation ratio (the ratio of annual evapotranspiration to precipitation) of catchments from a range of climatic regimes in a number of studies. It has been shown that aridity index alone can be used to obtain an estimate of ratio of standard deviation of annual evapotranspiration estimates to that of precipitation (the evaporation deviation ratio). At present, there are at least five functional forms available, which describe evaporation ratio as a function of aridity index. This study assesses data from Canadian Centre for Climate Modelling and Analysisʹ (CCCma) third-generation atmospheric general circulation model (AGCM) against these five functional forms. Evaporation ratio and evaporation deviation ratios from an AGCM simulation are compared against these five functional forms and it is shown that the primary control of available energy and precipitation over annual partitioning of precipitation, and interannual variability of evapotranspiration, is preserved well in the AGCM. The aridity index is further used to obtain an analytic equation, which can be used to estimate change in runoff given annual changes in precipitation and available energy. This equation is validated using data from control and climate change simulations of the CCCma coupled GCM (CGCM1) and shown to perform fairly well. The correlation between CGCM1 simulated annual change in runoff and the values obtained using aridity index is consistently around 0.95, and the average bias varies between 40.5 and 50.3 mm/year, for the five functional forms. The successful validation of this equation against data from a GCM climate change simulation illustrates the continued relevance of aridity index, and the primary control of precipitation and available energy in determining annual evapotranspiration and runoff rates.