Assessing the differences in sensitivities of runoff to changes in climatic conditions across a large basin

Long term average runoff depends largely on climatic conditions and is expected to change as climate change progresses, but to what degree and in which locations is uncertain. An analytical framework for assessing the sensitivity of runoff to changes in average precipitation and potential evaporation has recently been developed. This framework is based on Budyko's curve. It makes no assumptions about future climatic conditions and is simple to implement. Here we apply it spatially to the Murray Darling Basin (MDB) in Australia, for average 1981-2006 conditions, to gauge the differences in runoff sensitivity across different yield zones. The framework contains a catchment-specific parameter which is difficult to predict, and estimates of its value are explored and assessed.Results emphasise the importance of the spatial variation in runoff sensitivity. Due to the MDB's aridity, basin-average runoff is very low (around 28mmy^-1) and runoff is generally insensitive to changes in average precipitation and potential evaporation. Specifically, basin-wide runoff is expected to increase by 1mmy^-1 for a 10mmy^-1 increase in basin-average precipitation, and to remain unchanged for the same increase in potential evaporation. However, approximately 66% of basin flow originates from 12% of the basin's land area and in these few high yield catchments runoff is most sensitive to changes in climatic conditions. In the highest yield zone, runoff is expected to increase by 7mmy^-1 given a 10mmy^-1 increase in precipitation, and to decrease by around 4mmy^-1 for the same increase in potential evaporation. When applied spatially, this runoff sensitivity framework has the potential to help water managers and policy makers to target planning activities that seek to mitigate potential effects of a changing climate on water resources.