A probabilistic sensitivity analysis of water availability in the Budyko-space

One of the pending questions in the context of global change is whether climatic drivers or other factors have a stronger influence on water availability. Here we present an idealised approach that allows to estimate the probability that changes in the aridity index have a larger effect on water availability than any other factor. The analysis builds upon a recent probabilistic extension of the Budyko framework, which is subject to an analytical sensitivity assessment. The results highlight the nonlinearity of the coupled water and energy balance. We further show that changes in water availability are dominated by changes in the aridity index in humid conditions, without making any assumptions on future climate change. In arid conditions other factors are dominating. A global application suggests that changes water availability are dominated by the aridity-index in 73% of the global land-area, implying that climate change is likely to alter water availability in most populated regions.