The Unreasonable Effectiveness of the Budyko Hypothesis for Water Balance

The Budyko hypothesis, that the long-term evaporative fraction is primarily a function of the aridity index, is widely considered to be a hydrological success; perhaps this is because catchment hydrology has so few widely applicable constitutive equations! The apparent success of the Budyko hypothesis seems paradoxical, given the complexity of hydrological processes, and the myriad of interacting factors which drive the partitioning of the water balance. For any given value of the aridity index, there is a very significant range in many factors that also affect water balance, including climate seasonality, rainfall intensity, landscape structure, soil type, vegetation cover and hydro-geological setting. In this talk we use a comparative hydrology approach with global, continental and regional datasets to explore two competing schools of thought: (i) the Budyko hypothesis is essentially correct, because climate, soil, vegetation and topography are strongly interdependent; (ii) the hypothesis is incomplete and requires extension to account for the significant effects on water balance of factors other than aridity