The Budyko framework beyond stationarity

Abstract

Water availability is of major importance for a wide range of socio-economic sectors. Over land, the partitioning of precipitation {(P)} into evapotranspiration {(E)} and runoff {(Q)} is the key process to assess hydrological conditions. For climatological averages, the Budyko framework provides a simple first order relationship to estimate the evaporative index E / P as a function of the aridity index {(Ep} / P, with Ep denoting potential evaporation). However, a major downside of the Budyko framework is its limitation to steady state conditions, being a result of the assumption of a closed land water balance. Nonstationary processes coming into play at other than mean annual catchment scales are thus not represented. Here we propose an analytically derived new formulation of the Budyko curve including an additional parameter being implicitly related to the nonlinear storage term of the land water balance. The new framework is comprehensively analysed, showing that the additional parameter leads to an upward rotation of the original supply limit and therefore implicitly represents the amount of additional water available for evaporation. The obtained model is further validated using standard datasets of P, E and Ep. It is shown that the model is capable to represent first-order seasonal dynamics within the hydroclimatological system.