Catchments deviate less from their own Budyko curves over time than previously thought

Quantification of precipitation partitioning into evaporation and runoff is crucial for predicting future water availability. Over longer time scales, the widely used Budyko Framework, which is a curvilinear relationship between evaporative index (i.e., actual evaporation over precipitation) and aridity index (i.e., potential evaporation over precipitation), robustly quantifies precipitation partitioning under prevailing climatic conditions. Global long-term records indicate that catchments generally follow Budyko curves; however, a narrow scatter around these curves have been demonstrated in various studies, raising questions about the framework's applicability. To address this, we quantified (based on historical long-term water balance data of over 2000 river catchments world-wide) the global, regional and local distributions of deviations from parametric Budyko curves, between multiple 20-year periods over the last century. This process resulted in four 20-year distributions of deviation for each catchment. On average, it was observed that in 73% of the catchments, the long-term median deviation values across these distributions were not significantly different from zero suggesting minimal to no median deviations. Furthermore, it is found that for majority of the catchments (78%) the four 20-year distributions of deviations are not significantly different to each other implying consistency in deviations among different 20-year periods. Our analysis revealed that, for 80% of these catchments, the long-term median deviations, for the last century, fall within the range of ±0.02 with a very narrow spread in Interquartile Range values. These findings demonstrate that while catchments do not precisely follow the expected Budyko trajectories, the deviations are small and quantifiable. Consequently, by taking into account these deviations, the Budyko Framework remains a valuable tool for predicting future evaporation and runoff under changing climatic conditions, within quantifiable margins of error.