Catchment precipitation partitioning in the Budyko framework is controlled by root zone storage capacity

Abstract Quantification of long-term partitioning of precipitation into evaporation and runoff is a fundamental pursuit in catchment hydrology. The Budyko framework provides a theoretical basis for this and estimates the evaporative fraction based on the aridity index. However, deviations from the global-average Budyko curve point to additional controls on precipitation partitioning beyond the aridity index. We hypothesized that root zone storage capacity ( S r, max ), defined as maximum subsurface water volume accessible to vegetation roots, is a key driver of these deviations. The relationship between S r, max and precipitation partitioning in the Budyko space was investigated globally across >5000 catchments. S r, max was calculated using the memory method based on runoff observations and the water balance. The ω -parameter from Fu’s equation, which was used here to construct parametric Budyko curves, reflects deviations from the global-average Budyko curve and hence precipitation partitioning. Results revealed a globally stronger correlation (Spearman’s ρ = 0.68) of ω with S r, max , than with other potential controls, indicating S r, max as a dominant driver of precipitation partitioning. Further analysis based on Köppen–Geiger climatic zone classification revealed variations in the S r, max – ω relationship, with the strongest correlations observed in cold ( ρ = 0.87) and Mediterranean ( ρ = 0.83) climates, followed by temperate ( ρ = 0.76), tropical ( ρ = 0.64) and arid climates ( ρ = 0.61). Regional differences in S r, max indicate that, at a given aridity, E A / P largely reflects vegetation adaptation to the seasonal interplay between water supply and atmospheric water demand. This study provides strong empirical evidence on a global scale for S r, max as a governing factor in modulating catchment precipitation partitioning, as evident in the Budyko space. As a major implication our results provide a theoretical basis for the maximum values of S r, max found in nature, as constrained by the water and energy limits of the Budyko framework.