Differentiating Surface and Subsurface Runoff Elasticity to Climate and Landscape Changes Using a Modified Budyko Framework

AbstractUnderstanding the significant influence of climate and landscape changes is essential for managing water resources; however, the individual effects of climate and landscape on surface and subsurface runoff are not well understood. In this study, we quantified the contributions of precipitation (P), potential evapotranspiration (PET), and landscape parameter (n) changes to the elasticity of surface and subsurface runoff using a modified Budyko Framework (e.g., Choudhury–Yang equation) across six Ethiopian sub-basins. Trend and breakpoint analyses (Mann–Kendall and Pettitt tests) divided the long-term runoff data (1964–2023) into two distinct periods for attribution. We examined runoff elasticity, as the percentage change in mean annual runoff for a given percentage change in P, PET, and n. The results revealed that, on average, P decreased by 1.33 mm/year, and PET increased by 1.72 mm/year, resulting in a decline in the long-term total, surface, and subsurface runoff by 19.7 mm, 7.6 mm, and 12.4 mm, respectively, between the two periods. Crucially, the runoff components exhibited distinct hydrological responses; surface runoff elasticity was primarily governed by change in P (49.3%), followed by PET (26.2%) and n (24.5%), emphasizing its dominant link to water availability, particularly in arid sub-basins. The subsurface runoff elasticity showed greater sensitivity to landscape change and energy balance, being predominantly influenced by change in PET (33.6%) and n (39.7%). Large positive deviations in ∆n (n2-n1) rapidly shift runoff sensitivity from climate control to landscape domination. Overall, climate forcing (changes in P and PET) accounted for 66.7% of the runoff elasticity, confirming its primary role. These findings provide fundamental new insights into catchment partitioning behaviors, mandating that water management strategies adopt a component-specific approach tailored to the differential elasticities of surface and subsurface flow systems in complex catchments.KeywordsBudyko framework, Climate change, landscape parameter, Runoff components, Runoff elasticity