Development and Evaluation of a Novel Soil Water Balance Approach for Mountain Catchments

ABSTRACT Mountains are pivotal in the hydrological cycle and affect at least half the global population. However, quantifying the mountain hydrological cycle presents significant challenges owing to its inherent complexity and remoteness. Traditional soil water balance (SWB) methods, which assume sequential and hierarchical processes, fail to adequately capture the crucial contributions of vegetation and subsurface dynamics. A novel SWB method that combines a mass balance model and fully distributed water flow and storage simulation in unsaturated soil is used to address these limitations. By leveraging the concept of hillslope water sustenance, this innovative approach significantly enhances the assessment of water partitioning. It models the interactions between surface and near‐surface processes with greater accuracy. This new approach was evaluated in an Andean mountain catchment, where the impact of soil, vegetation and slope on the hydrological balance components was determined for 48 representative hillslopes. The disparities between the proposed method and conventional SWB approaches are significant, particularly in scenarios where available water allows competition among different hydrologic processes. The proposed approach results in higher soil water storage, approximately 5% higher annual recharge, and, more importantly, explicitly accounts for the interflow. Recharge and interflow can constitute as much as 25% and 11% of the annual precipitation, respectively, and the water deficit can exceed 46% of the reference evapotranspiration. Following the proposed replicable workflow, it is feasible to implement a non‐sequential SWB approach and better assess the competition between vegetation water use and water flow and storage on mountain hillslopes.