Assesing Soil-Moisture Storage and Discharge Dynamics at the Catchment Scale under Complex Wetting and Drying Conditions

Hysteresis in the soil water storage-discharge relation is an important phenomena observed during hydrologic response to rainfall events, which may vary spatially and temporally. In this study, we use a comprehensive physics-based hydrologic response model to simultaneously examine differences in hysteresis at the catchment scale and at the point scale for positions within the catchment hollow and higher up on the hillslopes. Using 11-years of observed climate data from a small headwater catchment dominated by saturation excess overland flow, we examine hysteresis for numerous rainfall events with different durations, intensities, and initial conditions. Additionally, we examine the annual water balances to explore the possibility of relating hysteretic storage-discharge trends to simple predictors of runoff generation. Simulation results did not show a dominant directionality in hysteresis across the catchment (clockwise vs. counter-clockwise) regardless of storm duration or initial conditions. However, the magnitude of hysteresis is consistently larger at hillslope positions as well as for short duration, higher intensity storms, which illustrates the importance of hillslope drainage limitations on the storage behavior within the catchment. Furthermore, results illustrate that a threshold value of hillslope soil moisture can be a useful indicator of the seasonal transition from ET-dominated to runoff-dominated response. Thus while soil moisture storage within the hollow remains near capacity throughout the year due to continuous lateral drainage, dynamic hillslope storage is only exceeded seasonally when PET is lower. Despite the complex temporal and spatial dynamics in the hysteretic responses to rainfall, results support previous assessments that a well-placed soil-moisture probe [e.g., Catchment Average Soil Moisture Monitoring site (CASMM)] could be a simple and useful predictor of storage within a watershed. The implication is that soil-water retention characteristics of hillslopes are key for predicting runoff generation potential in catchments dominated by saturation excess overland flow.