Soil drainage modulates climate effects to shape seasonal and mean annual water balances across the southeastern United States

Climatic forcing and landscape properties control catchments' hydrological responses over both seasonal and mean annual timescales. Controls on annual and seasonal water balances are usually studied separately which limits and fragments understanding of catchment behaviour. Establishing process controls on hydrological responses that act across multiple time scales could better unify hydrological theory. Here, we use streamflow and climate data from 56 catchments in the Ozarks and Appalachian regions (US) to test how climate (aridity index) and soil drainage together shape seasonal and mean annual water balances for these humid, snow-free catchments with little precipitation seasonality. We calibrate a simple conceptual model to observed seasonal streamflow and obtain an effective parameter that summarizes the nonlinearity of drainage of soil moisture to groundwater. Our comparative analysis of catchments in the Ozarks and the Appalachian regions indicates that catchments in the more humid climates have lower streamflow seasonality and higher mean annual flow, irrespective of the nonlinearity of soil drainage. In contrast, in relatively drier climates, more nonlinear soil drainage increases the seasonality of streamflow and reduces mean annual flow. Additional testing across 204 humid catchments with little precipitation seasonality and snowfall in the Southeastern United States further supports our hypothesis that soil drainage nonlinearity significantly modulates seasonal and mean annual water balances. These results reveal how soil drainage nonlinearity provides a process interpretation that connects seasonal and annual water balances and highlights the importance of nonlinearity of soil drainage in hydrological modelling.