How realistic are water‐balance closure assumptions? A demonstration from the southern sierra critical zone observatory and kings river experimental watersheds

Abstract

Abstract The water balance is an essential tool for hydrologic studies and quantifying water‐balance components is the focus of many research catchments. A fundamental question remains regarding the appropriateness of water‐balance closure assumptions when not all components are available. In this study, we leverage in‐situ measurements of water fluxes and storage from the Southern Sierra Critical Zone Observatory (SSCZO) and the Kings River Experimental Watersheds (KREW) to investigate annual water‐balance closure errors across large (1016–5389 km 2 ) river basins and small (0.5–5 km 2 ) headwater‐catchment scales in the southern Sierra Nevada. The results showed that while long‐term water balance in river basins can be closed within 10% of precipitation, in the smaller headwater catchments as much as a quarter of precipitation remained unaccounted for. A detailed diagnosis of this water‐balance closure error using distributed soil moisture measurements in the top 1 m suggests an unaccounted deeper storage and a net groundwater export from the headwater catchments. This imbalance was also found to be very sensitive to the timescales over which water‐balance closures were attempted. While some of the closure errors in the simple water balance can be attributed to measurement uncertainties, we argue for a broader consideration of groundwater exchange when evaluating hydrological processes at headwater scales, as the assumption of negligible net groundwater exchange may lead to an overestimation of fluxes derived from the water balance method.