What an unclosed catchment water budget can tell us

An imbalance in a catchment water budget does not identify a missing pathway, and exact closure does not validate every component. This evidence review synthesises two multi-basin closure studies and measurement-uncertainty guidance to develop a diagnostic approach for northern catchments. It examines boundary definitions, compatible units and periods, snow carryover, storage, and dependence between products. A fictional annual account shows why a residual can be small relative to combined uncertainty and why replacing an independent evapotranspiration estimate with a remainder is not validation. Seasonal residual patterns and targeted independent observations are considered as ways to discriminate explanations. The review supplies no universal closure threshold and performs no local assessment. Its conclusion is that retaining the residual and component histories makes a budget more informative than forcing an apparently complete account.