How Much Water Does a Basin Hold?

Terrestrial water storage is the key entity that determines flows in river channels, climate and the fresh water ecosystem. It is not possible to directly measure storage, say in a drainage basin, with the use of available technology. For example, GRACE (Gravity Recovery and Climate Experiment) satellites can measure storage fluctuation, but cannot measure absolute storage. Therefore, hydrologists generally use conceptual models to estimate storage, particularly by analyzing recession flows or streamflows during no-rain periods, because during these periods discharge (observable entity) in river channels is controlled only by storage in the basin. However, one major problem with recession analysis is that late recession flows, particularly for large basins, are usually not observed, and early recession flows indicate that storage is infinite, which is not realistic. We address this issue by using geomorphological recession flow model (GRFM), which suggests that storage-discharge relationship is exponential for the early recession phase and power law for the late recession phase, being distinguished from one another by a sharp transition. Then we obtain a simple expression for the 'active' (drainable) storage within a basin in terms of early recession curve characteristics and basin geomorphology. The predicted storage matches well with the observed storage (R^2=0.96), indicatiung the possibility that GRFM can be used to reliably model storage in basins to answer many practical and scientific questions related to water resources. The plot of modelled initial storage vs. observed initial storage for 124 recession events from 27 USGS basins. The good correlation (R^2 = 0.96) indicates that the predictions are reliable.