Water use regimes: Characterizing direct human interaction with hydrologic systems

The sustainability of human water use practices is a rapidly growing concern in the United States and around the world. To better characterize direct human interaction with hydrologic systems (stream basins and aquifers), we introduce the concept of the water use regime. Unlike scalar indicators of anthropogenic hydrologic stress in the literature, the water use regime is a two‐dimensional, vector indicator that can be depicted on simple x‐y plots of normalized human withdrawals (h_out) versus normalized human return flows (h_in). Four end‐member regimes, natural‐flow‐dominated (undeveloped), human‐flow‐dominated (churned), withdrawal‐dominated (depleted), and return‐flow‐dominated (surcharged), are defined in relation to limiting values of h_out and h_in. For illustration, the water use regimes of 19 diverse hydrologic systems are plotted and interpreted. Several of these systems, including the Yellow River Basin, China, and the California Central Valley Aquifer, are shown to approach particular end‐member regimes. Spatial and temporal regime variations, both seasonal and long‐term, are depicted. Practical issues of data availability and regime uncertainty are addressed in relation to the statistical properties of the ratio estimators h_out and h_in. The water use regime is shown to be a useful tool for comparative water resources assessment and for describing both historic and alternative future pathways of water resource development at a range of scales.