Downstreamness: A Concept to Analyze Basin Closure

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In many places in the world, increasing water demands have led to the development of infrastructure for freshwater storage and irrigation. Especially in water-scarce regions, this development has led to a growing number of concerns about basin closure. These concerns ask for a structured approach for analyzing the occurrence of basin closure to facilitate sustainable responses. In this paper, the downstreamness concept is proposed for analyzing the availability and commitments of freshwater in river basins. The downstreamness of a location is the ratio of its upstream catchment area to the entire river basin area. The downstreamness of a function on the basin, such as water availability or water demand, is defined as the downstreamness-weighted integral of that function divided by its regular integral. An approach to determine the downstreamness of surface water storage capacity, stored surface water volumes, and water demands is described. Applying the proposed approach is helpful for the spatiotemporally explicit assessment of basin closure and its drivers. Water management policies rely on the valuations of trade-offs between commitments of upstream and downstream uses and technical implementations of upstream and downstream measures. The concept of downstreamness can assist in analyzing subbasin points of view as an integral part of a basin perspective. In this way, the effects of anthropogenic processes driving basin closure can be better understood. To illustrate the use of the concept, the authors describe its application to the Jaguaribe Basin in the semiarid northeast of Brazil.