A spatially explicit framework for assessing the effects of weather and water rights on streamflow

Abstract Policies that establish availability of water for human uses are key tools for ecologically sustainable water management. In places where water management is decentralized and dispersed across a catchment, spatially explicit tools are essential for evaluating the cumulative effects of many instream diversions on discharge and water availability through the drainage network. We developed a spatially explicit model to evaluate the cumulative impacts of surface water rights under a decentralized management regime, and apply this framework to the Navarro River catchment, in northern California, and evaluate impacts relative to formal policies to determine whether water is available for further appropriation given environmental needs. Model results show that upstream water rights comprise a small fraction of normal-type winter discharge; but they comprise a much larger portion in dry years and all of discharge during the summer dry season. In a normal-type rainy winter season, water rights comprise less than 5 percent of the average discharge during the through almost all of the drainage network: by policy standards, water is widely available for further appropriation during this winter period. Most stream reaches where upstream water rights comprise more than 10 percent of discharge are small headwater streams (zero- and first-order). During other periods such as November and April (the beginning and end of the rainy season, respectively), impairment by water rights remains low under average conditions; but under dry conditions, most streams order 0–2 with upstream water rights are impaired more than 10 percent (with some more than 50 percent). During average summer conditions, 45 percent of the drainage network with upstream water rights is impaired by more than 20 percent; and 25 percent of these streams are impaired by more than 50 percent. These results indicate that water is available for further appropriation in winter; but water rights can cumulatively impair discharge by more than ten percent in small and large streams under dry-type conditions, and by more than 50 percent basinwide during the dry season.