The consequences of the Sierra losing its Nevada: Inconsistent water year classification and a nonstationary hydroclimate alter water allocation for humans and the environment

Abstract:

In California, many of the larger reservoirs and water projects are operated for multiple uses, such as water supply, hydropower, flood control, environmental mitigation, and recreation. Human and natural systems form a complex web of competing demands for freshwater that requires careful decisions on the water allocations to meet these multiple resource demands. Management decisions of water allocation are typically based on the typology of annual runoff at the supplying facilities, commonly referred to as “water year type”. The classification of water years uses numeric thresholds supported by historic and/or forecasted hydrologic data. In the San Joaquin River basin, the classification of water years is calculated independently per facility, project or region, using a variety of methods (e.g., indices, models, numerical thresholds) producing asynchronous results regionally. The objectives of this study are to assess the differences of water year type classification and to examine how the current discrepancy in classification systems affects water allocation in different facilities and watersheds under a nonstationary climate. In the Central Sierra Nevada, there are five dimensions considered to assign and apply water year types. Classifications depend upon the location where inflows are forecasted, the methods and time period used to calculate it, the date(s) when it is performed and the duration of validity of water year classification. Simulated historic and future streamflow for the water years 1951-2059 from regionally downscaled biased corrected Variable Infiltration Capacity hydrologic model were examined under historical and future climate change scenarios using four representative GCMs. Preliminary results indicate a change in the distribution of water year types and that facilities operated under different water year type classifications will likely be affected unevenly due to the inconsistent categorization methods. This analysis identifies operational and policy strategies to adapt water year typology to include hydroclimatic alteration and its impact on water allocation, hydropower generation, and environmental flows.