Water Right Curtailment Analysis for California's Sacramento River: Effects of Return Flows

California's recent drought has brought economic hardship and attention to long-standing weaknesses in California's water management. One weakness is assessment of surface water shortages and resulting needs to curtail water diversions. This thesis presents an integrated method that mathematically combines basin hydrology with legally defined water right priorities and regulations in a spreadsheet model with an open source solver to suggest a legally-optimized water allocation for a network of sub-basins within a watershed. The resulting Drought Water Rights Allocation Tool (DWRAT) allocates water to every water right holder in a watershed according to riparian and appropriative water law and can be modified for any watershed with hydrologic and water use data. This method is applied here to the Sacramento River basin. DWRAT currently employs a statistical hydrologic model developed by the U.S. Geological Survey (USGS) to disaggregate estimated unimpaired flows from points with gaged streamflow to regions of unknown streamflow. To ensure water right priorities are maintained, DWRAT uses linear programs to apply riparian and appropriative water law doctrines. The hydrologic representation and linear programming are combined with monthly water use data for water right holders to create a DWRAT model for a basin. This report focuses on the Sacramento application and the effects of different representations of return flows. Current model results show that for the critically dry water year October 2014 - September 2015, there would be 13.2 million acre-feet of water shortage (63% of normal water diversions/use) in the Sacramento basin, with 65% of that shortage during the irrigation season. Including return flows decreased annual water shortage by 1 maf (from 65% to 60% of normal use). DWRAT provides a transparent accessible framework for integrating legal doctrines, water use, and hydrologic knowledge of a basin for water use curtailment during drought.