Water Year Classification in a Non-stationary Climate

Many water management decisions, such as environmental flow requirements and water supply allocations, are based on categorical water year type designations. Thresholds for water year type designations vary by region and index, but most are defined by some measure of runoff in the current water year compared to average historical runoff. Climate change is anticipated to alter the timing and volume of runoff, and change the relative frequency of water year types as presently defined. We use California's Sacramento Valley and San Joaquin Valley Indices to examine climatic changes. These indices provide a framework for allocating and transferring water among users. Streamflow estimates for 1951 - 2009 are from the climate-forced Variable Infiltration Capacity hydrologic model and are used to estimate potential changes in runoff and water year type frequency (using 6 GCMs for the A2 and B1 emissions scenarios). We show that if current water year type thresholds are maintained, more years are classified as dry and less water is allocated for environmental outflows. If thresholds are redefined to reflect the historical distribution of year types, the burden of climate change is shared among consumptive users, water exporters, and environmental outflows. This case study illustrates how water policy and allocation frameworks were designed assuming climatic stationarity, and that adapting water policy (or maintaining the status quo) affects which users are harmed from climate change.