Projected climate change-driven streamflow declines present challenges in meeting rising water demand across the Southwestern United States

Abstract:

Across the Southwestern United States, water demand is expected to increase while climate change is anticipated to reduce supply. Future streamflow in seven water resource regions was estimated using a new SPAtially Referenced Regressions On Watershed attributes (SPARROW) streamflow model to project water supply in support of management decisions. Streamflow change was estimated by comparing projections resulting from seven climate models and two greenhouse gas concentration pathways (RCP4.5 and 8.5) for three, thirty-year intervals centered on the 2030s, 2050s, and 2080s to a historic thirty year period centered on the 1990s. Across all water resource regions, streamflow declines of 35-80% are projected for the 2080s relative to the historic period, with 51% and 67% of models projecting declines for RCP4.5 and RCP8.5, respectively. Although most models project moderate to substantial declines in streamflow in many water resource regions, especially for the 2080s and RCP8.5, a few models project streamflow increases. Streamflow increases are most prevalent in the 2030s under RCP4.5, although the total area experiencing increases is projected to diminish with time. Moreover, projected streamflow increases may still be insufficient for meeting future water demand. Streamflow at sites along the Colorado River used for measuring compliance with interstate and international water sharing agreements are projected to decline between 4-45% by the 2080s for both RCP4.5 and RCP8.5. Declines of this magnitude likely preclude the ability to meet water sharing agreements given that these agreements are already difficult to meet. By applying a spatially referenced modeling approach and comparing projected streamflow changes to present water use, we can identify which regions may be most impacted. These results support developing strategies to balance water demand and supply and to reduce future water scarcity and conflict.