Sustaining river flows for biodiversity in the arid southwestern U.S.: Effects of current and future municipal water use

Background/Question/Methods

Public discourse on water management in Arizona has focused largely on the distribution and quantity of water needed to support population growth. Sustainability in Arizona is often discussed as “safe yield”, the concept that groundwater withdrawals not exceed annual recharge. However, safe yield does not protect perennial stream-flow from groundwater pumping, raising the potential for rivers to be de-watered as groundwater withdrawals approach the volume of annual recharge. We compared current and projected future municipal water demand with river flows to identify rivers at risk and to estimate the magnitude and temporal nature of future changes. Our comparison focused on 26 sub-watersheds that support the state’s remaining perennial rivers. We used published estimates of average annual base flow where available, and otherwise use the model Indicators of Hydrologic Alteration. We used estimates of per-capita water use and census block population data to calculate current municipal water use. To estimate future use, we used a spatially explicit regional growth model to distribute county-level population projections to each sub-watershed by decade through 2050.  

Results/Conclusions

Our preliminary results show that the volume of municipal water demand already exceeds base flows in two San Pedro River sub-watersheds, and is projected to exceed base flows in all San Pedro River sub-watersheds in future decades. Projections also reveal that municipal water demand will exceed base flows in the upper Verde River and upper Agua Fria River sub-watersheds. These rivers are critical to Arizona’s freshwater species, as the San Pedro River provides habitat for 12 federally listed or candidate species, and the upper Verde River supports 17 listed or candidate species and 50% of the state’s extant native fish species. In contrast, our comparison shows that projected water demand will not exceed base flows for sub-watersheds located on the main-stems of the Gila and Salt rivers. Our analysis points to the need for: 1) basin-wide groundwater models that provide temporal and spatial projections of stream-flow depletion based on specific water management scenarios; 2) data to document flow alteration-biological response relationships for Arizona’s rivers; and 3) water planning that incorporates a portion of annual recharge as base flow to remain in situ for environmental benefits.