Sustaining water demands to urban ecosystems in the southwestern United States

Background/Question/Methods:  It is critically important that we understand the effects of climate change, both past and future, on streamflow in order to best manage future water supplies.  This connection is complicated by both obvious and nuanced secondary effects—including disturbance regimes and land use change—on climate/streamflow interactions. In the desert Southwest, a sustainable future is intricately tied to water supply and availability. In this paper we connect long-term climate and streamflow data to present and future water availability in the Central Arizona-Phoenix Long-Term Ecological Research Program (CAP LTER) study area. Specifically, we relate 60 years of headwater streamflow data in a source watershed to precipitation and to direct and indirect measures of evapotranspirative losses. We then relate these past trends to our growing understanding of water use and management within the city itself. Retrospective analyses of these relationships are not optimal predictors of future conditions, particularly given the many uncertainties associated with how climate and human decision-making will change in the future. Still, we argue that we can inform decisions about future sustainability by better understanding critical hydrologic functions and how they respond to climate change and human intervention in the greater social-ecological water system of south-central Arizona. 

Results/Conclusions:  Our climate-biophysical analysis of data from Sycamore Creek, AZ was part of a multi-site synthesis that used the Budyko curve energy-balance approach.  Budyko curves relate potential evapotranspiration (PET) and actual evapotranspiration (AET), with both indexed to precipitation (P), and indicate whether AET is water- or energy-limited. PET was calculated from air temperature (T), P, and stream discharge (Q) while AET was simply P-Q. Over the 60-year period of record we found no change in streamflow at any time of year or in precipitation. Over this time AET/P was lower than PET/P and variation in AET/P was lower than in PET/P; however, both metrics were highly variable among years compared with other sites. The streams providing urban Phoenix with about 50% of its water appear to be more efficient at supplying water than simple temperature-precipitation models would predict. Within the city, outdoor water use dominates and irrigation-based evapotranspiration mitigates Urban Heat Island effects. We are currently using a network of micrometeorological towers and an urban eddy flux tower to investigate these climate–water-use linkages, to determine if the same AET/P < PET/P pattern holds for the human-designed oasis landscapes that dominate Phoenix.