ESTIMATING CLIMATE CHANGE IMPACTS ON WATER RESOURCES IN ARIZONA'S UPPER SANTA CRUZ RIVER BASIN: THE ROLE OF CLIMATE MODEL DOWNSCALING

We evaluated the impact of two climate model downscaling methods on a water resources assessment for Arizona’s Upper Santa Cruz River (USCR) Basin under projected climate change. Downscaling is the process of translating Global Climate Model (GCM) projections with scales of one to two degrees latitude and longitude to a spatial resolution suitable for basin-scale hydrologic modeling. The approach compares an empirical method based on historical observations (statistical downscaling) to a physics-based method using GCM output as the input to a high-resolution Regional Climate Model (dynamical downscaling).

We focused on the “microbasins” portion of the watershed that contains a series of four relatively small, shallow alluvial aquifers bounded by the low permeability Nogales Formation. Intermittent flows on the Santa Cruz River recharge the microbasins and provide the preferred water supply for the city of Nogales, Arizona.

We found that dynamically downscaled climate projections provided larger estimates of climate change, either wet or dry, than their statistically downscaled counterparts. As statistically downscaled projections are constrained by the assumption of stationarity, it may be prudent to develop dynamically downscaled projections for important water resources assessments. We also found that small changes in precipitation were magnified through conversion to runoff and then to a stored water resource. Thus a minor change in projected precipitation may be of concern to water resources planners. The results provide important guidance for estimating the range of uncertainty associated with future water supplies in arid basins.