| Summary: | This study assesses the effects of a changing climate and population growth on water resources by modeling groundwater supplies in the Tucson Active Management Area. The finite-difference flow model, Modflow, is used to incorporate agricultural, municipal, and industrial well pumping along with natural and artificial recharge. This study expands on a Modflow model created by the Arizona Department of Water Resources to determine the impacts from limited water supplies and increased demand (Mason and Bota, 2006). Groundwater conditions and pumping in the Upper Santa Cruz and Avra Valley sub-basins are modeled starting in the year 1940 and continue to 2009. The model predicts pumping and recharge for the period of 2010 to 2050. During this projection period, nine scenarios based on various climate and population conditions are evaluated. Climate impacts are reflected in the amount of recharge entering the groundwater system. Local and regional climate conditions are incorporated since a large portion of the Tucson water supply is provided by the Colorado River water delivered along the Central Arizona Project (CAP). A decrease of 10% to the mean natural flow in the Colorado River over the next 50 years is used to predict Colorado River flows and shortages. Additionally, a 20% streamflow reduction case and two scenarios that evaluate the local and regional shortages individually are presented. Operational rules for the deliveries of the CAP water during shortage conditions are utilized to represent the system. The percentage of population growth is varied around the current case, which is extrapolated from data provided by the Arizona Department of Water Resources. Water demand is based upon the initial population, annual population growth, and gallons per capita day, which is a measurement of water use per person. The three population scenarios are limited growth, current case, and high growth. Results indicate groundwater depletion conditions are the worst during the high growth/shortage scenarios and best for the limited growth scenario. The change in storage of the aquifer is greatly driven by the pumping, which is dependent on population. For the shortage condition, the decline in natural recharge has a much larger effect on the change in water storage compared to the artificial recharge reductions due to shortages of CAP water.
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