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Climate Change and U.S.-Mexico Border Communities 367Figure 16.8 Water use in Mexico’sborder region (includes Region 1,Peninsula Baja California; Region II,Northwest; and Region VI, Rio Bravo).Adapted from CONAGUA (2011, annexes,128-129, 133).Rio Grande WATERSHED. The Rio Grande has its headwaters in the San Juan Mountainsof southern Colorado, flows through New Mexico, forms the international boundarybetween the United States and Mexico (Figure 16.9), and terminates in the Gulf ofMexico. Its watershed is divided roughly equally between the United States and Mexico.The Upper Rio Grande is defined as the headwaters area in Colorado downstream toFort Quitman, Texas (about 60 miles downstream from El Paso). The Lower Rio Grande,from Fort Quitman to the Gulf, takes in the river’s largest tributaries, including the PecosRiver and Devil’s River in Texas and the Río Conchos, Río Salado, and Río San Juan inMexico (CDWR 2009). The Upper Rio Grande system has two large storage reservoirs,Elephant Butte and Caballo Reservoirs, as well as smaller dams. xxx Overall, about half ofthe basin’s 19 million acre-feet (MAF) of storage is in Mexico and the other half in theUnited States (CDWR 2009). A 1938 interstate compact divides the waters of the UpperRio Grande among Colorado, New Mexico, and Texas. Two treaties between the UnitedStates and Mexico govern allocation of water from the river’s international reach. AboveFort Quitman, the United States is required annually to deliver 60,000 acre-feet of RioGrande water at Ciudad Juárez, in accordance with the Convention of 1906.Significant shared groundwater resources that are critical supply sources for cities inthis area include the Hueco Bolson and Mesilla Bolson aquifers in the El Paso–CiudadJuárez region which are shared among New Mexico, Texas, and Mexico. Overdraft andsalinity challenges are major issues for both sides of the border in this region (see Figures16.6 and 16.7). Groundwater levels and quality have declined precipitously in themost important aquifer, the Hueco Bolson, since 1940 (Granados-Olivas et al. 2012). Thewater supply for the Upper Rio Grande Basin is fully allocated. Its system of engineeredstorage and delivery requires precipitation “at the right time, right place, over time, andwith adequate quantity” in order to function properly (Lacewell et al. 2010, 105). Changesin the timing and amount of rainfall accompanied by an increase in temperature putsthe system in a vulnerable situation (Lacewell et al. 2010).