Reallocation of water and the hydrological effects of climate change: the upper Rio Grande Basin, Southwestern USA.

Abstract The Southwestern United States is a semi-arid region that has experienced rapid population growth in the past few decades. This growth shows little sign of abating, so much so that there is concern among water planners and managers as to where additional water supplies will be found. The few large existing perennial streams—such as the Colorado River and the Rio Grande—are overallocated, meaning that additional surface water supplies cannot be developed. Ground-water supplies are also heavily used, leading to excessive water-level declines and incipient land subsidence; some areas also have ground-water quality problems (e.g., arsenic). To exacerbate matters, the federal government's Endangered Species Act (ESA) may require that water be left in some streams to preserve aquatic and raparian flora and fauna, thus reducing the amount available for agriculture and municipal and industrial uses. Preliminary research indicates that parts of this region are entering a drought, which may not reach its driest period unitl 2020–2030. The upshot is that reallocation of water is inevitable. One approach to reallocation involves water marketing, which is already in effect in some areas. Water marketing will certainly not increase the total amount of water, but presumably will provide a market-based mechanism to sell or lease water rights. As an example, irrigated agriculture, the largest user of water, is perceived by some to be an inefficient and low-value use of water. Under the water market approach, a farmer might decide to retire some of her acreage, thereby freeing some water which could be sold or leased to a municipality or perhaps dedicated to instream ecological use. Clearly, increased population, ESA considerations and impending drought bode difficult times ahead for water managers and planners who must reallocate precious water supplies among competing uses. Reallocation decisions will be politically sensitive and must be done with as much certainty regarding water supply and availability as possible (note that “availability” pertains to both hydrological and legal availability). Informed decisions with strong scientific bases can mitigate some of the political issues. Sophisticated models that can simulate the effects of global change, drought, and reallocation are clearly needed. In response to that need, we propose a GIS-based (Geographic Information System) integrated physical hydrological—behavioral model of the upper Rio Grande Basin of New Mexico and Colorado. The model is unique in that it is process-based with the physical processes embedded within the GIS. Land-atmosphere interactions, surface water and ground water are represented, as is the legal availability of water. The physical model will determine the hydrological availability of water in the face of reallocation and climate change (drought); the behavioral model can assess stakeholders' responses to reallocation and reduced/increased hydrological flows. A preliminary model of a portion of the basin, the Conejos River Basin in the heavily agricultural San Luis Valley in southern Colorado, will be available by January 2003 for economic simulations. Future plans call for the integration of water quality and instream ecological flow components into the basic physical hydrological-behavioral model. Our approach will find applicability elsewhere, whether they be arid or humid regions; there is nothing inherent in the model that restricts its use to semi-arid climatic zones. The model will provide a stronger scientific foundation for the difficult water reallocation decisions facing humankind in the 21 st century.