From any close look at the major demands on western water resources emerges a sense of the inevitability of conflict. Evidence of pressure on water is everywhere. Most streams and rivers in the West have been fully appropriated. Salinity and toxic elements threaten traditional agricultural practices and impose high costs on subsequent water users. Groundwater tributary to a watercourse cannot usually be tapped unless equivalent surface flow rights are retired, and nontributary groundwater is being mined at high rates. In fact, use exceeds average streamflow in nearly every western subregion, and the deficits are being offset with groundwater and water imported from adjoining basins. (See Figure 9.1.)

Several major southwestern cities, including Tucson, are now mining groundwater aquifers to meet everyday demand. Reliable yields from these sources are falling, and economic exhaustion of the resource will soon be approached. Other cities, such as Los Angeles, face a reduction in long-held supplies as upstream states claim their compact-apportioned shares of Colorado River water. As cities attempt to increase supplies to meet growing demands, instream flows may be diminished and water use in other offstream sectors may be threatened politically, if not legally. Conflicts over water supplies and water-quality degradation are not limited to a particular state; they thrive in every state in the West and even extend across state borders.

Although the focus in this volume is on water demand in the municipal and agricultural sectors, overall competition for western water has been growing as strong demands emerge in other sectors as well.