Water Value and Sustainable Use in the American SW

As elsewhere in the world, anticipated population growth in the next 50 years, climate change and reduced surface water availability, water “productivity”, or water use efficiency (WUE) must continue to improve in the American Southwest. Beyond the intrinsic value to life, water takes on additional value as food and fiber, fisheries and ecosystem benefits that are linked such that emphasis of one over the other benefit often results in losses neglected in the past. For example, development of upstream water storage exchanges downstream fisheries and ecosystems benefits for crop production, while reservoir evaporation losses further reduce possible downstream resource values. Unlike WUE improvements in the municipal sector, possible through metering and technological changes in flow devices and washing appliances, improved WUE in crop production is hampered by unidentified achievable water use targets. In terms of water use, the dominant crops in the Southwest are alfalfa and sudangrass hay and cotton lint production. The water use characteris tics, average planted areas and yields, and water values are examined for these crops in Arizona, California and Idaho to determine possible target WUEs and assess possible onfarm water savings in the region based on actual production information from 1988-2000. Field -based WUEs of 1.7, kg/ha-mm for alfalfa and sudangrass hay and pima cotton, and 2.1 kg/ha-mm for upland cotton lint production appear to be practical target values from which to determine appropriate water use. Based on FAO #56 estimated and yield-based water use for these three crops, possible water savings of up to 50% exist with the greatest water savings potential in desert regions where current water values as hay or lint crops are low relative to other regions. Such high water savings in the desert region are unlikely and targets of 20-30% corroborated by research trials, are more likely. The greatest water values and least possible water savings occur in the southern San Joaquin Valley, CA where the combination of relatively high ET and some rainfall occur. This research is a starting point for assessing water use/savings at the field scale for hay and cotton productions and should be extended to other crops. Additional work may also be required considering water savings at the district scale associated with the water distribution systems.