The climate of the RMGB region is arid (dry) to semi-arid. Water is a limited resource and the water limitation is a major reason why the region's population did not originally increase in the same manner as in the eastern two-thirds of the Nation. Water is essential to all aspects of life in the RMGB. While agriculture is the biggest user, consuming about 80% of the total available water, urban, industrial, recreational, and historic Native American rights are intensifying competition for this limited and already totally appropriated resource (i.e., all available water is already allocated to some use).

The snowpacks of the region's mountain ranges are the sources of many of our Nation's rivers, including the Missouri, Yellowstone, Platte, Arkansas, Rio Grande, Colorado, and Snake. Rocky Mountain waters flow into the Mississippi and Columbia River systems, and subsequently into the Pacific Ocean, the Gulf of Mexico, and the Gulf of California. Thus the RMGB contributes to the water needs of municipalities outside the region, including Los Angeles, San Diego, Phoenix, and Albuquerque. Most of the Great Basin is an interior drainage basin. Its streams, never reaching the oceans, largely drain internally into the Great Salt Lake and numerous, seasonally drying lake beds called "playas."

About 85% of the water used by the population of the RMGB is derived from surface water, namely streams. Approximately three-fourths of the region's stream-flow comes from melt and run-off of the yearly snowpacks on its mountain ranges. Thus, the effects of climate change on the availability of water resources needed to supply the region's human requirements depend importantly on future trends in total precipitation, its seasonal timing (winter or summer), and its state (rain or snow), which is controlled by atmospheric temperature

As discussed above, the GCMs are projecting major increases in precipitation in the region by the end of the 21^st century. The largest increase is projected to occur in winter. At first glance, this would seem to imply increased snowpacks and greater water availability. The increased precipitation of the 20^th century does appear to be reflected in increased flow of streams in the northern portions of the region and rising groundwater tables in eastern Nevada as shown by one study.

However, what is likely to happen to snowpacks and seasonal stream-flow patterns is not entirely clear. Warmer temperatures could cause rainfall to change to snow later in the fall, and make spring snowmelt earlier. Along with snowpack retreating to increasingly higher elevations, the overall result could be reduced snowpacks, increased winter stream-flow, lower and earlier spring run-off, and longer summer and fall low flows.

One still undetermined factor in the water-resources equation is evapotranspiration (loss of water by the soil and by plants). This would likely increase with rising temperatures and offset to some degree the gain from increased precipitation.

If it turns out that, contrary to GCM projections, precipitation actually decreases in the RMGB, there will be increasing competition for already limited water supplies and all water-using sectors will come under increasing stress; some will surely be seriously affected.

The relative scarcity and the variability in time and space of water resources in the Intermountain West were smoothed out in the early 20^th century by an extensive engineering infrastructure that impounded and distributed water to what was then a small population. Nearly all, major western streams have been dammed, many of them numerous times. A legal and administrative infrastructure established the legal and regulatory ways those water resources were allocated to the growing population. Because the western American culture was largely rural in the early decades of the century, traditions and policies were established to allocate most (about 80%) of the water to agriculture, even though crop production is small and marginal compared with the Midwest and other highly productive regions in the US.

Since World War II, the populations of the western states have been among the fastest growing in the country. Most of that growth has been in the cities. Many of these states are now among the most urban in the country as measured by the percentage of their populations in cities.

This surging, urban population growth is now applying pressures to a water-allocation system that was never designed to accommodate it. Urban users are prepared to pay more (and already are doing so) for water than agricultural users. Cities buy up farmers' water rights, often from willing sellers, but often to the consternation of neighbors unwilling to sell and fearful that their water rights and way of life could some day be preempted. To many water managers, the 80% agricultural water use is looked upon as a sponge that can absorb growing urban-industrial needs as well as heightened environmental standards, recreational use, and historically allocated, but often not accommodated Native American water rights.

If the projected 50-100% increase in precipitation becomes a reality, it will relieve at least some of these pressures. The RMGB population is projected to double by the year 2050. Urban-industrial use is presently accounting for about 20% of the total water use. A 50-100% increase in the total water resource would accommodate that growth, the unfulfilled Indian water rights, while still allowing improved environmental standards. If rising temperatures increase evapotranspiration, that could reduce the water resource to some degree. However, if the increase approaches 100% (doubles), there should be plenty of water to go around by the end of the 21^st century.

If seasonal stream-flow timing is significantly changed, it could add confusion and difficulty to the growing pressures for water. Existing engineering and policy infrastructures are designed to distribute historical flow patterns to irrigation and hydropower uses. Significant change in seasonal stream-flow, and markedly increased flow from greater precipitation levels, would require new management plans and conceivably new engineering superstructures.

Existing reservoir and delivery systems are designed to perform during a wide range of current levels of stream-flow. If climate change alters the timing and amount of such flow, existing infrastructure might not be able to handle the new flow patterns and could require costly reengineering. Also, any changes in the timing and variability of water supply could increase conflicts over water use and have the potential to incite expensive lawsuits over water distribution, costs that would likely affect consumers. Because the region's water resources are greatly subsidized, consumers also could be economically affected if the true costs of these resources were passed-on to them. Assuming stable demand and increased availability, government expenditures might have to be increased, e.g., there might be a requirement for additional reservoir capacity.

An increase in extreme weather events, which could accompany climate change, would create its own economic challenges. For instance, an increase in the frequency of intense storms, and resulting flooding, could wreak major damage on this region's water-delivery systems, homes and other buildings, and the agriculture industry. In contrast, increased frequency of drought during vulnerable times of the growing season would seriously compromise the region's crops. Should this occur, a likely result would be the reduction of irrigated acreage, an economic concern for the region's agriculture industry which is relatively more prone to drought stress.

Finally, the economic affect of potential climate change on the region's many water-dependent recreational opportunities could be felt. Decreased snowpack and changes in snowmelt timing could affect the lucrative ski and water-recreation industries along with their associated support businesses such as travel, lodging, and restaurant businesses. Alternatively, potential increases in camping, hiking, and fishing would draw different outdoor recreation enthusiasts to the area and those activities might be available for a longer period of time during the year.

Several strategies are available if climate change causes significant impacts to water resources of the RMGB. These strategies include efforts to:

• Better adapt water-management options to address the needs of multiple users and to develop strategies to encourage optimal use of existing water supplies of differing qualities (for example, delivery of non-potable supplies, such as reclaimed water, for irrigation).
• Apply available technologies that are designed to increase water-use efficiency to all uses, including areas that remain in irrigated agriculture. And/or shift to regionally sustainable agricultural practices and reduce irrigated acreage.
• Adopt agricultural and land-management practices that better retain soil-moisture.
• Identify innovative methods of increasing water storage and storage techniques such as groundwater storage reservoirs.
• Improve flexibility in future water management to account for potential changes in timing and variability.
• Price water to more accurately reflect real costs.
• Explore modernization of western water-law. The "use it or lose it" tradition can encourage water wastage. The tradition of agriculture being "the highest and best use" needs to be re-evaluated in the light of changing western culture -- including rural-to-urban population shifts and the variety of costs attached to water.