I recently read a publication from the USGS titled: Water Availability for the Western United States. The following represents my modified and summarized version.
Markets are defined by supply and demand. The supply (water) is scarce and the demand (people) is increasing in the Western Water Market.
1. The West is the fastest growing region of the country. The map below shows the population growth rates (%) of Western States and their respective rank in the U.S. between 1990-2000 (Perry and Mackum, 2001).

2. These trends in population growth are projected to continue, as depicted by the estimated population growth for the Southwestern United States (AZ, CA, CO, NV, NM) (Campell, 1996).

3. This rapidly growing region happens to represent the driest portion of the county (NOAA).
4. The implications of this growing population and limited supply is people use and need water (Hutson et al., 2004)
5. This growing scarcity and increasing demand is driving the western market.