An increase in temperatures throughout this region will lead to widespread drought-like conditions. This will result in greater evaporation of surface water resources, thus reducing water supply and water quality throughout the region.  Many water utilities will face additional challenges in meeting peak summer demands, as surface water becomes less available in the hottest months of the year.

Drought conditions will also impact vegetation throughout the regions’ watersheds. When combined with more frequent wildfires, Western and Southwestern watersheds will experience greater erosion and flooding than in the past. This will also lead to a reduction in groundwater recharge rates as more water is lost to surface runoff.

Both total annual precipitation and the number of extreme precipitation events are expected to increase in the West and Southwest. However, the temperature increase in this region is forecasted to be severe enough that additional precipitation will not augment surface water resources. In fact, climate models estimate that a 3.8°C increase in the West and Southwest would require 15–20% more precipitation in the Colorado River Basin to maintain current water supply and quality.

Warmer temperatures will also lead to a reduction in snowpack throughout the West and Southwest. As temperatures rise, precipitation will tend to fall as rain rather than snow, and existing snowpack will begin to melt earlier in the year. This will result in peak stream flow occurring earlier in the year. This has already been observed in the West and Southwest, where annual peak stream flow has recently occurred one to four weeks earlier in the year, when compared to the 1970s.  Earlier melting of snow will also impact groundwater recharge, since the majority of western groundwater is derived from mountain precipitation. Rapid melting of winter snowpack, as a result of rain-on-snow events, will further reduce groundwater supplies.

Decreases in snowpack have already been observed throughout the West and Southwest. In both the Colorado River Basin and Rio Grande River Basin, snowpack was consistently below historical averages during the period from 1990–2006. Warmer temperatures will continue this pattern and reduce the amount of available water in river basins that rely on snowmelt.

Surface water will also become warmer due to higher air temperatures. This will reduce dissolved oxygen levels, concentrate contaminants, and promote algal and pathological outbreaks. Water quality will also be impacted by an increase in intense rainfall events. Extreme precipitation events will increase the turbidity of surface water, along with creating additional flooding throughout the region.

Finally, coastal areas in California and Texas will face the additional challenge of sea-level rise. Tide gauges in California have measured an increase in sea level of 17–20 cm during the 20^th century. Model simulations show an increase in the rate of sea-level rise in California, with sea-level rise expected to range from 30–45 cm relative to 2000 levels, by 2050. Higher sea-levels will provide additional challenges to coastal water utilities, as larger storm surges and saltwater inundation threaten both infrastructure and water supplies.

Overall, the impacts of climate change in the West and Southwest are forecasted to be fairly severe. Warmer temperatures, resulting in drought and a decrease in snowpack, will reduce both available surface water and groundwater. This will make it difficult for water utility managers to maintain system reliability, as future demand may exceed water supply. Water utility managers may need to implement demand management activities during the summer months when water supply is lowest. They may also need to develop new sources of water supply and innovative ways to store and treat existing resources.