According to the Intergovernmental Panel on Climate Change (IPCC), increasing average global temperatures will result in a number of impacts to the hydrological cycle, including changes in precipitation. Precipitation will be directly impacted by changes in atmospheric circulation and increases in water vapor and evaporation associated with warmer temperatures. This will result in an overall increase in precipitation, though the magnitude of this increase is uncertain.

Precipitation changes are expected to differ from region-to-region, with some areas becoming wetter and others becoming dryer. However, most models agree that precipitation will increase the most over high-latitude regions, while precipitation will decrease in most subtropical areas. Equatorial regions show a high level of uncertainty in forecasting changes in precipitation.

Any change in precipitation amount will result in corresponding regional changes in runoff, thus impacting water supply management regimes. Water resource managers in semi-arid regions will be most vulnerable to changes in precipitation, since runoff and river flows in these areas are particularly sensitive to changes in precipitation. Additionally, changes in average rainfall will impact groundwater recharge rates, thus potentially impacting water supply. 

Water resource managers who experience a decrease in precipitation may have to explore new sources of supply, implement demand management activities, or invest in new treatment techniques. Water resource managers who experience an increase in precipitation may need to make infrastructure investments to mitigate an increased risk of flooding and higher reservoir levels, along with developing new treatment processes. However, these resource managers may also benefit from an increase in water supply.

Changes in rain and snowfall can result in a number of impacts for water resource managers that depend on snowpack for water supply, including increases in flooding, decreases in summer water supply, and changes to both groundwater and surface water quality. These impacts may require water resource managers to develop alternative sources of water supply and water treatment and invest in new flood infrastructure.