According to the Intergovernmental Panel on Climate Change (IPCC), precipitation intensity and variability have increased over most land areas. This is a result of warmer temperatures intensifying and accelerating the hydrological cycle, leading to an increase in evaporation that causes an increase in total annual precipitation. However, evidence from climate change models suggests that precipitation events will occur less frequently, resulting in an increase in the number of extreme storm events. In fact, meteorological records have shown an increase in the number of days with precipitation greater than 2 inches during the 20th century (Karl et al. 1996).
An increase in extreme events increases the risk of flooding, drought, erosion, turbidity, debris in reservoirs, nutrient and pollutant loading, and wildfires. It also impacts surface runoff and groundwater recharge rates. The severity of these impacts will range geographically. For example, increased variability in semi-arid regions will increase aquifer recharge rates, while greater variability in humid regions will decrease aquifer recharge rates as more water is lost to runoff.
Climate models also project a likely increase in the intensity, wind speed, and precipitation associated with tropical cyclones and hurricanes, due to warmer ocean temperatures. However, it remains unclear how warmer ocean temperatures will affect the frequency of these extreme weather events. An increase in the intensity of these storms will affect coastal utilities by exposing them to greater storm surges and flooding.
Changes in precipitation frequency and storm intensity will pose a number of problems for water resource managers, including an increased risk of both coastal and inland flooding that threatens water infrastructure, a more volatile water supply requiring development of new water sources or innovation of storage techniques, and changes to both groundwater and surface water quality that necessitate new or upgraded treatment processes. Water resource managers may also have to make greater user of biological monitoring and assessment techniques. Importantly, increased storm variability may render the current hydrological “baseline” obsolete when making future water resource management decisions.