Changes in Storm Intensity and Frequency

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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 20^th 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.

Observed Increases in Extreme Storm Events Metrological observations have shown an increase in total annual precipitation throughout the United States. Analysis of metrological data suggests this change in precipitation can be associated with an increase in extreme storm events. The Great Lakes, Southwest, and Midwestern United States have experienced the largest increase in extreme storm events over the last century (Mortsch et al. 2003). Water resource managers in this region will be faced with additional challenges pertaining to water treatment.

Changes in Storm Intensity – Belgium and Denmark In Belgium, climate change models forecast an increase in extreme precipitation events throughout the country. However, due to natural variability and Belgium’s small size, climate change models display a wide range of uncertainty in projecting the frequency of future extreme events. Projections range from a trivial increase in the number of extreme events to a much more significant increase.

Similarly, Denmark is forecasted to experience an increase in annual precipitation, but fewer total rain events. Consequently, Denmark is expected to experience an increase in the frequency of extreme storm events throughout the 21st century. (European Environmental Agency 2007)

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