ERMUSR MISC 02-20-2007 - Laserfiche WebLink

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W®rld continued from the cover snowmelt or longer drought periods could cause wildfires to become more frequent and extensive. Where that occurs, land cover and watershed runoff characteristics may change quickly and dramatically as wildfires reduce forest cover, thereby alter- ing the runoff response. Less dramatic but equally important changes in runoff could arise from the fact that the amount of water transpired by plants will change with changes in soil moisture availability, and plant responses to elevated carbon dioxide concentrations. In addition, changes in the quantity of water percolating to groundwater storage will result in changes to aquifer levels, in base flows entering surface streams, and in seepage losses from surface water bodies to the groundwater system. There is a high level of confidence in projections of warmer temperatures over most land surfaces. Unlike their projections of precipitation change, climate models are fairly consistent in predictions of regional surface temperature. Because tempera- ture is central in determining the accumulation and melting of snow and ice, these scenarios are especially relevant to regions where snowpack or glacial runoff dominate the hydrology. With rising temperatures, it is very likely that a greater portion of winter precipitation will fall as rain rather than snow, especially in areas where winter temperatures are now only slightly below freezing. An increase in rain events would increase winter runoff but result in smaller snowpack accumulations. Temperature also determines the timing of melt-off, and a warmer climate will likely result in an earlier melt season. Many regions are likely to see an increase in winter or spring flows and reduced Increased global precipitation does not necessarily translate to increases in regional water availability. A change that appears most likely is that global average pre- cipitation will increase as global temperatures rise. Evaporation will increase with warming because a warmer atmosphere can hold more moisture. The net result is basically "what goes up, must come down" in the form of more precipitation. This moisture-holding capacity is governed by the exponential Clausius-Claperyon equation, which states that for a one degree Celsius increase in air temperature, the water-holding capac- ity of the atmosphere is increased by about seven percent. Ori average, current climate models suggest an increase of one to two percent per degree Celsius due to warming forced by carbon dioxide. However, an increase in global average precipitation does not mean that it will get wetter everywhere and in all sea- sons. Climate models tend to agree in projecting precipitation increases over high-latitude land areas, much smaller and less certain increases over equatorial regions, and decreases over some subtropical areas. Climate models differ considerably whether precipitation will increase or decrease over the middle latitudes, including the continental US. Additionally, scientists differ on how precipitation intensity and frequency might change. Some say that average precipitation will tend to be less frequent but more intense. This implies a greater incidence of flooding, as well as droughts, with their consequent adverse effects to water quality, water availability, and structural damage. However, increased global precipitation does not necessarily translate to increases in regional water availability. All climate simulations predict substantial areas where there are large decreases in runoff, despite an increase in precipitation. These different simulations produce quite different regional impacts, however, demonstrating the uncertainty related to climate projections. summer flows. In fact, there is evidence that this is already occurring. Studies document the fact that the peak spring run- off has been arriving earlier in the last few decades. One of the most important impacts of global warming is a rise in sea level. Miami-Dade County in Florida has measured a 12-inch rise in sea level since 1848. Rising sea levels impact coastal water utilities in six key ways: (1) lowland inundation and wetland displacement; (2) altered tidal range in rivers and bays; (3) changes in sedimentation patterns; (4) severe storm- surge flooding; (5) saltwater intrusion into estuaries and fresh- water aquifers; and (6) increased wind and rainfall damage in regions prone to tropical cyclones. Water utility infrastructure, such as water intakes, are particularly vulnerable to these effects of rising sea levels. Given the uncertain nature of climate impact analysis, it maybe tempting to disregard climate change in water resource planning. Rather, the uncertainty introduced by climate change emphasizes the importance of incorporating flexibility or no-regrets options in water resource planning. Next month this column will discuss utility planning and adaptations to climate change. Edited from Climate Change and Water Resources: A Primer for Municipal Water Providers by Xathleen Miller and David Yates, National Center for Atmospheric Research. Available from American Water Works Association, $95 members, $143 retail. No. 91120. _. _ __..~- n .. ~ T - - To order call 1.800.926.7337 or go to www.awwa.org/bookstore