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The overwhelming scientific consensus is that global warming - the rise in global temperatures caused by the buildup of carbon dioxide and other emissions in the atmosphere that trap the sun's heat like a blanket - poses a significant threat to our health, our economy, and our environment. Read on to learn what global warming means for our rivers and water supply - and what steps we can take to meet the challenges ahead. Climate change is likely to have significant impacts on the availability of fresh water. Already in short supply throughout many parts of the world, water for human consumption, agriculture, and industry will be a major factor in economic growth, ecological sustainability, and global conflict.

Research was undertaken to make initial assessments of potential impacts of climate change on stream flow and water balance in the western United States-a region characterized by the shortage of water. Additionally, research was conducted to address the need for models, which account for the spatial magnitude and extent of hydrologic processes. The models need to handle key parameters such as precipitation, soil moisture, and evaporation, in response to changing climatic conditions. The models must account for vegetation interactions with soil moisture. This is particularly important for simulating regional vegetation response to climate change since vegetation distribution is controlled in large part by the availability of soil moisture.

Research focused on developing and refining detailed watershed scale hydrology models to address stream dynamics and water storage. Regional-scale modeling research was directed toward developing physically and mechanically-based water balance models, which can be spatially distributed at watershed, regional, and continental scales. The research effort contributed to developing methods for spatially distributing climatic data at scales appropriate for the models, and providing these data bases to the climate change research community. This ORD project has been completed; extensions of this research are continuing within the US Geological Survey.

So what does all this data and modeling mean? The increasing demand for water by population and industrial growth is creating chronic water shortages throughout the world (Revenga 2000). Add to this the potential impacts of global climate change on water supplies and chronic shortages could reach crisis levels. Throughout much of the western United States the supply of water for human consumption, agriculture, and industry depends on snow pack and reservoir storage. Most global climate warming scenarios suggest warmer winters with more rainfall and less snowfall for much of the western United States, which would substantially reduce snow accumulation and shift the high flow season for many rivers from the spring to the winter (Lettenmaier et. al. 1992).

A substantial amount of the natural storage of winter precipitation that presently occurs in the snow pack would be lost resulting in increased spills in the winter and lower reservoir levels in the summer and fall (Lettenmaier and Sheer 1991). A significant increase in flood hazard in the western US could result from climate change, primarily due to an increase in rain-on-snow events (Lettenmaier and Gan Water 2 1990). Such events occur when warm, wet storms move over existing snow pack. Rapid melting of the snow pack is the result of a combination of warm air temperature, high wind and high humidity, which cause significant condensation on the snow, and is particularly severe in forest openings and forest clear-cuts (Marks et al. 1998). This research suggests that some mitigation of the adverse effects of global climate change may be achieved by adapting land and water management practices to changes in runoff patterns and maximizing the protective effects of natural vegetation.

Global climate changes are expected to be regional in nature, and affect land cover and land use. Key to understanding such regional effects on water supplies is the response of vegetation. Plant communities play a significant role in regional energy and water balance. While hydrologic models designed to simulate large river systems are good for operating reservoirs systems, they are not adequate for predicting changes to regional water balance and, hence, changes in regional vegetation (Marks et al.1993). Dolph et al. (1991) developed a spatially distributed regional water balance model to evaluate the sensitivity of large river basins to climate change. The model was exercised for the Columbia River Basin. This research demonstrated that the existing Historic Climate Network of climate monitoring stations underestimate precipitation primarily because mountainous areas are underrepresented. With climate warming, the model predicted increased evaporative loss, decreased runoff and soil moisture.

These conditions could have profound effects on vegetation distribution and subsequently regional water resources. The ability to predict changes in regional vegetation is necessary to evaluate the effects of climate change on forest resources, agriculture, and water supplies. Changes in soil moisture and evapo-transpiration resulting from climate will have large impacts on water and vegetation. If changes in the regional water balance are significant, major shifts in vegetation patterns and condition are a likely (Marks et al. 1993). Neilson and Marks (1994) incorporated a distributed water balance model with a vegetation model to produce a biogeographic model, MAPSS (Mapped Atmosphere-Plant-Soil System). This model was used to predict changes in vegetation leaf area index, site water balance and runoff as well as changes in biome boundaries.

When applied to potential climate change scenarios, two areas exhibiting among the greatest sensitivity to drought- induced forest decline were determined to be eastern North America and Eastern Europe to western Russia. How will global warming affect rivers in these and other areas? Global warming is projected to have far-ranging effects on rivers across the United States and worldwide. Although these changes will vary from region to region, scientists expect higher average global temperatures over the next century to cause higher river temperatures, resulting in harm to freshwater fish like salmon and bass and significant changes in aquatic plant and animal habitat.

In addition, rainfall patterns will shift -some areas will get more precipitation, some less. Higher temperatures will cause mountain snow pack to melt earlier in the year, causing significant changes to river flow patterns - with less water available during the warmer and drier summer months. Changing water levels in our rivers poses greater challenges for farming, manufacturing, drinking water supplies and wildlife habitat. The supply of and demand for water will be affected dramatically by these changes, as regions of the country that currently have wet climates are expected to become drier and vice versa.

Some places may experience prolonged periods of drought, while others could see a dramatic increase in rainfall and more frequent flooding. These changes will have significant implications for a wide range of water uses, including agriculture, industry, energy production, recreation, water infrastructure/storage, waste disposal, and of course, healthy watershed functions.

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