Options for Improving Climate Modeling to Assist Water Utility ...

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Water Utility Climate Alliance White Paper Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change 5. Using Climate Model Output for Water Resources Analysis This chapter briefly addresses how climate model output can and should be used in the analysis of water resources. The topics discussed below are the appropriate use of model data for water analysis and how best to convert climate model output into hydrology. 5.1 What Are Appropriate Impact Assessment Uses for GCMs and RCMs, and How Could They Be Incorporated into Water Utility Planning Frameworks? In general, the projections from climate models are more reliable for large areas and when averaged over many simulated years. In addition, temperature projections are more reliable than precipitation projections. So, the most reliable output from GCMs are long-term estimated changes in average global temperature. As we move to finer spatial or temporal resolution, the model projections become less reliable. This is not to say that projections at less than a global scale are necessarily unreliable. Indeed, in recent years the quality and quantity of GCMs have improved to the extent that the IPCC now feels there is enough confidence to project long-term average changes in sub-continental climate. Christensen et al. (2007) conclude that in North America: … annual mean warming is likely to exceed the global mean warming in most areas. Seasonally, warming is likely to be largest in winter in northern regions and in summer in the southwest…. Maximum summer temperatures are likely to increase more than the average in the southwest. Annual mean precipitation is very likely to increase in … the northeast USA, and likely to decrease in the southwest…. Snow season length and snow depth are very likely to decrease in most of North America….” (p. 850) 1 Projecting long-term average changes in climate at the sub-continental scale (e.g., the Southwest) is as far as the IPCC goes. Such projections are generally not at the scale water managers desire. 1. “Likely” means the IPCC finds there is at least a two-thirds chance the projection is correct. “Very likely” means the IPCC finds there is at least a 90% chance the projection is correct.
Water Utility Climate Alliance White Paper Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change The question being asked here is what are appropriate impact assessments using GCMs and RCMs. This question should be rephrased as “What are relatively appropriate impact assessment methods?” It could be argued that the climate models do not currently yield sufficient reliability to make forecasts of change in climate at the scale of river basins. Yet, there are relative degrees of reliability. The basic principles stated above apply: the larger the area, the longer the time period, and the more reliable the model output. Projections over hundreds of miles are more reliable than projections over tens of miles. Projections of average annual change are more reliable than projections of average seasonal change, which are more reliable than projections of monthly change, which are more reliable than projections of submonthly change, and so on. And as noted above, projections of temperature change are more reliable than projections of change in precipitation. Table 5.1 provides an analysis of a number of decisions on water resource management and the relative appropriateness of using climate model output. The decisions or issues regarding water resource management are split into three categories: water supply, flooding, and water quality. For each category, we give specific examples of outcomes water managers may be interested in, identify variables from climate models that can be used to estimate potential impacts, and give our judgment about the relative reliability of climate model output. Note that the reliability is stated as relative, not absolute. Even a rating of high should not be interpreted as having high confidence in the projections. Table 5.1. Climate model variables and relative reliability for water resource analysis Relative reliability of Water management issue Climate model variables climate model output Water supply Long-term supplies – mean annual basin yield Annual average temperature and precipitation High on temperature Precipitation depends on geographic scale, higher at sub-continental scale RCM precipitation projections are more reliable than GCM projections Long-term demand Warm-season temperature and Same as above precipitation Shift in seasonality of runoff – Monthly temperature Medium-High snowmelt-dominated areas Shift in seasonality of runoff – non-snowmelt-dominated areas Seasonal precipitation Medium-Low Long-term supplies – variability in yield Monthly temperature and precipitation Medium-Low Page 5-2
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