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 2. Perspectives from Water Utilities: Current Uses of Climate Models and Desired Output Scales 2.1 What Methods Have Water Utilities Used to Analyze Climate Change Impacts on Water Resources? Several WUCA member utilities have already engaged in studies to determine how projected changes in climate may affect their utility operations and planning needs. A brief review of these efforts is instructive, as it indicates the direction and strategy these utilities have already taken to address the potential impacts of a changing climate on their utility operations and planning. 2.1.1 Denver Water Denver Water has engaged in several climate initiatives: a sensitivity analysis, a paleoclimate extrapolation from tree ring data, and the Joint Front Range Climate Change Vulnerability Study. Sensitivity analysis. Denver Water conducted a simplified sensitivity analysis of how water supplies may be affected by climate change using a hydrology model and Denver Water’s Platte and Colorado Supply Model (PACSM) water allocation model. Denver Water worked with the Colorado River Basin Forecast Center using the Sacramento Soil Moisture model coupled with the Anderson Snow-17 model to calibrate Blue River basin streamflows using historical temperature and precipitation to mimic historic streamflows. They then ran a sensitivity analysis by altering temperature to determine percentage changes in Blue River streamflow, and applying those same delta streamflows to the South Platte, Fraser, and Williams Fork basins. These altered streamflows were then fed into PACSM to determine the effect on Denver’s water supply. The sensitivity analysis included runs for (1) a 5°F (3°C) temperature increase, and (2) a 2°F (1.1°C) temperature increase, with no precipitation change. This analysis found that a 2–5°F (1.1−3°C) increase in temperature could cause a 7–14% decline in water supply yield. The hydrology models adjusted historical streamflows using forecasted temperature changes. Ultimately, Denver Water determined that using the delta streamflows for the Blue River basin as a proxy for Denver Water’s other basins was insufficient, which led to the Front Range study described below (Robert Steger, Manager of Raw Water Supply, Denver Water, personal communication, June 3, 2009). Paleoclimate reconstruction. Model reconstructions of 400 years of streamflows using tree ring data (Woodhouse and Lukas, 2006) were completed within Denver Water’s collection system. This provided Denver Water with a broader perspective on vulnerability due to natural
Water Utility Climate Alliance White Paper Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change variability within the climatological record. The adjusted streamflows were analyzed in PACSM to quantify potential impacts to Denver’s water supply should severe prehistoric droughts be repeated. This gave Denver Water a broader perspective of climate vulnerability based on historical drought conditions that exceed anything in the instrumental record. Joint Front Range Climate Change Vulnerability Study. Denver Water leads the Joint Front Range Climate Change Vulnerability Study in cooperation with the City of Aurora, the City of Boulder, Colorado Springs Utilities, the City of Ft. Collins, and the Northern Colorado Water Conservancy District. Other water agencies, including the Colorado Water Conservation Board, the Water Research Foundation, and the Western Water Assessment – a joint effort between the University of Colorado’s Cooperative Institute for Research in Environmental Sciences and the National Oceanic and Atmospheric Administration’s (NOAA’s) Earth System Research Laboratory also participate in this effort (Laurna Kaatz, Climate Scientist, Denver Water, personal communication, April 1, 2009). This project focuses on assessing potential changes in the timing and volume of runoff from selected climate change scenarios centered about the years 2040 and 2070, using 1950–1999 natural streamflow as a baseline. The project partners are calibrating two hydrologic models – the Water Evaluation and Planning model (WEAP) with the National Center for Atmospheric Research (NCAR) and the Anderson Snow Model and Sacramento Soil Moisture Model with Riverside – as the primary tools to convert statistically downscaled temperature and precipitation data from GCMs into streamflow, which can then be used by the individual water utilities in their own operational system models. This project focuses on assessing potential changes in the timing and volume of runoff by selecting five model runs from the total of 112 statistically downscaled GCM runs available for the region (Maurer et al., 2007). The runs selected were based on idealized hot and dry (90th percentile temperature, 10th percentile precipitation – all percentiles refer to the distribution of the 112 scenarios), hot and wet (70th percentile temperature, 70th percentile precipitation), warm and dry (30th percentile temperature, 30th percentile precipitation), warm and wet (10th percentile temperature, 90th percentile precipitation), and median (50th percentile temperature, 50th percentile precipitation) scenarios. These scenarios were chosen to incorporate a wide array of model run results without being extreme, and to make sure the model run selection process was straightforward and easily repeatable. The project compares baseline data from 1950 to 1999 with two projections averaged over 30 years centered around 2040 and 2070. Average monthly temperature and precipitation projections will be used to calculate changes in hydrology for the complete grid of the study area (Laurna Kaatz, Climate Scientist, Denver Water, personal communication, August 14, 2009). The project partners are calibrating two hydrologic models – the Water Evaluation and Planning model and the Sacramento Soil Moisture model coupled with the Snow-17 model – as the primary tools to convert statistically downscaled temperature and precipitation data from GCMs into streamflow, which can then be used by the individual water utilities in their own operational system models. Page 2-2
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