75E - PH - WATER MANAGEMENT PLAN

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2015 URBAN WATER MANAGEMENT PLAN Approximately 40 million people rely on the Colorado River and its tributaries for water with 5.5 million acres of land using Colorado River water for irrigation. Climate change will affect future supply and demand as increasing temperatures may increase evapotranspiration from vegetation along with an increase in water loss due to evaporation in reservoirs, therefore reducing the available amount of supply from the Colorado River and exacerbating imbalances between increasing demands from rapid growth and decreasing supplies. Four water supply scenarios were developed around these uncertainties, each representing possible water supply conditions. These four scenarios are as follow: • Observed Resampled: future hydrologic trends and variability are similar to the past approximately 100 years. • Paleo Resampled: future hydrologic trends and variability are represented by reconstructions of streamflow for a much longer period in the past (approximately 1,250 years) that show expanded variability. • Paleo Conditioned: future hydrologic trends and variability are represented by a blend of the wet -dry states of the longer paleo- reconstructed period. • Downscaled General Circulation Model (GCM) Projected: future climate will continue to warm, with regional precipitation and temperature trends represented through an ensemble of future downscaled GCM projections. The Colorado River Basin Water Supply and Demand Study(Study) assessed the historical water supply in the Colorado River Basin through two historical streamflow data sets, from the year 1906 through 2007 and the paleo- reconstructed record from 762 through 2005. The following are findings from the study: • Increased temperatures in both the Upper and Lower Colorado River Basins since the 1970s has been observed. • Loss of springtime snowpack was observed with consistent results across the lower elevation northern latitudes of the western United States. The large loss of snow at lower elevations strongly suggest the cause is due to shifts in temperature. • The deficit between the two year running average flow and the long -term mean annual flow that started in the year 2000 is more severe than any other deficit in the observed period, at nine years and 28 MAF deficit. • There are deficits of greater severity from the longer paleo record compared to the period from 1906 through 2005. One deficit amounted to 35 MAF through a span of 16 years. • A summary of the trends from the observed period suggest declining stream flows, increases in variability, and seasonal shifts in streamflow that may be related to shifts in temperature. Findings concerning the future projected supply were obtained from the Downscaled GCM Projected scenario as the other methods did not consider the impacts of a changing climate beyond what has occurred historically. These findings include: 75E -37 arcadis.rcrrn 3 -3