United States Department of Agriculture Natural Resources Conservation Service
Idaho Water Supply Outlook Report April 1, 2013
This month we examine how average reservoir storage changed as result of the 30-year comparison periods shifting from 1971-2000 to 1981-2010. Previous reports this year demonstrated the result of dropping the wet 1970s and adding the dry 2000s made the new “normal� snow and streamflow amounts decline. Unlike the snowpack and natural streamflow, people manage reservoirs to prevent flooding and store as much water as possible for various uses. Due to management, reservoir storage averages were not affected the same as these other variables. Management adjusts each year to maximize storage, however managers are limited by the storage capacity of each reservoir. Reservoirs that store more than one season’s water allocation saw the biggest change in averages because storage levels continued to decline for multiple years during the dry 2000s. Examples include Bear Lake, Lake Owyhee, Salmon Falls and Oakley reservoirs, where monthly averages decreased about 20% from the 1971-2000 averages to the 19812010 averages. For Bear Lake, above, the new monthly averages are about 200,000 acrefeet less than before. For other reservoirs where capacity more closely matches annual water use, modest changes of less than 5% were common. As a result, the percent of average amounts for reservoirs tend to be more inflated for reservoirs with a multi-year capacity and less inflated for the others. Knowing where your water comes from will help you decide how to interpret the new percentages in a meaningful way.