Potential effect on water project yield of changed snowmelt runoff patterns

One of the potential changes of global warming would be a shift in runoff patterns in temperate zone mountains with less spring snowmelt and more direct winter rain runoff. The Mokelumne River, located in the central Sierra Nevada range of California, was examined to see how this shift in runoff pattern would affect water supply yield. The shift reduced April through July runoff by an average of 28 percent with a corresponding increase in winter runoff, some of which had to be released for flood control purposes. However, the loss in water project dependable urban supply was only 1 to 3 percent. Part of the reason for the small change was the fact that total reservoir storage capacity exceeds average annual runoff of the basin. Another factor was the relatively uniform monthly demand, which generates a smaller dry season draft than an equivalent annual agricultural demand. The level of demand also has an influence; water systems operating beyond their firm yield limit will be more stressed by changing runoff.