The Bureau of Reclamation's May 2010 water supply forecast for the Yakima Basin indicates a shortage for junior water rights irrigators and a full water supply for senior water rights irrigators, based on the Total Water Supply Available (TWSA).
"April precipitation was very good; 132 percent of average for the month. Many of the basin's snow stations have held or improved in April," said Chuck Garner, Yakima Project River Operations Supervisor.
"On the other hand, the Yakima basin reservoir system is lagging behind at only 64 percent of capacity," said Garner, indicating that number is 86 percent of average.
Senior water rights holders, which include farmers using the resources of the Sunnyside Valley Irrigation District, are expected to receive their full water supply this season.
Junior users, which include farmers using the services of the Roza Irrigation District, are expected to receive 78 percent of their entitlement from the beginning of pro-rationing until Sept. 30, 2010. The start of pro-rationing and water entitlement amounts will be determined later in the year. Those with water rights issued after May 1905 must cease diverting water as soon as pro-rationing begins.
"In spite of the good precipitation we will still be in a water short situation this season," Garner added. "Reclamation uses pro-rationing as a means to distribute water allocations. This simply ensures that water distribution is adequately shared among junior water users."
The TWSA forecast is based on flows, precipitation, snowpack and reservoir storage as of May 1, along with estimates of future precipitation. Other future weather conditions that determine the timing of the run-off and the demand for water are also critical in determining, stream flows, pro-rationing and the extent to which the reservoirs fill.
Garner said the water supply can be improved through prudent use of water.