April 10, 2001
David Murillo
509-575-5848 Ext. 213
Yakima Basin April Water Supply Forecast
Forecast of water supply based on snowpack and precipitation data available on the first of April indicates a significant water supply shortage for proratable users for the coming irrigation season. Non-proratable users will receive a full supply.
Converting the April 1st runoff forecast (natural flow) to total water supply available (TWSA) gives the following volumes for the April through September use period:
With 50% normal subsequent conditions 1..43 million acre-ft With 100% normal subsequent conditions 1.68 million acre-ft With 150% normal subsequent conditions 1.93 million acre-ft.
Total demand to be placed against this supply for irrigation, regulation and flows passing Sunnyside Dam averages 2.5 million acre-feet in a normal year.
The total water supply available for irrigation is the sum of natural flow, storage and return flow less residual storage and flow passing Sunnyside dam.
Though specific proration levels will not be determined until we near the time the system goes on storage control, the following percentages of proratable entitlements would occur if prorationing were to begin on April, 1 (as based on the forecast) for the respective subsequent precipitation levels:
50% subsequent conditions 8% pro-ratable supply 100% subsequent conditions 28% proratable supply 150% subsequent conditions 48% proratable supply
Non-proratable users will be fully supplied with their entitlement demands for all subsequent conditions listed above. Only proratable users will receive a proratable supply.
Prorationing is currently not in effect, since the system is not on storage control. However given this range of potential prorationing, irrigation districts and water users are urged to apply their own judgement in deciding which level of supply may be realized. Water demands requiring storage releases, even prior to storage control, may be charged to the users total entitlement.
Subsequent precipitation is only one factor affecting the amount of water supply finally available. Diversion rates and the timing of the run-off are critical in determining the storage control date and the resulting pro-rationing levels. If diversions during the run-off period can be restricted to nearer the amount of unregulated flows below the reservoirs and the run-off can sustain through May and or June, storage control could occur later. This can tend to improve the proration level that is ultimately adopted. If warm, drier weather prompts an early run-off, the situation could worsen.
On or near the beginning of storage control, the Bureau of Reclamation will announce the formal levels of pro-rationing.
Storage Conditions as of March 1 were as follows:
Reservoir Content
(acre-feet)Total Capacity Percent of Capacity Keechelus Kachess Cle Elum Bumping Rimrock Totals
1Keechelus capacity is currently restricted to 140,397 acre-feet. Normal capacity is 157,800 acre-feet.
2Reservoir is currently 396,979 as of April 6, 2001