Use of Idaho's Surface Water Supply Index to Predict Irrigation Shortages and Surplus Volumes at the Local Watershed Level

Since the mid-1990s, the Idaho Natural Resources Conservation Service (NRCS) Snow Survey and Water Supply Forecasting Program have successfully used the Surface Water Supply Index (SWSI) to monitor agricultural surface irrigation shortages. SWSI provides a means to compare forecasted water supplies to historical data allowing NRCS to forecast irrigation shortages which help irrigators to plan and plant accordingly. With ever-tightening water supplies there is increasing interest in forecasting surplus volumes that may occur. These forecasts assist water managers and users to be proactive in managing irrigation delivery schedules and mitigating flood impacts.

The monthly SWSI value is calculated by combining historic reservoir storage and summer streamflow volumes. The current years reservoir volume is combined with the five NRCS monthly exceedance streamflow forecast volumes and compared to historic water supplies in each basin. Interpretations of the current year monthly SWSI values provide a greater understanding of the potential for deficit, surplus or adequate water supplies. Knowing and understanding the demand thresholds when agricultural shortages start to occur in your local watershed allows water managers and irrigators to make wise crop planning decisions. These agricultural irrigation thresholds were determined by examining historic SWSI values and then meeting with local irrigation districts to understand which years had shortages.

With the increase in climate variability, users are also interested when a surplus of water will be available. Idaho NRCS is now in the process of meeting with local irrigation districts to determine these surplus thresholds. Knowing when these surplus thresholds are exceeded provides key information to others in the region. A SWSI value above the surplus threshold serves as an early warning for some reservoirs that seldom fill or when excess water may be available for aquifer recharge and other uses. These surplus thresholds may also be incorporated into criteria to suspend cloud seeding operations in a basin because of abundant snow, projected streamflows and limited reservoir storage to store more runoff.

Because each basin is unique, the surplus threshold definition varies from basin to basin and only defines that basin. The key is understanding your watershed and knowing the indicators to determine when excess volumes may occur. In some watersheds, simply knowing the potential exists for reservoirs to fill provides adequate and much needed information for water managers to be proactive in wisely managing excess water in Salmon Falls and Oakley basins. In the Boise basin, determining years when flows through Boise exceeded 6,700 cfs may be the appropriate threshold to use as the COE and USBR strive to keep flows below the 7,000 cfs flood stage level. In eastern Idaho, years that reached the flood stage at the Snake River near Blackfoot or approximately 21,000 cfs, may be a valid measure to indicate there is plenty of snow in the mountains, or to suspend cloud seeding because of the likelihood of excess water that cannot be stored by the upper Snake reservoir system.

To determine the surplus thresholds, all pieces of the water supply picture such as snowpack, streamflow forecasts, reservoir storage or flood stage are integrated. Inclusion of reservoir storage carryover is the unique parameter in this water surplus index. By including current reservoir storage, SWSI takes the reservoir's ability to store more water into account. Thus, in a year like 2014 with low carryover storage, SWSI accounts for the filling of that reservoir, and downstream users are informed that a surplus streamflow may not occur. In contrast, when carryover storage is high from an extremely wet year like 2011, SWSI accounts for the reservoir's ability to store less snowmelt runoff, which resulted in higher downstream surface water supplies. All five NRCS exceedance streamflow forecasts are used to forecast the chance to exceed the surplus thresholds and provide water managers more time to be proactive in managing excess water to mitigate flood impacts.

Each basin is unique and different from another. By meeting with local water users to determine the SWSI thresholds for surplus and shortages, SWSI incorporates unique local situations in each basin. Incorporating surplus thresholds into Idaho's current SWSI should provide as much information about the potential for high runoff volumes, as the shortage thresholds did to assist users in drought mitigation. With the increase in climate variability and potential for weather patterns to change in mid-winter as they did in 2014, it is critical to understand when shortages and surplus volumes may occur in your local watershed to wisely manage water as a natural resource. SWSI provides a forecast tool to inform local managers and water users about reservoir operations, water distribution and potential drought and flood impacts.