Potholes Reservoir

9722-9EK - Simulation of Unregulated Runoff and Irrigation Return Flows into Potholes Reservoir, Washington - Completed FY2008

Problem - The U.S. Bureau of Reclamation (USBR) manages the diversion of Columbia River water into Potholes Reservoir and the canal system for delivery of the water to irrigators. Through the USGS/USBR collaborative Watershed and River Systems Management Program (WARSMP), the USBR will be developing a river-management model to more efficiently distribute water to lakes and reservoirs for storage and to irrigators for growing crops. An important input to the river-management model will be forecasted snowmelt runoff and irrigation return flows into Potholes Reservoir for an upcoming runoff season from Crab Creek, Rocky Coulee, and Lind Coulee. Currently, there is no tool to make these runoff-season forecasts.

Objectives - The general objective of this study is to provide the USBR with a tool to help in the efficient operation of Potholes Reservoir. This tool is a watershed model of the study area that can be used to forecast daily values of unregulated streamflow discharge and irrigation return flows at specific points. The forecasts can be used as input data by the river-management model to be constructed and run by USBR.

Relevance and Benefits - The study is part of an interagency effort that fits the objectives of the federally funded program, WARSMP. Those objectives include the development and demonstration of new Decision Support Systems for water management on USBR-managed river systems. Developing Decision Support Systems is also an objective of the USGS Strategic Plan, and understanding watershed systems is one of five issues addressed in the USGS Washington Water Science Center Science Plan.

Specific benefits provided by the work are opportunities to:

1. Validate new MMS computer modules that were recently developed by WARSMP. The modules simulate diversions, irrigation applications, return flows, and ground-water flow between ground-water reservoirs.
2. Understand frozen soil hydrology through the development and testing of a new MMS module based on previous USGS work in North Dakota.
3. Provide tools to the USBR to aid in the water management of Columbia River Basin surface water. Also, the Crab Creek Watershed Planning Unit, a group of stakeholders in the basin sponsored by the Washington State Department of Ecology, expressed interest in the watershed model as a possible tool for its watershed planning.
4. Demonstrate the application of a data-centered decision support system to optimize water use in highly developed agricultural systems in arid landscapes.

Approach - A watershed model will be constructed for the study area using the Modular Modeling System (Leavesley and others, 1996) to simulate unregulated runoff and return flow into Potholes Reservoir. Input to the model consists of historical and newly collected meteorological data. New model modules will be added to simulate frozen soils; irrigation diversions, applications, and returns; and ground-water flow. The existing streamflow record will be split into two time periods: one period will be used to calibrate the model and the other to test the model.

The completed model will be turned over to the USBR, who will use it to forecast unregulated runoff and link the forecast to their river management model. The completed model will include a hydrological database and a graphical user interface for user-friendly model operation.