State Water Resources Research Institute Program

Project Id: 2010ID161B
Title: Improving estimates of tributary underflow in the Eastern Snake River Plain
Project Type: Research
Start Date: 3/01/2010
End Date: 2/28/2011
Congressional District: ID-001
Focus Categories: Methods, Hydrology, Water Quantity
Keywords: Eastern Snake River Plain, Basin Yield, tributary underflow, water budget
Principal Investigators: Taylor, Stacey; Contor, Bryce
Federal Funds: $ 13,482
Non-Federal Matching Funds: $ 28,664
Abstract: Water yield of a basin is the manageable part of the water supply that is potentially available for consumptive use. Underflow from tributary basins is an important portion of the water yield for basins connected to the Eastern Snake River Plain Aquifer. Tributary underflow is a quantity that cannot currently be measured, thus several techniques have been developed for use in estimating it. Until recently, there has been an inadequate amount of climate data, such as precipitation and evapotranspiration, to estimate underflow.

Tributary underflow is an important component of recharge in the water budget for the Eastern Snake Plain Aquifer Model, version 2 (ESPAM 2). There are 22 basins considered in the model in which the estimates of underflow are based on the 1992 Regional Aquifer-System Analysis (RASA) study performed by the USGS. Basin-yield equations were used to calculate average annual underflow rates from the tributary basins. The characteristics of the basins incorporated include drainage area, mean annual precipitation, and percentage of forest cover. As part of the water budget balancing process, all tributary underflow estimates were scaled by a factor of 0.97 (a net 3% reduction) in ESPAM 1.1. Tributary underflow varies seasonally and from year to year, so the average annual underflow values were scaled using normalized values based on measured discharges at Silver Creek. Silver Creek was chosen because it is almost entirely spring-fed and reflects spring discharge from a basin similar to many of the Snake Plain tributary basins. Although this was chosen as the best method of estimating tributary underflow, ESPAM 1.1 has a degree of limitation and uncertainty. One of three components of the aquifer budget for ESPAM 1.1 mentioned in the final report that has the greatest uncertainty is tributary underflow. Unfortunately, time was not allotted to improve this aspect of the water budget for ESPAM 2.

Methods of estimating tributary underflow have been utilized for the Raft River Basin of Idaho and Utah and the Little Lost River Basin of Idaho. One such method used in these basins was developed by W.B. Langbein in which it is assumed that there is a relationship between precipitation, potential evapotranspiration, and water yield. This relationship was developed in basins in which the entire water yield can be measured as surface runoff. This method has provided reasonable estimates of underflow for the basins previously mentioned and this method may provide the newest version of the model, ESPAM 3 with improved estimates of underflow.

The Langbein method will be used to calculate estimates of water yield for tributary inflow basins for the water budget based on two ratios: (1) precipitation to potential evapotranspiration and (2) water yield to potential evapotranspiration. The estimates of precipitation will be based on PRISM data for the 22 tributary basins in the Eastern Snake Plain. Given the water yield estimates per basin, tributary underflow will be calculated by taking the difference between the water yield and the combined value of streamflow, diversions, and returns. While it is believed that this method of estimating tributary underflow will be an improvement over the previous estimates, a comparison between the ESPAM 1.1 and 2 estimates of underflow and the newly calculated values will be made. This assessment will provide insight as to how sensitive underflow values are relative to the estimation technique.

Progress/Completion Report, 2010, PDF

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