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Development of a Numerical Model for the Walla Walla Basin using IWFM (Groundwater-surface water modeling of the Walla Walla Basin Using IWFM)

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  • The Walla Walla basin lies in an arid region on the border of Eastern Washington and Oregon. A large portion of the area is devoted to agricultural production, relying on irrigation water diverted from the Walla Walla River and underlying aquifers occurring within Quaternary gravel and Mio-pliocene basalt formations. Heavy water demand over summer months has resulted in a fully allocated surface water supply and significant drawdown in groundwater levels. This has led to several proposals for water management aimed at increasing the efficiency of water use and the potential for seasonal storage using shallow aquifer recharge. Specific research questions relate to the interaction between surface and groundwater with regard to agricultural use, aquifer recharge, and factors such as leakage through permeable canal beds. There is currently an ongoing effort to develop a hydrologic model using Integrated Water Flow Model (IWFM) software to simulate surface and subsurface flows over a portion of this watershed. This work is a collaborative effort between a research team from Oregon State University and the Walla Walla Basin Watershed Council (WWBWC). The modeling process includes model setup, data collection and input, parameter estimation, estimation of initial and boundary conditions, model calibration, error analysis, and validation. This application of IWFM uses grid with average spacing of 100 * 100 meters. Data sources include federal and state agencies as well as WWBWC staff. Parameters have been determined with field measurements when possible, and otherwise are estimated using established methods of hydrologic analysis or values drawn from previous studies within the region.The model is being developed using data from 2007 through 2009. Analysis using the Nash-Sutcliffe method yields a value .75 for surface. Simulated groundwater elevations at 88 well locations show a mean discrepancy of 2.9 meters, with a standard deviation of 4.2 meters, when compared to recorded data. Upon validation of this model, it is intended as a tool for informing decisions related to water resource management in this region. Hypothetical scenarios may include the further development of aquifer recharge sites, lining or piping of irrigation canals, and systemic responses to climate change.
  • Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.
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