Using MODFLOW to Predict Impacts of Groundwater Pumpage to Instream Flow PDF Download

Author: Zoe Oriel Futornick
Publisher:
ISBN:
Category : Groundwater
Languages : en
Pages : 127

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Book Description
From the Abstract: Surface waters in the Yakima River Basin in central Washington are considered over allocated. Since 1960, new water demands have been met through groundwater withdrawals, with most groundwater users holding a later priority date than senior and junior surface water users. As a result of the discussions surrounding this issue, the Upper Kittitas Groundwater Rule has been in effect since 2010. Pumping from new domestic (i.e., permit-exempt or “exempt”) groundwater wells in Upper Kittitas County is not allowed unless mitigation is used to offset the groundwater use. The United States Geological Survey (USGS) has already created a basin-wide model for the Yakima River Basin for the period October 1959 through September 2001; however, the hydrogeology of Upper Kittitas County is coarsely represented in the USGS model because individual bedrock units are not delineated. Based on the USGS Yakima River Basin groundwater flow model (hereafter the YRB-GFM), an Upper Kittitas County groundwater flow model (hereafter the UKC-GFM) was extrapolated to refine the Upper Kittitas County modeled region. This new model constitutes an M.S. thesis, done in collaboration with the USGS. The UKC-GFM contains 246 columns and 195 rows, with 1,000 foot grid cells, and five layers representing three basin fill units, basalt, and bedrock; it is populated with model information for the period October 1991 through September 2001. Refinements to the UKC-GFM include: (1) using a newer version of MODFLOW (MODFLOW-NWT) with the new Newton Solver and the Upstream Weighting (UPW) package. The YRB-GFM used MODFLOW-2005, the PCG2 Solver, and the Hydrogeologic-Unit Flow (HUF) Package; (2) incorporating zone arrays with multiple hydraulic properties into model bedrock layers; (3) extending streamflow-routing cells into smaller headland creeks; (4) changing simulated monthly reservoir stages from steady state to time variant; and (5) estimating new parameter values. The UKC-GFM was calibrated using trial-and-error methods and automated parameter estimation with the software PEST. Groundwater model calibration involves comparing measured water levels and streamflow observations with simulated water levels and streamflow values. At 116 well observation points, the calibrated model produces a root-mean-square (RMS) error divided by the total difference in water levels of 1.5 percent, an acceptable error. Annual differences for measured and simulated streamflow ranged from 7 to 11 percent (percent difference) along the Yakima River, and ranged from 19 to 49 percent along tributaries. Once calibrated, the UKC-GFM was run as three scenarios to assess responses of the flow system to potential changes in stresses. These scenarios are: (1) Existing Conditions without All Pumping, (2) Decrease Recharge by Fifteen Percent, and (3) Increase Pumpage by Fifteen Percent. The scenario with the greatest impacts to stream leakage is Scenario 2, where the annual difference in streamflow for the most downstream gage in 2001, the end of the model simulation period, is approximately 80 ft3/sec. This is a 4.7 percent decrease in streamflow, versus Scenario 1 (all pumping removed), which produces a 0.17 percent increase in streamflow. A comparison of the applied scenarios suggests that potential climate changes that decrease recharge have more impacts on streamflow than groundwater pumping.