Event Title

Updated MODFLOW Ground Water Model of Cache Valley, Utah and Idaho

Presenter Information

Thomas E. Lachmar
Barry Myers

Location

Space Dynamics Laboratory

Event Website

http://water.usu.edu/

Start Date

3-25-2004 4:00 PM

End Date

3-25-2004 4:20 PM

Description

An updated ground water model of Cache Valley has recently been created using MODFLOW. Steady-state calibration of the model indicated that subsurface recharge to the lower of two confined aquifers may occur along the bases of the Wellsville Mountains and Bear River Range, and that discharge from the unconfined aquifer overlying the two confined aquifers may occur along the eastern and western margins of the valley in both Utah and Idaho. Two predictive simulations were run with increased pumping from the lower confined aquifer, the first using the average annual precipitation and the second using the lowest annual precipitation from 1984 to 1997. The increased pumping was largely offset by an increase in subsurface recharge along the eastern margin of the valley for both simulations. Discharge to springs, streams, evapotranspiration and from the unconfined aquifer all remained unchanged during the first simulation, suggesting that the two confining layers separating the unconfined and lower confined aquifers may be an effective barrier to flow, but all discharges decreased during the second simulation, suggesting that droughts may have a greater impact than additional pumping. Recharge to the lower confined aquifer along the western margin of the valley did not increase for either simulation, indicating that true steady-state conditions were not achieved. A sensitivity analysis was performed during steady-state calibration of the model. Altering the vertical hydraulic conductivity of the two confining layers produced the greatest head changes. The hydraulic conductivities of the aquifers produced minimal head changes. The other hydraulic properties (specific storage, porosity, effective porosity and specific yield) produced no significant head changes. Water levels in the unconfined aquifer were more sensitive to the head of spring, stream and evapotranspiration cells than the conductance value or evapotranspiration rate.

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Mar 25th, 4:00 PM Mar 25th, 4:20 PM

Updated MODFLOW Ground Water Model of Cache Valley, Utah and Idaho

Space Dynamics Laboratory

An updated ground water model of Cache Valley has recently been created using MODFLOW. Steady-state calibration of the model indicated that subsurface recharge to the lower of two confined aquifers may occur along the bases of the Wellsville Mountains and Bear River Range, and that discharge from the unconfined aquifer overlying the two confined aquifers may occur along the eastern and western margins of the valley in both Utah and Idaho. Two predictive simulations were run with increased pumping from the lower confined aquifer, the first using the average annual precipitation and the second using the lowest annual precipitation from 1984 to 1997. The increased pumping was largely offset by an increase in subsurface recharge along the eastern margin of the valley for both simulations. Discharge to springs, streams, evapotranspiration and from the unconfined aquifer all remained unchanged during the first simulation, suggesting that the two confining layers separating the unconfined and lower confined aquifers may be an effective barrier to flow, but all discharges decreased during the second simulation, suggesting that droughts may have a greater impact than additional pumping. Recharge to the lower confined aquifer along the western margin of the valley did not increase for either simulation, indicating that true steady-state conditions were not achieved. A sensitivity analysis was performed during steady-state calibration of the model. Altering the vertical hydraulic conductivity of the two confining layers produced the greatest head changes. The hydraulic conductivities of the aquifers produced minimal head changes. The other hydraulic properties (specific storage, porosity, effective porosity and specific yield) produced no significant head changes. Water levels in the unconfined aquifer were more sensitive to the head of spring, stream and evapotranspiration cells than the conductance value or evapotranspiration rate.

https://digitalcommons.usu.edu/runoff/2004/AllAbstracts/39