Directly Assessing Hydrologic Effects with Drawdown Models

Assessing hydrologic effects of developing ground-water supplies in the western United States requires ground-water models.  Hydrologic effects typically are the timing and magnitude of capture from streams, springs, wetlands, phreatophytic plants, and water-table decline.  Assessments initially were limited to simulating water-table decline with a Theis solution and capture with a Glover-Balmer solution.  Modern numerical models allow for the inclusion of many additional hydrologic features and can be quite complex.  Additional model complexity also has made model results more uncertain, which encourages more controversy in a historically contentious process. 

Direct simulation of drawdown can reduce model complexity and uncertainty because fewer hydrologic features need to be simulated.  Model input, other than the proposed pumpage, is limited to hydraulic conductivity, storage coefficient, and ground-water discharge distributions.  Drawdown models simulate changes so relatively unchanging quantities, such as recharge and existing pumpage distributions, are not simulated and do not need to be defined.  The absence of these features simplifies presentation of model results and avoids the large uncertainty associated with recharge and historic pumping estimates. 

Reductions in discharge to streams, springs, wetlands, and phreatophyte stands where water availability is limited can be accurately simulated with wells and drains in MODFLOW.  Observed discharge rates are injected into the model with wells and removed with drains.  Drain elevations are consistent with the extinction depth below the existing water table and conductances are set to equal discharge rates divided by extinction depths.  Differences between injected and drained water, simulate the reduction in ground-water discharge that pumping captures.  This approach limits the amount of captured ground water to measured discharges.  Uncertainty is reduced by rejecting realizations where simulated transmissivity greatly departs from aquifer test results.  The direct-drawdown approach is applied to an assessment of potential hydrologic effects from water-supply development in Snake Valley, White Pine County, Nevada.