Bruce G. Campbell

How processing digital elevation models can affect simulated water budgets.

For regional models, the shallow water table surface is often used as a source/sink boundary condition, as model grid scale precludes simulation of the water table aquifer. This approach is appropriate when the water table surface is relatively stationary. Since water table surface maps are not readily available, the elevation of the water table used in model cells is estimated via a two-step process. First, a regression equation is developed using existing land and water table elevations from wells in the area. This equation is then used to predict the water table surface for each model cell using land surface elevation available from digital elevation models (DEM). Two methods of processing DEM for estimating the land surface for each cell are commonly used (value nearest the cell centroid or mean value in the cell). This article demonstrates how these two methods of DEM processing can affect the simulated water budget. For the example presented, approximately 20% more total flow through the aquifer system is simulated if the centroid value rather than the mean value is used. This is due to the one-third greater average ground water gradients associated with the centroid value than the mean value. The results will vary depending on the particular model area topography and cell size. The use of the mean DEM value in each model cell will result in a more conservative water budget and is more appropriate because the model cell water table value should be representative of the entire cell area, not the centroid of the model cell.

A unique approach to estimating lateral anisotropy in complex geohydrologic environments

Aquifers in fractured rock or karstic settings are likely to have anisotropic transmissivity distributions. Aquifer tests that are performed in these settings also are frequently affected by leakage from adjacent confining units. Finite-difference models such as MODFLOW are convenient tools for estimating the hydraulic characteristics of the stressed aquifer and adjacent confining units but are poor tools for the estimation of lateral anisotropy. This limitation of finite-difference methods can be overcome by application of the spin method, a technique whereby the positions of the observation wells are rotated about the production well to estimate anisotropy and orientation. Formal parameter estimation is necessary to analyze aquifer tests because of the number of parameters that are estimated. As a test, transmissivity, anisotropy, and orientation were successfully estimated for a simple hypothetical problem with known properties. The technique also was applied to estimate hydraulic properties of the Santee Limestone/Black Mingo (SL/BM) aquifer and a leaky confining unit beneath Charleston, South Carolina. A 9-day aquifer test with an average discharge of 6441/min was analyzed numerically. Drawdowns in the SL/BM aquifer and confining unit were simulated with a 12-layer MODFLOW model that was discretized into 81 rows of 81 columns. Simulated drawdowns at seven observation wells that ranged from 23 to 2700 m from the production well were matched to measured drawdowns. Transmissivity estimated along the minor axis ranged from 10 to 15 m2/day and along the major axis ranged from 80 to 10Om2/day. The major axis of transmissivity was oriented along compass heading 116° (degrees clockwise from north), which agrees with geologic interpretations. Vertical hydraulic conductivity and specific storage estimates for the overlying confining unit were 4 x 10-5 m/day and 2 x 10-4 1/m, respectively.