Tuesday, April 13, 2010
The Spokane Valley-Rathdrum Prairie Capture Spreadsheet is a Microsoft Excel spreadsheet utility that allows lay users to identify a location, magnitude, and timing for aquifer withdrawal or recharge (a stress or stresses) and determine the impacts on surface water bodies interconnected with the Spokane Valley-Rathdrum Prairie (SVRP) aquifer of Washington and Idaho. The spreadsheet is based on the SVRP aquifer model which represents our best understanding of surface and ground water interactions in the basin.
The surface and ground water interactions forming the basis of the spreadsheet tool were developed from a double-precision version of the SVRP MODFLOW model consisting of 120 monthly timesteps (10 years) followed by a steady state condition. The simulation represented average hydrologic conditions for the October 1995 through September 2005 period.
The spreadsheet utilizes capture response functions which are determined from the SVRP aquifer model. The use of response functions requires that the governing equations are linear so that effects of multiple stresses are additive and that the magnitude of the effect is proportional to the magnitude of the stress (recharge or discharge). Unconfined conditions and piecewise linear representations of streams and lakes creates the potential for some non-linearity in the SVRP aquifer model. Non-linearity errors are minimized when this spreadsheet is used to represent a) aquifer conditions (e.g. aquifer water levels) that are similar to those of the 1995 to 2005 period, and b) additional recharge and discharge stresses that do not represent major changes to the water budget of a region of the aquifer. Recharge and discharge stresses less than about 10 cfs should be acceptable in most areas, however, further testing is being performed to better identify thresholds of concern.
The surface and ground water interactions forming the basis of the spreadsheet tool were developed from a double-precision version of the SVRP MODFLOW model consisting of 120 monthly timesteps (10 years) followed by a steady state condition. The simulation represented average hydrologic conditions for the October 1995 through September 2005 period.
The spreadsheet utilizes capture response functions which are determined from the SVRP aquifer model. The use of response functions requires that the governing equations are linear so that effects of multiple stresses are additive and that the magnitude of the effect is proportional to the magnitude of the stress (recharge or discharge). Unconfined conditions and piecewise linear representations of streams and lakes creates the potential for some non-linearity in the SVRP aquifer model. Non-linearity errors are minimized when this spreadsheet is used to represent a) aquifer conditions (e.g. aquifer water levels) that are similar to those of the 1995 to 2005 period, and b) additional recharge and discharge stresses that do not represent major changes to the water budget of a region of the aquifer. Recharge and discharge stresses less than about 10 cfs should be acceptable in most areas, however, further testing is being performed to better identify thresholds of concern.