The sensitivity analysis provides for the evaluation of how sensitive the model results are to the variation in any given parameter. This process allows the modeler to determine which parameters are most important in controlling the geometry of the delineation and therefore which should be determined to the greatest degree of accuracy. Porous Media Assumption. As indicated in the accompanying flow chart, an evaluation of the porous media assumption relative to the aquifer must be made. In a granular or porous medium, groundwater will move directly downgradient, assuming no significant preferential flow paths exist.
Most analytical models assume such a condition.
In fractured rock, however, groundwater will flow preferentially along fractures, which frequently are not oriented perpendicular to hydraulic head contours. Variations in Hydraulic Gradient. In many hydrogeologic settings, the gradient may change significantly in direction and magnitude on a seasonal basis, owing to changes in pumping levels and recharge. If the magnitude of that variation is known, delineations are performed under wet and dry conditions and overlaid to produce the final delineation. If the magnitude is not known, the gradient direction is rotated through an angle of 25 or 45 degrees, depending on whether the gradient is based on field measurements or regional data, respectively.