Computer Determination of Aquifer Parameters using Slope- Matching

Various numerical methods have been developed to solve aquifer tests and eliminate the subjectivity of traditional graphical type curve methods. The slope-matching method, which matches the slope of the field data to the slope of the Theis type curve, can be used to numerically solve pump tests for both leaky and nonleaky aquifers. The slope-matching method was selected for implementation into a C/C++ computer program. The computerized version has the following advantages over the original hand calculator version: (1) it eliminates human error inherent in hand calculations; (2) it has the ability to quickly analyze large data sets and large numbers of data sets; and (3) it can be used on portable computers to analyze test data in the field. When performed by hand, the slope-matching method requires the interpolation of values from tables for values of Theis curve slopes and the Bessel function Ko. For the computer implementation, an iterative 'guess-and-correct' algorithm was developed to eliminate the need to interpolate from tables. The computer program shows a good correlation with results obtained by hand calculations for both leaky and nonleaky aquifers, using the same method. Discrepancies with hand calculations are traced to rounding errors in the estimation of W (u) and elimination of the need to interpolate values from tables.