ABSTRACT
The well function defined in Equation 19.9 is a function of eight dimensionless parameters. If the aquifer thickness and screen locations of all wells are known, four parameters are unknown: t5, t_y, ~. and cr. The number of dimensionless parameters can be reduced from three to two if the aquifer specific yield can be assumed to be much larger than the storativity (Sy >> S5m) so that cr = 0. For this case the method of analysis proceeds by dividing the drawdown data into two parts for analysis: early data and late data. For the early data, the effects of gravity drainage above the water table on groundwater flow to the pumping well are negligible; for the late data, the effects of elastic storage within the aquifer are negligible . The result is two asymptotic families of type curves, one for early times (the Type A curves) which is primarily influenced by elastic storage, S5, and one for late times (the Type B curves) where storage from gravity drainage, Sy, becomes important. Equation 19.8 can be written as
(19.19)
The following match-point method was developed based on this concept:
Prepare plots of W(t5, ~) vs. t5 and W(ty, ~) vs. ty on logarithmic paper. These plots are called type curves. The curve W(t5, ~) vs. t5 (the Type A curve) will be used to interpret early drawdown data and the curve W(ty, ~) vs. ty (the Type B curve) will be used to interpret late drawdown data. A separate set of Type A and Type B curves must be prepared for each well with different combinations of do and 10 • The type curves can be plotted using values computed by the computer program TYPE14 (Appendix B).
