For many these ideas represent a new perspective. One must understand the dynamics of the groundwater system to assess development impacts. The static information on what was the undisturbed recharge is not nearly as important as the dynamics of capture. This comes as a shock to many who consider undisturbed (“natural”) recharge to be all important in evaluating sustainable groundwater development (Bredehoeft, 2007; Bredehoeft et al., 1982). It is a common misconception that groundwater development is “safe” if the average annual rate of groundwater withdrawal from an aquifer does not exceed the average annual rate of natural recharge (Alley and Leake, 2004). Bredehoeft et al. (1982) call this the “water-budget myth” and clarify that hydraulically maintainable development depends on pumping becoming balanced by capture.
Describing the dynamics of how a particular groundwater system functions is best done today with a well-calibrated numerical groundwater model, as is illustrated later in this book with two case studies. Such a model solves the groundwater flow equation for the specified boundary and initial conditions and provides the global mass balances (water budgets) described by Equations 1 through 4.