Prior to development (Q = 0) one assumes that on average the recharge to a groundwater system is balanced by the discharge, and there is no long-term change in storage (∆V = 0). This is especially true when considering that groundwater systems evolve over geologic time. Admittedly there are wet and dry years, as well as seasonal and even daily variability in precipitation and recharge, but over the long-term it is reasonable to assume that the annual fluctuations balance out. Under this assumption:
|R0 = D0||(2)|
for conditions prior to development (Figure 2). In many investigations considerable effort is spent in trying to estimate the undisturbed (or “natural”) recharge, R0. Usually it is best to estimate both the undisturbed recharge and discharge simultaneously, as they must be equal and constrain one another. As explained in the “A New Perspective” section of this book, the undisturbed recharge is not as important in analyzing groundwater development as many people think.
This is not to say that some aquifer systems cannot continue to undergo slow transient changes in their water balance over tens of thousands of years or longer if they are relatively large and somewhat slow to respond to major changes in climate and/or sea level during “recent” geologic times. If climate change near the end of the Pleistocene has caused a substantial reduction of average recharge rates to the aquifer, the new recharge regime cannot balance or maintain the discharge from the system. One implication of this imbalance is that the system will continue to drain (and water in storage will be reduced over time) as the system tries to reach a new equilibrium condition and discharge is reduced accordingly. If the properties and boundary conditions of the aquifer are such that reaching this new equilibrium would take centuries to millennia, then we can today see groundwater systems that are still responding to the vast changes in climate, glacial extent, and sea level 10,000 to 20,000 years ago. Groundwater occurring in such systems is sometimes referred to as “fossil water” because it is essentially nonrenewable under present climatic conditions. One example is the Nubian aquifer of North Africa, where modern natural predevelopment discharge (and storage depletion) was estimated to be about 86 m3/s (2.7 km3/yr) prior to development (about 1960 and earlier), while natural recharge was close to zero (Voss and Soliman, 2014). But such examples are the exception rather than the rule – most aquifer systems were in balance (between recharge and discharge) prior to human development.