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Ground-Water Budgets Under predevelopment conditions, the ground-water system is in long-term equilibrium. That is, averaged over some period of time, the amount of water entering or recharging the system is approximately equal to the amount of water leaving or discharging from the system. Because the system is in equilibrium, the quantity of water stored in the system is constant or varies about some average condition in response to annual or longer-term climatic variations. This predevelop- ment water budget is shown schematically in Figure SA. We also can write an equation that describes the water budget of the predevelopment system as: Recharge (water entering) = Discharge (water leaving) A Recharge The water leaving often is discharged to streams and rivers and is called base flow. The possible inflows (recharge) and outflows (discharge) of a ground-water system under natural (equilib- rium) conditions are listed in Table 1. Tabte 1. Possible sources of water entering and leaving a ground-water system under natural conditions Inflow (recharge) 1. Areal recharge from precipita- tion that perco- lates through the unsaturated zone to the water table. 2. Recharge from losing streams, lakes. and wetlands. Outflow (discharge) 1. Discharge to streams, lakes, wetlands, saltwater bodies (bays, estuaries, or oceans), and springs. 2. Ground-water evapotranspiration. Ground-water system Discharge B Increase in recharge Pumpage Removal of water stored in the system Decrease in discharge Figure 8. Diagrams illustrating water budgets for a ground-water system for predevelopment and developmellt conditions. (A) Predevelopment water-budget diagram illustrating that inflow equals outflow. (B) Water-budget diagram showing cllanges in flow for a ground-water system being pumped. Tile sources of water for tile pumpage are changes in recharge, discharge, and the amount of water stored. Tile initial predevelopment values do not directly enter the budget calculation. 17