Illustration of drawdown and recovery terms and processes. The figure is not to scale, and is intended to represent the basic concepts, not the relative or absolute magnitudes. Drawdown near the well can be a number of tens of feet, and the zone of influence can greater than a mile. In areas where the water table is declining, the aquifer may become progressively less useful for various types of withdrawals as the water level decreases to and past the elevation at which high-rate drawdowns would be in excess of the available saturated thickness. This would be the amount of water in storage that would trigger mandatory use reductions if a transition plan is not already in effect.
Resource use zones and progressive aquifer exhaustion.
As pumping continues, the rate of local drawdown decreases and eventually stabilizes as the withdrawal is compensated for by inflow of groundwater from the surrounding area. As this happens, the measurable decline in the water table spreads outward. The pattern of water table distortion is called the cone of depression , and the area over which the depression can be detected is called the zone of influence of the well. This zone of influence can easily have dimensions of a mile or more, depending on the characteristics of the aquifer. Figure 3 illustrates the estimated distribution of zones of influence of non-irrigation wells in GMD3.