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tesian water will require that we distinguish between two types of recharge areas. In one, the aquifer is exposed at the land surface or is covered only with porous sand through which water may infiltrate to the aquifer quite readily. (See figs. 12 and 13.) In such / an area a large percentage of the rainfall is available to the aquifer. Where the aquifer is not full, it accepts the water offered to it, leaving little or none to run off in streams. (See fig. 14.) Where the aquifer is full to overflowing, however, it rejects a part of the rainfall. Of that rejected, some runs off in streams and some returns to the atmosphere through evaporation and transpiration. Where the aquifer is rejecting water, recharge may be increased by the simple expedient of drilling wells and pumping water for use, thereby unwatering the aquifer and providing space to be occupied by additional recharge. When this is done, some of the streamflow and some of the water that otherwise would be evaporated or transpired is captured by the wells. Certain species of vegetation, when so robbed of their perennial supply of water, become sparse.
In the second type of recharge area the aquifer is overlain by a blanket of relatively impervious material that tends to confine the aquifer and preclude recharge. Here it is only where the blanket is breached that appreciable quantities of water reach the aquifer. In certain areas the blanket of impervious material is perforated with sinkholes, which form when limestone caverns, growing ever larger as the limestone gradually dissolves away, eventually collapse (fig. 15). These sinkholes are the avenues through which water finds its way down into the artesian aquifer (String-,
FIGURE 13.-Model explaining the piezometric surface.
In this laboratory model, the piezometric surface is the plane passing through the water surfaces in the tank and in the vertical tubes. In nature, it is an imaginary surface coinciding everywhere with the height to which water will rise in wells, owing to its pressure head. Water moves in the direction of downward slope on the piezometric surface.