WHAT'S HAPPENING TO WATER? — ROBINOVE 379 



soil can hold moves downward under the force of gravity. The me- 

 chanics of the movement of water in this zone are complex and not 

 so well understood as the movement of water in some other phases of 

 the hydrologic cycle. Because water must move through this zone, 

 which is not saturated with water, in order to enter the saturated zone 

 where the ground water moves laterally through completely filled 

 pore spaces, further research on the mechanics of water movement in 

 the zone of aeration is essential. 



Fluid movement in the zone of aeration involves movement in three 

 phases ; water, water vapor, and air. Water is retained in the zone of 

 aeration as films of water surrounding particles of the rock or com- 

 pletely filling some of the void spaces between the grains. Water 

 completely filling the voids can move downward until it reaches the 

 water table as groimd-water recharge. Downward movement of 

 water in the zone of aeration is primarily in the water phase; the 

 transfer of water vapor does not contribute significantly to ground- 

 water recharge. 



A particular gi'ound-water reservoir, such as the Dakota Sandstone 

 of the northern Great Plains, may underlie tens of thousands of 

 square miles, while another ground-water reservoir, such as the sand 

 beds underlying Long Island, may be confined to relatively small 

 areas. The movement of ground water in small aquifers (water- 

 bearing beds or strata) may be only part of a large pattern of ground- 

 water movement throughout a larger area. The effects of water use 

 and development must be studied in both large and small areas in 

 order to understand fully the regimen of gromid water. 



Water moves through ground- water reservoirs until it is discharged 

 as springs, by seepage into streams and lakes, and through with- 

 drawal by man. Such discharge allows a continual movement of 

 water through aquifers and provides room for recharge. The amount 

 of water moving through the ground and the total amount of water 

 removed from the ground-water reservoir in any specific period of 

 time are known only approximately. The water that seeps into 

 streams and lakes provides the base flow of the streams — that is, the 

 low flow that is sustained through the driest part of the year. If 

 water is diverted from a ground-water system and withdrawn for use, 

 such as irrigation, or the water is evaporated back into the atmosphere, 

 the base flow may be reduced substantially or even eliminated. The 

 complex interrelationsliip of water on the surface and under the 

 ground is one part of the hydrologic cycle which we need to study 

 more intensively in order to make the best use of both sources of 

 water. 



Evaporation and transpiration are two phases of the hydrologic 

 cycle that are difficult to study quantitatively. Only in the last few 

 years have instruments and mathematical techniques been developed 



