HYDROLOGIC PRINCIPLES 17 



face and form springs, seeps, or lakes; some may flow directly 

 into streams or into the oceans. Of the water in streams, some 

 may accumulate in lakes and surface reservoirs; some may be 

 lost by evaporation or transpiration of riparian vegetation; 

 some may seep downward into ground-water reservoirs, and 

 some may continue on to the oceans. The hydrologic cycle is 

 completed by evaporation from the oceans and circulation of 

 water vapor in the atmosphere. 



Lest it appear that because of these apparent multiple 

 choices, the path followed by a particle of water is entirely for- 

 tuitous, it should be stressed that there are definite priorities 

 for that movement. Except for the water that evaporates at the 

 surface, the soil or mantle-rock has top priority upon the water 

 that falls as precipitation. Overland runoff does not occur 

 unless or until precipitation exceeds the capacity of that sur- 

 face layer to absorb the water. The soil holds water against 

 the force of gravity until its field capacity is reached, that is, 

 its capacity for holding water by molecular attraction, and 

 only then does water start to percolate downward under the 

 force of gravity. In the intervals between storm periods soil 

 moisture may be depleted by evaporation and transpiration, 

 and this depletion must be made up during subsequent storms 

 before there can be additional downward percolation. 



Ground-water reservoirs, including those perched upon im- 

 permeable rock layers, receive the water that percolates down- 

 ward from the soil zone. These reservoirs, or aquifers, are 

 composed of materials sufficiently permeable that water can 

 move through them by gravity. Water accumulates until the 

 reservoir is filled sufficiently to cause underground flow, which 

 may ultimately be discharged into lakes or stream channels 

 or oceans, or at the land surface by springs or seeps. Where 

 ground water is at shallow depth, it may be discharged by evap- 

 oration or transpiration. 



Streams are the spillways of the hydrologic cycle and carry 

 off the surplus water that is not stored in lakes or underground 

 or returned to the atmosphere by evapotranspiration. They 

 have the lowest priority on water that falls as precipitation, for 



