OUR WATER SUPPLY— MEINZER 299 



its water in dead storage insofar as the roots of the plants are con- 

 cerned. Between these extremes are the productive soils of inter- 

 mediate texture, such as the loams formed from the loess, which hold 

 considerable water against the pull of gravity and yield it freely to 

 the plants. 



Dry-farming methods consist largely in utihzing the reservoir 

 capacity of a soil by storing in it the rain and snow water of one or 

 more years and making it available to a crop that is grown in a much 

 shorter period. Unfortunately, soils do not generally have the 

 capacity to store the quantity of 'SMiter that is needed to produce a crop 

 without replenishment by rains or irrigation at more or less frequent 

 intervals during the growing season. In the eastern and especially 

 the southeastern part of this country it frequently happens that the 

 soil moisture is fully replenished early in the winter and that for many 

 weeks thereafter water from the rain and snow percolates through the 

 soil without adding to its water content. In the ensuing summer, 

 however, the soil moisture may become depleted long before the crops 

 have matured, and severe drought damage may result. 



Agriculture is, from the viewpoint of this discussion, one of the 

 greatest of all achievements of man in the utilization of our water 

 supply, but soil erosion, lilte a dread disease, gnaws at its roots. The 

 erosion of the soil removes a part of the water reservoir that it utilized 

 in crop production, and especially the upper part, which generally 

 has the greatest capacity for holding water available to plants. Thus 

 the measures imdertaken to check soil erosion are measures of water 

 conservation. 



Storage oj water in the rock jormations. — The systems of rocks that 

 form the outer part of the solid earth are the products of all the 

 diverse and variable geologic processes that have been operative 

 through the ages. The description and interpretation of these rock 

 systems, with their almost infinite complexity, is the task of the 

 geologists. The rock systems constitute natural systems of water- 

 works with many reservoirs of great variety, some of which have very 

 large capacity. The study of these natural waterworks and their 

 operation is a task of the hydrologists. It is a task that has requu-ed 

 the development of a distinctive technique in the application of the 

 science of fluid mechanics and hydrauhc engineering to the geologic 

 structure of the rocks. 



The porous and permeable rock formations which constitute the 

 underground reservoirs are saturated below a certain level with water 

 that is under the control of gravity. In other words, then nder- 

 ground reservoirs are filled to the level of the water table. In most 

 places the roots of the plants do not extend downward to the water 

 table or to the capillary fringe, which occurs directly above the water 



