268 



CONSERVATION 



Thus moisture, in pursuing the great 

 globe-encircling routes of the winds, 

 may be influenced by local conditions to 

 stop or to hasten on. There is a cycle 

 of reactions which begins with rain, is 

 continued by ground water and vegeta- 

 tion, and leads back to rain. The show- 

 ers sink in, the seed germinates, the 

 plant grows in the moist soil, and the 

 cool, moist air above the green surface 

 provokes a shower ; or the downpour 

 dashes off, the bare surface radiates the 

 heat of the brilliant sun, and the shower 

 that might fall passes over. 



These reactions are not the great 

 ones, but they are effective enough in 

 many regions to make a garden or a 

 desert, as the experience of our race, 

 viewed in the light of modern knowl- 

 edge, proves. 



GROUND WATER 



If anywhere in humid regions a well 

 be sunk deep enough water will com- 

 monly be found. The water fills the 

 soil or the spaces in the rocks. It flows 

 into any opening and is governed by 

 gravity, so that it runs down grade 

 under ground. It is "ground water" 

 and its surface is the "ground-water 

 table." 



In humid regions the ground-water 

 table corresponds with the surface level 

 of swamps and streams in their imme- 

 diate vicinity, but it rises above them in 

 uplands, following in a measure the alti- 

 tude of the surface, though it does not 

 reach an equal height. From the high- 

 er parts the ground water flows down 

 to the lower ; that is, from the uplands 

 underground to the streams. If it be 

 replenished by frequent rains it feeds 

 the streams steadily, if not, the sup- 

 ply diminishes at first rapidly and then 

 very slowly. It is the principal source 

 of moderate and low-water flow in riv- 

 ers of the eastern United States. 



In arid regions the ground-water 

 table usually lies at considerable depth 

 or is wanting. Underground water 

 may even be restricted to deep water- 

 bearing strata, which it enters along 

 the outcrop and in which it is under 

 artesian head. 



In all regions, except excessively dry 

 deserts, the soil above the ground-water 

 table contains some moisture, which 

 envelopes the grain with a thin film 

 only, if the soil be very dry, but which 

 otherwise occupies the minute spaces 

 between the grains. This moisture does 

 not fall or rise in obedience to gravita- 

 tion, it being under control of forces 

 due to surface tension, which exist in 

 tubes of very small bore (capillary 

 tubes) and similar minute spaces. 

 These forces of surface tension, over- 

 powering the effect of gravitation, draw 

 the moisture upward or sideways or 

 downward in the direction of least 

 moisture and thus tend to establish a 

 balance throughout the mass so far as 

 the capillary films are continuous. If 

 there be evaporation from the surface 

 the moisture is drawn upward, and this 

 is the principal effect due to capillary 

 attraction. 



In humid regions the ground water 

 and the soil moisture are in contact. 

 From the surface of the ground to the 

 water table the soil is more or less 

 moist, and if evaporation takes place 

 the upward movement may extend 

 down to the water table, and some part 

 of the ground water may be drawn into 

 the capillary circulation. 



In arid regions the ground water and 

 soil moisture are commonly not con- 

 nected, but are separated by a greater 

 or less space in which the capillary films 

 that envelope the soil grains are not in 

 contact and there is consequently no 

 capillary circulation. This is true un- 

 der much of the great plains. Streams 

 are fed only by immediate flood run-off 

 or by underground waters from remote 

 mountains. 



Both ground water and soil moisture 

 are derived from precipitation ; the soil 

 moisture in any acre being part of the 

 rain or snow that falls on that acre ; and 

 ground water also being often accu- 

 mulated from the surface which it un- 

 derlies, but frequently coming from 

 higher grounds near by or sometimes 

 far away. This fact places in the farm- 

 ers' hands the power to regulate the 



