VARIATION IN LEVEL OF GROUND WATER. 425 



The variation in the level of ground water due to this cause may be excep- 

 tionally 2 or 3 meters, but in general is but a fraction of a meter. 



(d) The cyclonic intervals of varying precipitation in the north 

 temperate zone are usually from three to seven days. However, in various 

 regions, at various times, several of the cyclonic periods may rapidly follow 

 one another, and give conditions of almost continuous rainfall. In certain 

 regions, as in the Tropics, a chief variation in precipitation is diurnal, there 

 being in the region of the doldrums abundant showers almost daily. 

 Ordinarily the variation in the level of ground water due to the cyclonic 

 period is but a few centimeters; but locally, and where there is nearly 

 continuous precipitation for a day or more, the level of ground water may 

 rise, due to this variation in rainfall, by amounts as great as a meter, or, 

 very locally, several meters. 



The rise of ground water for a given amount of precipitation in any 

 cycle depends not only upon the quantity of the rainfall, but upon the 

 amount of seepage into the soil, and this depends upon many factors.^ For 

 instance, low declivity, veg-etation, rough surface, and large openings are 

 favorable to entrance into the soil; while steep declivity, lack of vegetation, 

 and small openings are favorable to large run-off and evaporation. Where 

 there is a high percentage of seepage a comparatively small precipitation 

 may raise the surface of ground water a considerable amount, for the pore 

 space at the level of ground water may be small. In case the pore space 

 were 10 per cent, a fall of 1 cm. would produce a rise in ground water of 

 5 cm., provided one-half of it reached the level of ground water through 

 seepage. In many districts increased seepag'e, due to irrigation, has mark- 

 edly raised the level of ground water. 



In so far as the water evaporated is derived from the belt of cementa- 

 tion, this tends to lower the level of ground water. The amount of this 

 lowering, due to the upward transportation of water through molecular 

 attraction and vegetation, is likely to be great in proportion as the level of 

 ground water is near the surface. Where the level of ground water is 

 many meters below the surface the variation due to this cause is probably 

 small. The variations of the level of ground water due to evaporation are 

 considered in connection with precipitation and seepage; for in many 

 regions the abundant precipitation occurs at the same periods of the year 



