DISPOSAL OF RAINFALL 49 



are likely to occur on such reservoirs through evaporation. That 

 the losses on lake areas are very great, and often of greater extent 

 than precipitation, is well known. 



The map in Fig. 14 shows the mean average evaporation in the 

 United States from open waters. It is compiled from observa- 

 tions of the United States Weather Bureau in 1887 and 1888. 1 



Absorption. A considerable part of the rain which falls on 

 the earth is absorbed by the ground. The amount varies, how- 

 ever, greatly, depending on the rate of precipitation, texture of 

 soil, slope of drainage surface, temperature and vegetation. 



A light shower will usually be quickly evaporated, while a 

 heavier rain may be absorbed, and if lasting for some time there 

 will be an excess amount of water which will run off to the nearby 

 stream. On the other hand, less may be absorbed during a heavy 

 rain than during a light, gentle rain, tacause each type of soil has 

 a certain rate of absorption due to its porosity, and if the water is 

 supplied more rapidly than it can be taken up, the excess runs off. 

 A deep, porous, sandy soil naturally will absorb and hold water 

 more than a compact, shallow one, such as a clayey soil. 



If the slope of the watershed is very steep, the water may 

 drain off before any can be absorbed by the soil, and if the slopes 

 are rocky practically no water is absorbed. 



Temperature necessarily also affects absorption. A high tem- 

 perature increases it while the opposite is the case at low tem- 

 peratures as when the ground is frozen. 



On slopes, vegetation and forest are of the greatest importance 

 in that they retard part of the drainage water during heavy rains, 

 which gives the soil time to absorb the same. They are, there- 

 fore, of great value in reducing the intensity of floods after severe 

 storms. The absorbed water seeps into the ground, which it sat- 

 urates, and some of it percolates still further into the pores and 

 fissures and trickles slowly toward the stream. 



These ground waters have a most important bearing on the 

 stream flow. Areas of little or no underground flow are subject 

 to violent floods and extreme droughts, while areas with a large 

 proportion of underground storage are comparatively free from 

 floods. The greater part of the low-water of streams having no 

 lakes or swamps in their watershed is also supplied by this under- 

 ground flow. 



1 Monthly Weather Review, September, 1888. 



