THE WATER OF THE HABITAT 21 



loss, place the three soils in a water-bath and dry them out completely. 

 Make a final weighing. Compute the percentage of water in the satu- 

 rated soils at the beginning, the rate of loss from each as compared 

 with the water surface, and the amount of water in each at the time 

 the sand showed no further loss in the air. 



26. Influence of precipitation upon water content. In all 



habitats except those where the supply of water is constant, owing 

 to the presence of springs, streams, ponds^ or other bodies of water, 

 the dependence of water content upon rainfall is absolute. Soil' 

 and slope determine how much of the latter finds its way into the 

 ground, but their action is secondary. Daily rains are able to keep 

 practically any soil saturated, regardless of its character or the 

 slope. All habitats not covered with water reach their maximum 

 water content immediately after a heavy rain or during the rainy 

 season. The water decreases gradually throughout a dry period 

 or season, only to again approach the maximum when precipitation 

 takes place. The latter occurs in various forms, such as rain, 

 hail, dew, frost, and snow. Of all these, rain is by far the most 

 important. In spite of local exceptions to the rule, hail is too 

 infrequent to be taken into account. Frosts have at best only 

 a slight and fleeting effect upon water content, especially in view 

 of the fact that they usually fall outside the growing season. Snow- 

 fall is often of great importance. It not onl}' acts as a cover to 

 prevent evaporation, but upon thawing it also enters the soil 

 directly just as rain does. The loss by run-off from slo})es is much 

 greater, owing to the frozen condition of the ground. The exact 

 importance of dew is not easily determined. Dew is ahnost always 

 too small in amount and too fleeting to add directly to the water 

 content of the soil. By its own evaporation it doubtless decreases 

 in a slight degree the amount of water lost by the soil and by 

 bedewed plants. In studying the water content of habitats a 

 knowledge of the amount of rain usually suffices, though in the 

 study of habitats in spring, the amount and distribution of snows 

 must also be taken into account. 



27. Measurement of rainfall. Rainfall is measured by means 

 of a rain-gauge, an instrument which collects in a narrow vessel the 

 rain falling upon a large surface. In the standard instrument the 

 ratio of surface between receiver and tube is 10 to 1. A direct 

 measurement of the water in the tube must be divided 1)\- 10 to 

 give the rainfall, or a standard measuring-rod, upon which this 



