THE WATER OF THE HABITAT 23 



29. The influence of slope. By slope is meant the inclination 

 of the surface of a habitat with respect to the horizon. The 

 principal effect of slope is in controlling run-off and drainage, 

 and through them water content, although they are at the same 

 time affected by surface and soil texture. Slope has also a less 

 direct influence through its action upon heat and wind, which 

 in turn modify humidity and thus affect the water content. Slope 

 is expressed in degrees of the angle made by the intersection of 

 a line bounding the surface and the base line. It is measured 

 by means of a clinometer, a simple instrument in which a line 

 and plummet indicate the angle of slope upon a semicircle 

 graduated in degrees. In making a reading, it is desirable to 

 use a basing strip, a piece of wood 1 meter long and 5 centimeters 

 wide, with a true edge. The basing strip is placed upon an area 

 typical of the slope, and is pressed down firmly to equalize irregu- 

 larities. The clinometer is moved gently along the upper edge, 

 causing the marker to swing freely. When the latter comes to 

 rest, the instrument is carefully turned upon its back, and the 

 angle of slope read directly in degrees. Two or three readings 

 in different parts give a very satisfactory mean for the entire 

 habitat. The angle of slope can not l)e directly connected with 

 the amount of water content, because of the other factors con- 

 cerned. The rule is that the rainfall lost by run-off increases 

 with the angle, and the water absorbed correspondingly decreases. 

 In two or more areas essentially alike in soil, cover, and rainfall, 

 differences in water content are directly determined by differ- 

 ences in slope. 



30. The influence of surface. The surface of a liabitat often 

 shows irregularities which retard the movement of run-off and 

 cause more of the rainfall to soak into the soil. The soil itself 

 often shows such irregularities, e.g., the rocks of boulder and 

 rock fields, the hummocks of meadows and bogs, the mounds 

 of prairie-dog towns, the raised tufts of prairies and sand-hills, 

 the minute gullies and ridges due to erosion, etc. The influence 

 of these is usually not great, but it is always appreciable, and 

 in some cases of considerable importance. Their effects are often 

 measurable by means of soil samples, but it is impossible to (ex- 

 press the character of the surface in definite terms. It must 

 suffice to describe the surface as even or uneven, and to indi- 

 cate the kind and amount of mievonncss. 



