10 TIMBER OF THE EDWARDS PLATEAU OF TEXAS. 



STRUCTURE OF THE EDWARDS PLATEAU AS TO INFLUENCING THE 

 BEHAVIOR OF THE PRECIPITATED WATER. 



We have just seen wherein the structure of this formation renders 

 it an efficient catchment area. The same structure operates to convey 

 a large part of the water to the deeper strata, whence it reappears as 

 spring water to feed the rivers flowing from the region. 



The topography has, however, from our present point of view, an 

 even more important bearing; since by it are determined in large 

 measure the rapidity of run-off and the percentage of precipitated 

 water lost in this way. Since the border region of the plateau is very 

 rough and deeply dissected, the water will flow off after heavy rainfall 

 before it has had time to enter the limestone formation; and with such 

 volume and velocity as to cause swift and destructive floods, unless 

 detained by some agency other than the limestone structure of steep 

 hillsides. This is exactly the function which a timber or heavy grass 



covering performs (PL II, fig. 1). 







CLIMATIC CONDITIONS OF THE REGION. 



The climatic conditions are of significance in this discussion: First, 

 because they determine the actual amount of rainfall and influence its 

 behavior after falling, e. g., its rate of evaporation; second, because 

 they determine very largely the character of the vegetation, with 

 whose efficiency as a water conserve!* we are mainly interested. 



RAINFALL. 



The region under consideration lies in the belt of transition from 

 moderately heavy rainfall to arid climate, with a very pronounced dif- 

 ference between the eastern and the western borders. Thus, at Austin, 

 the average is nearly 33 inches annually, while on the Pecos side it 

 sinks to about 15 inches. But the more important part of the area for 

 our purpose receives an average annual rainfall not much below 25 

 inches, which, if well distributed and available, would be ample for 

 agriculture. 



The manner in which the rainfall comes is next in importance to the 

 total quantity of it, because of its relation both to the percentage and 

 velocity of run-off and to the checking power which a forest covering 

 could exert. It happens that the Edwards Plateau region is in a zone 

 of fluctuation as regards the type of rainfall; so that when, for example, 

 the Eastern Gulf type prevails, as during the winter of 1902-3, a very 

 heavy season of rainfall may occur entirely unaccompanied by destruc- 

 tive floods, while, on the other hand, an equally heavy season's rain 

 may fall in a very few heavy downpours between April and July. An 

 example of the latter was in 1900, when repeated destructive floods 

 occurred. The amount of percipitation within a brief period may be 



