10 TIMBER OF THE EDWAKDS PLATEAU OF TEXAS. 



STRtlCTUKE OF THE EDWARDS PLATEAU AS TO IXFLUENCIXG THE 

 BEHAVIOR OF THE PRECIPITATED AVATER. 



We have ju.st seen wherein the structure of this formation renders 

 it an efficient catchment area. The same structure operates to conve^^ 

 a larg-e part of the water to the deeper strata, whence it reappears as 

 spring water to feed the rivers flowing from the region. 



The topograph}^ has, however, from our present point of view, an 

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

 measure the rapidit}^ of run-off and the percentage of precipitated 

 water los't in this way. Since the border region of the plateau is very 

 rough and deeply dissected, the water will flow off after heav}^ 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 heav}" grass 

 covering performs (PL II, flg. 1). 



CLIMATIC CONDITIONS OF THE REGION. 



The climatic conditions are of signiflcance 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 conserver we are mainlv interested. 



RAIXFALL, 



The region under consideration lies in the belt of transition from 

 moderately heav}^ rainfall to arid climate, with a ver}^ 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 

 agriculfure. 



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

 total quantit}' 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 thetvpe of rainfall; so that when, for example, 

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

 heav}^ season of rainf till may occur entirely unaccompanied b}^ destruc- 

 tive floods, while, on the other hand, an equally heav}^ season's rain 

 ma}" fall in a very few heavj' 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 



