\VATKK SI'PPLV OK THK EDWARDS PLATEAU. 11 



extraordinary. Tims, at Fort Clark, toward the western bonier of 

 our area, tbe June rainfall of IDOO was *2*2.'-\'2 indies, of which ovei' 1 1> 

 inches fell within twenty-four hours. During the same month, in 

 IXJM.i, occurred tlie great I>ra/os River Hood, in connection with which 

 the rainfall of a period of three or four days at Waco exceeded 18 

 inches. 



Mr. Hill, in The Physical (icography of Texas (p. 11), calls atten- 

 tion to the recurrence of flood periods: 



This conil>ination of overlapping conditions in the provinces mentioned, and the 

 accompanying meteorologic phenomena, produce peculiar climatic conditions pro- 

 ductive of erratic floods, which have an important bearing on the agricultural inter- 

 ests, and have, no doubt, been a factor in the peculiar erosion of the denuded central 

 province. 



ATMOSPHERIC MOISTURE. 



Tbe relative humidity and the evaporation capacity of the air have 

 significance, also, both for the behavior of the precipitated moisture 

 as affected by its rate of evaporation and for the occurrence of a pro- 

 tective vegetation; since the drier the air the more open and stunted 

 the timber or other growth. 



During most of the year the Edwards Plateau climate is dry. Even 

 at Austin the average number of actually humid days during the year 

 is small. From Burnet, Fredericksburg, and San Antonio westward 

 the dry ness of the air is notable so much so that invalids requiring 

 dry atmosphere resort thither while in the Devils River country the 

 air has the dry ness and, in the summer, the parching heat of the 

 desert. On the average, the evaporation capacity of the air over this 

 region is sufficient to remove annually a free layer of water over 50 

 inches deep; which means that if the water which falls over the area 

 were continuously and freely exposed as the surface of a lake, this 

 thirsty air coidd drink it up twice over in one year. 



We shall see later how a heavy timber covering operates to check 

 this loss of water from the soil. 



WATER SUPPLY OF THE EDWARDS PLATEAU. 



This vast limestone country is the receiving area for an annual rain- 

 fall sufficient to cover its entire 15,000 square miles of territory with a 

 sheet of water 2 feet deep. The important question is, what becomes 

 of this water ( If most of it runs off speedily it is, of course, lost to the 

 region and its adjacent coast plain, so far as benefiting the growth of 

 vegetation is concerned. If, on the other hand, it is detained to be 

 doled out gradually by the limestone formation, its possibilities for 

 plant life are enormously multiplied. As a matter of fact, the quantity 

 lost b} T a rapid run-off no doubt materially exceeds that retained by 

 the earth stratu. Rut this, we have seen, ; s dependent upon certain 



