WATER SUPPLY OF THE EDWARDS PLATEAU. 11 



extraordinaiy. Thus, at Fort Clark, toward the western l^order of 

 our area, the June rainfall of 19U0 was 22.32 inches, of which over 16 

 inches fell within twent^-foiir hours. During the same month, in 

 1899, occurred The great Brazos River flood, in connection with which 

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

 inches. 



Mr. Hill, in The Physical (Jeog-raphy of I'exas (p. 11), calls atten- 

 tion to the recurrence of flood periods: 



This combination of overlapping conditions in the provinces mentioned, and the 

 accompanying meteorologic phenomena, prodnce pecuUar cUmatic conditions pro- 

 ductive of erratic floods, wliich have an important bearing on the agricuhural inter- 

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

 province. 



ATMOSPHERIC MOISTURE. 



The relative humidit}^ and the evaporation capacity of the air have 

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

 as affected 1)y 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 numl)er of actuallv humid da3^s during the 3^ear 

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

 the dryness of the air is notable — so much so that invalids requiring 

 dry atmospliere resort thither — while in the Devils River country the 

 air has the dryness 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 annualh^ a free la3^er of water over 50 

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

 were continuousl}^ and freely exposed as the surface of a lake, this 

 thirsty air could drink it up twice over in one 3'ear. 



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 A^ast limestone country is the receiving area for an annual rain- 

 fall sufficient to cover its entire 15,000 square miles of territor}^ with a 

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

 of this water ? If most of it runs off' speedil}^ 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 graduall}^ b}^ the limestone formation, its possibilities for 

 plant life are enormoush^ multiplied. As a matter of fact, the quantit}^ 

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

 the earth strata. But this, we have seen, is dependent upon certain 



