122 Report S.A.A. Advaxckment of Sciexck. 



otliei-s show a lessened flischurge, and a cessation of ilow in winter 

 (the tlry season). The ultimate result may be extinction. 



Along the Missouii bottoms in north-western Iowa and near 

 Canton, South Dakota, there are numerous large cotton-wood (Popu/nif 

 deffoide,s) groves, which have been planted for the special purpose of 

 drying the soil, and this has been done very effectively. This is a 

 large-leaved tree, and probably has a very great evaporating powei'. 



Many works on forestry state that in flat country forests tend 

 to lower the natural water plane ; but it seems possible that the result 

 may at least partly be attributed to broad-leaved trees that grow better 

 on the plains, to smallei- rainfall due to lower elevation, and to a 

 greatei- depth of absorbent soil. 



Although the amount of moisture absorbed by forest trees is large, 

 the quantity required by field crops is considerably greater.* Kisler, 

 from a lengthy series of investigations, reached the conclusion that 

 forests actually take up less than one-half as much water from the soil 

 as the average agricultural crop, and the following table by him is 

 frequently quoted : — 



Daily Consumption of W'atkk for Different Crop.s. 



Crop. 

 Lucerne ... 

 Meadow grass 

 Oats ".. 

 Indian corn 

 Clover 

 Vines 

 Wheat ... 



%e 



Potatoes ... 

 . Oak trees 

 Fir trees ... 



SiltviKj of Re!<('vv(>irt<. — It is eas}' to see how engineering works are 

 affected by the freshets in an unprotected distiict, and how serious a 

 question it is for irrigation engineers. In South Africa so many 

 ditticulties have been encountered from the silting up of tanks and 

 reservoirs that many people ha\e formed the idea that large storage 

 leservoirs are impi-acticable. This is a very e.xaggerated idea, and is 

 largely due to the silting up of vevy small farm dams on the Karroo, 

 where e\ ery rain washes oil' the surface soil that has been loosened by 

 the sheep and goats. A reservoir was constructed at Beaufort West 

 in 187D, aiul in 1905 its capacity was found to ha\(' been diminished 

 to nearly one-half. In 1875 the storage capacity was appioximately 

 510,(»0(),000 gallons: but the capacity at the present time is only 

 208, 1()0,00(* gallons. The catchment area, some 70 square miles, is 

 typical Karroo veld, with ]>ractically no \egetation to break the rush 



Mfniiiof i>f Forestry, by "W. Schlich, CLE. p. 44. 



