16 CHAPARRAL. 
fall. In one locality, just above the Sweetwater Dam, the yearly 
evaporation is equal to 56 inches, while the rainfall is hn 10 inches. 
Conclusions drawn from such ficure would be erroneous, however, 
since much depends upon the distribution of the rainfall. Less than 
10 inches of rainfall might produce a flowing stream and fill a reser- 
voir, while a much heavier annual total might be entirely lost 
by evaporation if it came gently and intermittently. Intermittent 
showers, with periods of sunshine, permit all the moisture to be 
withdrawn from the surface soil. The ground is usually so dry that 
an hour’s hard rain will not wet more than an inch or two of the 
surface. If followed by a hot wind or by sunshine, it might easily 
be that none of this rainfall would be available for the use of man. 
Over the chaparral area, as a whole, including the forest area, present 
conditions of precipitation, run-off, and evaporation are estimated 
in round figures as follows 

Area-of chaparral Covers: 32s) 5 ese ese ee 2 eee acres__ 5, 500, 000 
Area Of Torest™COVer soe Maan a ele ee eee ee ee do_s_= 2,000,000 


Potal-area Of ‘covered watershed sae= seen eee doS.= =" 5005008 

Average annual precipitation : 
At ‘Sea. level: 28 seo ee ee ee inches__ 15 
West and south slopes 25 
At 200058 et_{} Ee aU feh 8 Dh) SC aes 7 
East and north slopes a de a 
West and south slopes 43 = 
At 5,000 feet_{ ee ae ak 
Kast and north slopes aa ee 2g 
At 8,000 aaa est and south slopes ot Getcge™ oe oun 53 
East and north slopes 45 
At the average elevation of the covered watersheds, 3,000 feet, 
TM@We ss yh hs ET ee a ec Oe 

Average annual precipitation on the covered watersheds__acre-feet__ 15, 000, 000 
Average annual run-off from the covered watersheds (10 per 
COME) a 2 es oe ee acre-feet__ 1, 500, 000 
Losses by evaporation and for plant food and unknown (90 per 
GENE) fee a ae BI cee RE aes eee acre-feet__ 13, 500, 000 
Since the precipitation can not be controlled, the preblem narrows 
down to the saving of the greatest amount of the precipitation and 
the control of its rate of run-off. 
Chaparral serves to conserve moisture and regulate the flow ot 
streams in two ways: (1) By the root systems, which penetrate the 
soil and assist the water to percolate, while making a binding mat to 
prevent erosion; and (2) by lessening the evaporation, (a) by break- 
ing the force of hot winds, and (0) by shading the ground. Every 
one of the chaparral species, great and small, contributes something 
toward these ends. There are, however, certain species, not numer- 
ous, which by their size, density of growth, and shade-bearing quali- 
ties are of first importance. Some of these are particularly desirable 
