24 CIRCULAR 925, U. S. DEPARTMENT OF AGRICULTURE 
of highest precipitation. This was more than six times the minimum 
yield of 75 pounds per acre, which occurred in October 1943 following 
two consecutive years with below-average precipitation. 
The total herbage yield on the winter range as measured in October 
was found to be closely related to the precipitation during the preceding 
12 months. This relation of ker bage yield to precipitation is shown in 
figure 9. Within the limits of variaticn in precipitation observed at the 
Desert Experimental Range, berbage production increased about 46 
pounds per acre (dry weight) with each additional inch of precipitation. 
(POUNDS PER ACRE) 
PRECIPITATION 
Ve 
/ 
\ 
PRECIPITATION (CINCHES) 
PRODUCTION 
HERBAGE PRODUCTION 
AVERAGE PRODUCTION 
AND 
PRECIPITATION 
6) 
1935 1937 1939 194] 1943 1945 1947 
FIGURE 9.—Average annual air-dry herbage production as measured in October in 
all range pastures at the Desert Experimental Range and precipitation during the 
preceding 12 months, 1935-47. 
The close relation between precipitation and herbage production 
provides a basis for estimating October herbage yields from the amount 
of precipitation received each year® (fig. 10). Reliable estimates of 
herbage yield for years when rainfall is between 4 and 11 inches can 
be made directly from the chart. Herbage production increases with 
precipitation as indicated by the slope of the estimating line. The 
relation between the measured herbage production and precipitation 
for individual years is shown by the dots. 
3’ Equation for estimating herbage yield from previous 12-month precipitation 
(calculated from data for 1938-45 and 1947) is: 
Ye = — 92.46 + 45.802 with a standard error of estimate of + 41.25 
where Ye is the expected air-dry herbage yield in pounds per acre and r is the 12- 
month precipitation in inches. Correlation betw een Hurheee production and pre- 
cipitation for this period of measurement was r = + 0.9 
