26 CIRCULAR 925, U. S. DEPARTMENT OF AGRICULTURE 
Seasonal distribution of precipitation also influences herbage yields. 
Most of the fall and winter precipitation filters into the soil, where it is 
retained and used by forage plants to make early spring growth. In 
contrast, much of the summer precipitation, which comes as localized 
torrential storms, is lost as surface runoff. Also, during the summer 
months losses by evaporation are large because temperatures are high 
and desiccating winds are common. Analysis of data from the pastures 
where the vegetation is chiefly shadscale, winterfat, galleta, and drop- 
seed indicates that precipitation between October and December was 
about twice as effective in the production of total herbage as that 
between April and September, and that precipitation during the other 
three months, January to March, was about one and one-half times as 
effective. 
YIELD OF INDIVIDUAL SPECIES 
Individual species respond differently to precipitation. Yield of 
shadseale and winterfat in October, like total herbage, was found to 
be closely associated with precipitation during the preceding 12 months 
(fig. 11). These two species produced more than half of the total 
herbage in the experimental pastures and therefore largely influenced 
total production. Production of these two shrubs fluctuated widely 
from year to year. Yields of shadscale followed changes in precipita- 
tion more closely than did winterfat or any other species. 
200 ) 
{3 
= 
y 
s (2 
160 
: rue 
k 
: | 2 
= SHADSCALE pee 3 
2 120 Aa So 
SS ! 9 = 
A PRECIPITATION ! 
= i We a A 3 
9 / \ /\ ! 8 nN 
c ‘ ie Lex I x 
G BOK = ’ 5 Sa ~ 
S ‘ \ i) \ ! Q 
S ‘\ ‘ ‘ 1A\ if \ : : 
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x he: \. p \ Ih \ '/ & 
- Sey oe \ if \ f 6 
ie) 
S si : BS \ ae 1 6 
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x WINTERFAT —=- 
wey 
O 
1935 1937 1939 194] 1943 1945 1947 
FicuRE 11.—Average herbage production in October of the major shrubs, winteriat 
and shadscale, in the 20 experimental pastures in relation to rainfall during the 
preceding 12 months, 1935-47. 
