26 



CIRCULAR 870, U. S. DEPARTMENT OF AGRICULTURE 



2,800 



2,400 



2,000 



1,600 



1,200 



? 800 



400 



TOTAL PRECIPITATION 

 DURING SEASON 



| 12"- 16" 



jj 16"- 20" 



g3 ABOVE 20" 



I 



Y 

 Yi 

 Yi 

 Yi 



OP 





YYt 

 YYt 

 YYt 

 YYt 

 YYt 

 YA 

 YYt 

 YA 



1935- '36- 

 '36 '37 



37- 

 '38 



38 



39- 

 39 '40 



PLANT 



40- 

 '41 



41- 

 '42 



42- 43- 44- 

 '43 '44 '45 

 GROWING SEASON 



Figure 13. — Estimated total production of air-dn^ herbage per grazable acre, and 

 total precipitation during plant growing season (September of one year to time of 

 plant maturity in next year), San Joaquin Experimental Range, 1935-48. 



In years with precipitation greater than 20 inches, the heavy rainfall 

 during the winter months was largely lost by seepage from the upper soil 

 before air temperatures became favorable for plant growth. Shallow- 

 rooted annuals could not take advantage of the excess precipitation 

 except on bottom-land soils, where ample seepage from the slopes pro- 

 vided moisture into the spring months. Thus, total herbage production 

 was approximately average in four of the six seasons when total precipita- 

 tion was above 20 inches. Greater than average yields were obtained 

 during 1937-38, with 32.09 inches of precipitation, when good rains oc- 



