USE OF ANNUAL PLANTS IN CALIFORNIA FOOTHILLS 27 



curred late in the spring. Exceptional yields were also observed during 

 1934-35, not shown in figure 13, when precipitation totaled 29.40 inches 

 and good rains occurred in both fall and spring. In 1936-37, however, 

 yields were below average, even though precipitation totaled 22.96 inches, 

 apparently because severe winter temperatures stunted the plants so that 

 full production was not attained later on. Winter temperatures may have 

 more effect on plant yields than is readily apparent because effects of 

 temperature are not easily dissociated from effects of precipitation. 



In 1942-43 and 1944-45, with total precipitation of 17.24 and 18.99 

 inches, respectively, soil moisture was considered adequate during the 

 winter and there was sufficient seepage into the swales to promote good 

 growth on these fertile sites. Yield in each of these 2 years was well 

 above average. These years were near the long-time average of 17 to 18 

 inches, calculated from isohyetal maps of the U. S. Weather Bureau and 

 from comparison of precipitation records at the experimental range with 

 records of longer duration at other foothill stations. Apparently average 

 or greater yields can be expected in most years when total precipitation 

 at the experimental range is within a range of 16 to 20 inches. 



In dry years, with precipitation between 12 and 16 inches, soil moisture 

 was not adequate at all times during the winter, and there was only 

 limited seepage into the bottom lands. Herbaceous vegetation was 

 retarded at some time during the winter or spring by lack of moisture, 

 and total yields were accordingly reduced. In each of the five grazing 

 seasons with total precipitation well below the long-time average, herbage 

 production was measured or estimated to be between 1,200 and 1,500 

 pounds per acre, as compared to the 13-year average of about 1,640 

 pounds. Often, half or more of the total herbage in dry years is pro- 

 duced after rains of 1 to 2 inches in late March or April, after some species 

 start to dry at a very short height. This ability to make rapid growth 

 during a short period of favorable weather in the spring allows production 

 of a fair total yield in years that are otherwise unfavorable, and enhances 

 the dependability of annual-plant herbage production. The critical 

 point of annual and monthly precipitation, below which annual-plant 

 production is reduced, will depend on the character of the soil. Observa- 

 tions indicate that plant growth is limited by below-average precipitation 

 to a greater extent on heavy, fine-textured soils than on lighter soils. 



In drought years, say with less than approximately 12 inches total 

 precipitation at the experimental range, herbage yields would probably 

 drop below 1,200 pounds; but how far below is speculative: annual 

 rainfall was more than 12 inches throughout the period of study. In fact, 

 study of long-time records at other foothill stations indicates that pre- 

 cipitation at the experimental range may have fallen as low as 8 to 12 

 inches in only 6 years out of 48; and only two of these occurred in the 

 40 years from 1909 through 1948. 7 Because the distribution of rainfall 

 in drought years ordinarily does not follow the normal pattern, some 

 drought years may have above-average spring precipitation and a fair 

 total production of herbage. To the knowledge of local ranchers, no 



7 Monthly and annual precipitation at the San Joaquin Experimental Range for 

 period September 1897-August 1934 was computed from U. S. Weather Bureau 

 records for Friant, Calif, (no record 1904-06), by regression equation based on correla- 

 tion of precipitation at San Joaquin Experimental Range and Friant during the period 

 September 1934-August 1946. Average annual precipitation at Friant was 13.24 

 inches for the period 1897-1945. 



