FORAGE-CROP EXPERIMENTS AT SAN ANTONIO. 5 



was as late as April 12, and in several years it has occurred in the 

 second or third week of March. The first autumn frost may be ex- 

 pected about November 26, though it often occurs as early as Novem- 

 ber 12 to 14. A minimum temperature of 4° F. was recorded in the 

 winter of 1899, though the temperature does not usually fall below 

 18° F. 



The weather records show that the region is one of short, mild 

 winters, with a long growing season, characterized, fortunately, by 

 a very early spring. During seasons of unusually heavy rainfall or 

 with irrigation it is possible to grow two different forage crops on the 

 same land in a single season. Where crops are grown without irriga- 

 tion the important point is to have the land in condition for planting 

 as soon as possible after the previous crop is removed, so that when 

 favorable rains come the next crop may be seeded promptly and the 

 moisture thus utilized before it is lost by evaporation. 



RAINFALL. 



The mean annual rainfall of the section for the past 20 years has 

 been about 25 inches. Much of this rain, especially during the sum- 

 mer months, comes in torrential storms and falls on soils that are 

 easily compacted, so that the proportion of run-off is large. There 

 are also many light showers, of one-half inch or less, that do not 

 moisten (he soil deeply enough to benefit the growing crops, par- 

 ticularly during the summer months. The potential evaporation, 

 as measured by the loss from a free water surface, is very high. It 

 has been shown by Briggs and Belz in their studies of dry farming in 

 relation to rainfall and evaporation in the Great Plains area ' that 

 the rainfall at San Antonio corresponds approximately to a 20-inch 

 rainfall in Minnesota or South Dakota, the average annual rainfall 

 at San Antonio being approximately 24| inches. These figures are 

 based on the differences in evaporation in the two localities. The 

 annual evaporation at San Antonio averaged 09.2 inches during 

 the four years 1908-1911. 



The following table shows the rainfall records by months for each 

 year since 1907 and the average monthly rainfall for the past 20 

 years. It will be observed that while the average monthly rainfall 

 is fairly uniform, the actual rainfall for each month is extremely 

 variable and, furthermore, that there are no regular wet and dry 

 seasons. 



1 Briggs, Lyman J., and Belz, J. '). Dry Farming in Relation lo Rainfall ami Evaporation. Bulletin 

 188, Bureau of Planl Industry, U. S. Dept. of Agriculture. 1910. 

 [Cir. km; | 



