EVAPOTRANSPIRATION ETC., WITH IRRIGATED GRAIN SORGHUM 



15 



Table 9. — Effect of soil moisture level and nitrogen on the production of grain sorghum per unit of irrigation 



water applied annually, Bushland, Tex. 1 



Fertilizer treatment 



Increase at moisture level of — 



Average 

 for all 



No. 



M, 



M 2 



M3 



M< 



Ms 



M 6 



moisture 

 levels 



F 2 



Lb./ 



acre-inch 



208 



230 



225 



Lb./ 



acre-inch 



288 



314 



328 



Lb./ 



acre-inch 



226 



340 



323 



Lb./ 



acre-inch 



222 



360 



379 



Lb./ 



acre-inch 



173 



308 



334 



Lb./ 



acre-inch 



207 



330 



345 



Lb./ 

 acre-inch 

 220 



F t 



314 



F 6 



322 





Averages _____ 







221 



305 



296 



321 



272 



295 



286 









Based on the increase in yield over dryland yields of the same hybrid in 1957-59 and Early Hegari in 1956. 



Fallow Period Irrigations 



Irrigation before planting is practiced in the 

 High Plains to assure a stand, to maintain growth 

 until irrigation furrows can be made, and to 

 germinate grain not removed during harvest. Ir- 

 rigation of wheat is generally not economical after 

 May 20, as yields will not be materially affected. 

 Thus the irrigation wells can be used for preplant- 

 ing irrigations for grain sorghum several weeks 

 prior to the optimum time to plant. Also, because 

 of the low intake rates, a preplanting irrigation is 

 often made to store water in the 3- to 5-foot depth 

 of the soil profile. Storing water in the 3- to 5-foot 

 depth allows the farmer to irrigate more acres 

 with a given water supply. If irrigations are made 

 only after planting, the demand for water may be 

 greater than the capacity of the wells when E t 

 rates are high. Without water storage in the 

 3- to 5-foot depth, severe reductions in yield can 

 occur. 



Storage of rainfall during the fallow period is 

 usually 15 to 20 percent of the offseason precipita- 

 tion with dryland farming. About 25 percent of 

 the total precipitation at Amarillo comes from 

 storms bringing less than 0.25 inch each. Nearly 

 70 percent of the precipitation comes from storms 

 bringing less than 1 inch each (5). With these 

 light showers, penetration into the fine-textured 

 soil is limited and evaporation losses are high. 



Storage efficiency of precipitation plus irrigation 

 water applied offseason was also low. The 3-year 

 average fallow season precipitation was 11.18 

 inches on the Mi plots and 10.95 on the M 4 plots. 

 However, because of high evaporation losses, 

 preplanting irrigations were necessary to wet the 

 soil profile to a depth of 6 feet. The average 

 depth of preplanting irrigation was 5.5 and 5.2 

 inches for a total of 16.7 and 16.1 inches of pre- 

 cipitation plus irrigation water on the M x and 

 M< moisture levels, respectively. The average 

 net gain in soil moisture from harvest to planting 

 was 5.5 and 4.2 inches. Thus, the efficiency of 

 storing precipitation plus irrigation water was 



33 percent on the Mi plots and 26 percent on the 

 M 4 plots. This loss of water, primarily by 

 evaporation, was approximately one-half the 

 amount required to grow a crop of winter wheat 

 during the same period. The total depth of 

 water evaporated and transpired annually on the 

 grain sorghum plots with optimum soil moisture 

 was about 34 inches. 



Irrigation Water Management 



Irrigation water management practices for 

 grain sorghum will vary with each farm unit, 

 depending upon the crops grown, available water 

 supply, general level of production desired, and 

 facilities and labor for irrigating. Some general 

 irrigation guidelines can be derived from the 

 results of this study. 



Preplanting Irrigations 



Under normal climatic conditions and recom- 

 mended irrigation practices, the soil profile will 

 be near the wilting percentage in the top 4 feet 

 at harvest. As indicated in table 1, about 6 

 inches of available water could then be stored in 

 the 0- to 4-foot depth. From table 3, the average 

 precipitation during the fall and winter months 

 in this area is about 7 inches. However, from 

 November through March approximately one- 

 half of this precipitation comes from storms 

 bringing less than one-half inch each, resulting 

 in high evaporation losses. Therefore, unless 

 large rains are received in April and May, the soil 

 will generally not be wet to more than 1 to 2 feet 

 by planting time. With these soil moisture 

 conditions, preplanting irrigations may be more 

 convenient than applying greater amounts of 

 water after planting. 



High Production Level 



If it is assumed that adequate fertilizer was 

 provided for near maximum production and a 



