EVAPOTRANSPIRATION ETC., WITH IRRIGATED GRAIN SORGHUM 



of the land area. Thus, even during the summer 

 season, about two -thirds of the area is nonirri- 

 gated. Evapotranspiration determinations made 

 in this 2.8-acre site surrounded by irrigated and 

 nonirrigated land should be representative of 

 irrigated fields surrounded by nonirrigated land 

 in the area. 



Seasonal Evapotranspiration 



A detailed summary of seasonal evapotranspira- 

 tion (E t ) and analysis of variance for three fertility 

 subplots on each moisture level are presented in 

 table 12 in the appendix. Some of these data have 

 been summarized and published earlier (6, 7, 8). 



The largest yields and the highest water use 

 efficiency were obtained on the M 4 moisture level. 

 Therefore, the M 4 moisture level will be referred 

 to as the optimum moisture level in the rest of 

 this report. Cumulative E ( averaged for the 

 F 4 and F 5 fertilizer plots in 1957-59 and all fertilizer 

 treatments in 1956 on the M 4 moisture level is 

 presented in figure 2. The 4-year average seasonal 

 E t was about 22 inches. 



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 1957 

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JUNE 



AUGUST SEPTEMBER OCTOBER 



Figure 2. — Cumulative evapotranspiration for grain 

 sorghum at Bushland, Tex., with optimum soil moisture 

 conditions (M4 moisture level). 



Climatic conditions during the 1956 growing 

 season were nearly normal except in September 

 and October, when solar radiation and air tem- 

 perature were above normal and rainfall was 

 below normal. Cumulative E t in 1956 closely 

 followed the 4-year mean except in September, 

 when it exceeded the mean. The total 1956 

 seasonal E t probably would have been higher if 

 an additional irrigation had been given in Septem- 

 ber. With below-normal rainfall in September 

 and October, most of the available soil moisture 

 was depleted by early October. 



Low cumulative E t in 1957 was a result of 

 below-normal solar radiation and air temperatures, 



771-798 0—65 2 



especially in October. Solar radiation and air 

 temperature were above normal in July 1957; 

 however, the planting date was about a week 

 later than average, wnich apparently resulted in 

 below-normal cumulative E t that month. 



In 1958 heavy July rainfall (7.79 inches) may 

 have caused some deep percolation beyond the 

 soil sampling zone. Soil moisture in mid-July 

 was greater in the 4- to 5-foot and 5- to 6-foot 

 depths than that at corresponding depths on 

 June 23. Also evaporation losses from the soil 

 surface may have been above normal in July 

 because of frequent rains. 



Use of nitrogen fertilizer increased yields sub- 

 stantially but had little influence on the seasonal 

 E t (appendix table 12). For example, with the 

 M 4 moisture level, the 3-year average yield (1957- 

 59) was 194 percent greater on the highest nitrogen 

 treatment than on the O-nitrogen treatment, but 

 the average seasonal E t was only 6.2 percent 

 greater, in 1959, with a lower yield on the 0- 

 nitrogen treatment, the yield was 262 percent 

 greater on the high-nitrogen treatment. The 

 larger yield in 1959 was obtained with an increase 

 of only 5.7 percent in E t . There was no signifi- 

 cant difference in E t between the 120- and 240-lb. 

 N rate. Similar results have been reported for 

 other areas (16). 



During August, grain sorghum can be considered 

 as having adequate evaporation and transpiring 

 surfaces so as not to limit E t . Consequently, an 

 estimate of mean evaporative demand or potential 

 E t should be approximately the same as the mean 

 E t determined on the M 4 moisture level during this 

 period. Potential E, as used here refers to evap- 

 orative demand in irrigated fields located in arid 

 and semiarid areas. The 4-year average E t for 

 August was 8.6 inches (fig. 2). The average total 

 radiation for August was 583 calories per square 

 centimeter per day, which would be equivalent to 

 the energy required to evaporate about 0.389 inch 

 per day. The mean air temperature was 76.9° F. 

 Jensen and Haise (4) obtained the equation E tp = 

 (O.UT-0.37)R s for estimating potential E t by 

 correlating measured E t from crops other than 

 grain sorghum with solar radiation and mean air 

 temperature. T is the mean air temperature 

 in degrees Fahrenheit, R s is solar radiation 

 expressed as evaporation equivalent, and E„ is 

 estimated potential evapotranspiration. With 

 this equation the estimated mean potential E t 

 during August at Amarillo is 8.5 inches. 



Rate of Evapotranspiration 



The average rate of evapotranspiration for 

 sampling periods on the M 4 moisture level for the 

 4 years is presented in figure 3 along with estimated 

 mean potential E t . Estimated mean evaporative 

 demand or potential E t is high in June and July up 

 to about the first part of August, then it begins to 



