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Show CONSUMPTIVE USE OF WATER IN IRRIGATION 1363 ing a 2-month lag period-the time required for the seepage losses to appear in the springs-Crandall found the increases in the spring discharges from all irrigation sources and from deep percolation from farms, and knowing the total deliveries to irrigators, he determined the consumptive use as shown further in Table 5. Crandall believes that delay in water deliveries until the middle of May in 1917, due to maintenance work, is the cause of the smaller consumptive use during the first year of the measurements. 2.-Crandall's Soil Moisture Measurements By taking samples of soil before and after irrigation, Crandall found that the moisture content of the forage-crop soils increased from 10 to 20% of the dry weight of the soil and that the moisture content of the grain-crop soils increased from 8 to 20%-each to a depth of 4 ft. The water-free soil weighed 80 lb. per cu. ft.; hence, the upper 4 ft. of the forage-crop soils absorbed 0.5 ft. per irrigation, or 2.5 ft. for the five irrigations; the same depth of the grain-crop soils absorbed 1.8 ft. from three irrigations, or 0.6 ft. in each irrigation. About 54% of the project was in forage crops and 45% in grain crops; hence, the consumption of irrigation water on this basis was 2.2 ft., and adding the crop-season rainfall, 0.15 ft., and the draft on soil moisture, about 0.25 ft., the consumptive use becomes 2.6 acre-ft. per acre TABLE 5.-Project Consumptive Use as Determined by Crandall on the Twin Falls North Side Project, Snake River Valley, Idaho. From June, 1917, to May, 1918, included, in acre-feet. From June, 1818, to May, 1919, included, in acre-feet. Increase in springs discharge from all irrigation sources...... Losses from cana! system py seepage plus spring water contributions from other irrigation projects................. Contributions to spring discharge by seepage from irrigated lands........................................................... Deliveries to irrigators less run-off or recovered wastes........ Irrigation water consumed J Total on the project............. by the crops...............} Acre-feet per acre............... Rainfall during crop season, April to September, inclusive---- Draft on moisture in soil........................................ Project consumptive use, UP.................................... 816 800 612 627 878 900 650 439 204 173 228 461 389 909 484 8S8 185 736 256 877 2.1 2.5 0.15 0.15 0.25 0.25 2.5 2.9 3.-Crandall's Computations on Basis of Water-Requirement Studies At the beginning of this Committee's consideration of experimental observations, the difficulties and common sources of error in the use of water requirement studies from pot or tank experiments as a basis for consumptive use determinations over large areas were briefly enumerated. That determinations of the consumptive use on this basis could be considered only as approximations was also asserted. Despite these facts, it is interesting to note the agreement obtained by Crandall using this method with the two methods just presented. The analysis made by him is quoted herewith from his report: |