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Show CONSUMPTIVE USE OF WATEE IN IBEIGATION 1357 transpiration ratio. Most of the evapo-transpiration studies have been conducted in tanks or potometers. Kiesselbach (10) found that the limitation of the amount of soil used, because of the small capacity of the potometer, may be a great source of error in pot experiments. It seriously affects not on]y the transpiration relationships, but the entire development of the plant. Other difficulties in determination of consumptive use by means of potometers are: (a) Obtaining the same atmospheric environmental conditions around pots as exist in ordinary cropped land. (&) Obtaining the same number of plants per unit area of land as occur under field conditions. (c) Getting the soil into tanks in the same physical conditions as exist in the field, thus permitting the same aeration and water movement. The influence of these factors on the consumptive use is great enough to render the pot method of determining such use of questionable Value. Consumptive Use as Determined on Field Experimental Plots Direct measurements of consumptive use in field plots are believed to be more dependable than measurements with pots or tanks. In order to permit measurements in the field, it is clearly necessary to use land in which the water-table is at a considerable distance below the surface, because when working with small field plots it is usually impractical to measure the quantity of water absorbed by the crop from the ground-water. It is also necessary that the irrigation water be applied in small units not to exceed a depth of 5 in. in a single irrigation on ordinary soils. Even a smaller quantity will often result in percolation beyond the depth of feeding roots in coarse-textured soils. Either very large unit applications of water at ordinary time intervals, or excessively frequent irrigations resulting in the maintenance of a high moisture content, is likely to cause an appreciable downward movement of moisture in the gravitational or capillary form, and thus give apparent values for the consumptive use higher than the true values. It is impracticable, by direct means, to measure the amount of such downward movement, and hence an attempt to measure the consumptive use under these conditions may give, in reality, the farm consumptive use. When the percolate equals zero, the farm consumptive use equals the consumptive use; therefore, it follows that the consumptive use equals the supply less the surface run-off. In most of the field determinations of net duty of water, the run-off has been either carefully measured or reduced to zero by the proper preparation of the experimental plots. The supply factors, in general, have not been measured with precision even on plots where the great depth of the water-table has precluded the possibility of the crop receiving any ground-water. This is due to the fact that precise measurements of the quantity of water taken from capillary soil-moisture storage is practical only in comparatively homogeneous soils free from gravel, and that even under the most favorable soil conditions dependable soil moisture tests add greatly to |