Study on economic duty of water and optimum scale of irrigation facilities

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The results obtained in 1992 are summarized as follows: 1. Measurement and estimation of evapotranspiration.... Evapotranspiration from a turf was measured by heat balance method and consumptive use was presumed by tensiometer measurement in the turf soil. Furthermore, consumptive use was analyzed for evapotranspiration and water quantity passing through the effective soil layer. 2. Amount of capillary water from the subsoil layer.... Evapotranspiration and capillary rising flow were measured accurately by using a soil column apparatus. A cylinder 30cm in diameter and 100cm in height was filled uniformly with soil, and porous cups of seven tensiometers were set at depths of 5, 15, 25, 35, 45, 55, and 65cm below the surface. Then a groundwater table was set up at the lower end of the cylinder and the quantity of capillary water as well as the evapotranspiration rate were measured. Soil water movement changed with different conditions of soil texture, water table, and effective soil layer. In the case of sandy soil, capillary rise was not significant. On the other hand, the direction of soil moisture movement in silt changed upwards six days after irrigation and the intensity of capillary rising flow was about 1.0mm/d, for eight days irrigation interval. The effect of water table may not be taken into account except when water table is very shallow, less than 1m below ground surface. 3. Adequate scale of water supply system....Main and sub-main irrigation canal system - Capacity of farm pond - Supplying dry field with irrigation water (restriction of water quantity to certain hours). Considerable adjustment was done on the above-mentioned three objectives. Then, the optimum scale of regulating the reservoir for field irrigation was investigated. 4. Effective rainfall in an area....For the purpose of grasping the actual conditions of effective rainfall, the author rearranged the solution of effective rainfall in the field and suggested new analytical method.