STUDY ON SUBTERRANEAN WATER IN SMALL FIELDS UNDER IRRIGATION

Abstract

1. Ground water in the truck fields surronding Sakai City The fields surrounding Sakai City have been irrigated from ancient times in Japan. The present report is based upon the investigation carried on by the writer concerning the irrigation method and the chemical composition of the ground water. The scale of irrigation is small, and the method of farming very intensive. The depth of the wells varies from 1 meter to 3 meters in the coastal region and in the upland from 6 ms to 10 ms, the depth of water being from 1m to 2 ms in the former and 2-6 ms in the latter. Windmills are chief means of supplying water. As to the soil, sandy soil and sand loam were found to exist in the coastal region and loam in the upland. The ground water can be classified relative to anion into the following three types according to the amount of salt contained in it: a.- Cl>NO3>SO4>HCO - in the upland. b.- SO4>Cl>HCO3>NO3 - in the reclaimed area in the northern part. c.- Cl>SO4>HCO3>NO3 or CL>SO4>NO3>HCO3 - in the coastal area. The residue in the upland is seldom under 300 p. p. m. In the northern part it is mostly above 1, 000 p. p. m. In the other parts on the coast it stands between those of the twe other areas. The varied amount of salt containd in the ground water in the respective areas is attributable to the quality of soils, quantity of fertilizers used, water waste drained out from factories, and sea water. 2. Chemical composition of irrigation water and the method of irrigation for eliminating salinity hazard in the coastal region of Osaka City For the truck fields covering 100 has of sandy soil area the following irrigation method has been used from ancient times in order to eliminate salinity hazard in the suburbs of Osaka City. When the tide is low, the flood-gate is shut, and then the draining-gate is opened allowing the saline water of the creek to flow out. The draining is completed in an hour. After that is done, the draininggate is shut and the flood-gate is opened for taking in the fresh water of the Yamato river. These are done once in three days during the summer and once in ten days in winter time. The concentration of salt in the fresh water for irrigation varies from 100 p. p. m. to 300 p. p. m. However, it is gradually increased as the water flows down towards the draining-gate, and it is as much as one third of the sea water. The figure shows the variation of concentration of salt contaiend in the water of the creek atd ifferent points (Fig. 2, Fig. 3). Relative to the varied concentration of salt in the creek water due to the encroachment of sea water, the fomula - Cl>Mg>SO4>Ca can be established. However, the composition of pond water and that of ground water are complicated, and chi efly the formula- Cl>Ca>Mg>SO4 is possible in their case. A large quantity of phoshate and nitrate is contained in them.