Studies on the Rationalization Water in the Sprawled Area : ―The Relation between the Transition of the Paddy Field Area and the Net Duty of Water―

The quantity of irrigation water calculated by the water balance law was 99,600m^3/day, and the net duty of water corresponded to 81,600m^3/day by subtracting main channel water loss 18,600m^3/day calculated throughout planting term from 99,600m^3/day. According to the method of water requirement in depth, the quantity of irrigation water was 14.6mm/day (61,000m^s/day) by testing evapotranspiration and percolation, and calculating the equation of Thornthwaite, but the net duty of water was assumed 24 mm/day (81,600m^3/day) in consideration of the safety factor for water loss and seepage of the groundwater. The net duty of water obtained by this method was corresponded to the reslt gained by the water balance law, therefore, optimum duty of water (81,600m^3/day) in this area have dicreased by approximately 34,400m^3/day (29.7%) in comparison with the result (116,000m^3/day) in 1972. Nevertheless, the paddy field area has only decreased by 12.8% (from 390ha to 340ha) during these three years. Accordingly, the surplus water is not proportinal to the decrease of the paddy field area. In this area such as low reclamation land, it is necessary to investigate the groundwater movement and the drainage from the other areas affecting to the water level of the Nakagawa water channel when we analyze the net duty of water only with the method of water requirement in depth, because the quantity of percolation is influenced by the seepage of the groudwater. (Received June 30 1976)