A Study of the Elements of Water Requirement for Rice Field Preparation : An Investigation and Experiment in the YAHAGIGAWA Water Use District

In the YAHAGI drainage basin which is situated in the eastern part of Aichi prefecture, the authors have researched the elements of water requirement for rice field preparation. Main results obtained are as follows, The elements of the water requirement for preparation are evaporation, the depth of flooding water, the saturation of the surface soil, and percolation. In a rough estimate, evaporation is 2-3㎜, the depth of flooding water is 40㎜, the saturation of the surface soil is less than 40㎜, percolation is 0-200㎜. Commonly, the water requirement for preparation is 80-120㎜ in a ill-drained paddy field, 100-150㎜ in a well-drained paddy field and 150-250㎜ in a leaking paddy field. Therefore, these differences are based on the total of the saturation of the surface soil and percolation. The saturation of the surface soil is influenced by the ground-water table. When the groundwater table is lower than 1m from ground surface, saturation has a maximum quantity of 40mm. Percolation is influenced by the groundwater table and the soil texture of the subsoil (particularly, the coefficient of water permeability). When the water requirement for preparation is calculated, it is an effective means to establish patterns for the groundwater table, and tire soil texture of the subsoil