In clayey paddy field, the estimation of drainage discharge is very important for rational planning for underdrainage. In this paper, the relationship between the area of ponded parts of the clayey paddy field and drainage discharge has been reported.
When clayey paddy field is partially ponded with water, the fundamental equation for calculation of drainage discharge was derived from the two dimensional view point (equation 13). This is based on the asumption that the soil surface of paddy field has random undulation, this equation (13) was applied for partially ponded paddy field to obtain the total discharge (equation 19).
In order to judge the linearity between the ponded area and drainage discharge the equation (20) has also been formulated. This linearity factor was calculated under several conditions and it was found that this linearity factor was applicable to actual paddy fields. This linear relationship has also been experimentally confirmed with clayey paddy field in Shonaka reclaimed land. Water balance calculations were carried out using this linear relationship and the result were compared with the experimental data obtained at Shonaka test field. Very good agreement on linearity factor was observed. If this linear relationship is established with extensive experiment, it would be very convenient for estimating soil moisture change after rainfall. Therefore, these results were very useful for rational planning of underdrainage.
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