Effect of slope on surface drainage in converted rice paddy

Due to the recent regulation of rice production in Japan, it has become necessary to convert rice paddy to other field crop production. To achieve this, drainage conditions, especially for surface drainage, must be improved. We proposed the introduction of a slight slope to improve surface drainage, but the optimal slope must still be determined in order to prevent soil erosion caused by excessive slope, as well as increased cost. In Japan, a 0.1% slope has recently come into widespread use and, therefore, the impact to surface drainage must be quantified. In this report, observations were carried out to quantify the impact of a 0.1% slope for converted rice paddy, and the following results were obtained: 1. An approximate 0.1% slope enables improvement of 46% of the soil surface saturation area compared with that of flat conditions about 10 hours after inundation. 2. Inundation water remains on a flat field, while that on a sloped field moves downward toward the slope end. These are important factors on which to base the introduction of a slight slope to rice paddy for surface drainage improvement. However, the optimal slope should be decided from various perspectives including engineering, agronomy and economics.