RESEARCH STUDY ON THE WATER QUALITY PURIFICATION FUNCTION OF PADDY FIELDS REGARDING DIFFERENCES IN IRRIGATION METHOD AND FARMLAND UTILIZATION

Purification of paddy field water in the low-lying areas around the INBA Lake shore by T-N (NH4-N, NO3-N, NO2-N and Org-N), T-P, etc. was discussed from different viewpoints of water management and land utilization. 1) A continuous penetration spill-over irrigation method proved to be very effective for the purpose of purifying irrigation water for the paddy fields where the irrigation water was highly concentrated and took a large amount of irrigation water. 2) If the concentration of irrigation water (water flowing to paddy fields) was high, it would purify the water in the paddy fields; on the other hand, if it was low, the discharging load quantity of irrigation water would increase and lead to the discharge of pollution. It was suggested that the purification of the water quality of the paddy fields was dependent on the concentration of influent water, and this research more clearly demonstrated that there was a critical concentration value that could exercise the purification function. 3) It was feared that deterioration of INBA Lake (a closed body of water) would be accelerated if the paddy field was converted to a field, because the denitrification action that worked as a reducing condition decreases and NO3-N, caused by rainfall, flows out.