抄録
We developed a numerical model to simulate N flow in an agricultural paddy field and upland field area and analysed scenarios for recycling the agricultural runoff, including field drainage, from an agricultural area with an irrigation/drainage system. In it, we considered N removal in paddy fields, a regulating reservoir, and canals. The results of analysis indicated that a large amount of the effluent load occurred during the transplanting period and just after fertilisation, and that recycling could reduce the effluent N load. In the case where paddy fields occupied the area, the effluent N load would be equal to the inflowing N load (net zero effluent) at a 48 % recycling rate. However, in the case where upland fields (33 %) and paddy fields (67 %) occupied, 95 % recycling rate was needed to achieve the net zero effluent. To implement the knowledge for reduction of effluent load from an agricultural area, we should solve the matter that the water environment deteriorates as pollutants accumulate with intensive recycling of runoff.
