Modeling the effect of rainfall intensity on soil‐water nutrient exchange in flooded rice paddies and implications for nitrate fertilizer runoff to the O ita R iver in J apan

This paper examines the effect of rainfall intensity on nutrient exchange at the soil‐water interface of rice paddy fields and the implications to nitrate runoff to the Oita River. The Oita River Basin on Kyushu Island in Japan covers 650 km 2 of which 11% are used for agriculture (rice). During the monsoon season in June/July, the heavily fertilized paddy fields are flooded and large amounts of NO 3 ‐N are discharged to the Oita River. A model has been developed for the NO 3 ‐N release in the rice paddy fields. The model focuses on the effect of rainfall intensity. It assumes that in addition to increased surface runoff and infiltration, the monsoon rain enhances pore water flow and causes nitrate release from the soil by dynamic pressure fluctuations at the soil/water interface. The magnitude of NO 3 ‐N release from paddy fields is described by the simulated soil/water exchange velocity (W) which increases with rising rainfall intensity and hydraulic conductivity, and is on the order of 10 −2 to 10 −6 cm/s. When the river flow rises due to precipitation (monsoon), the NO 3 ‐N load rises almost proportionately to the river discharge, and with little delay. Measured unit NO 3 ‐N loads in the Oita River per day and m 2 of paddy fields were nearly proportional to precipitation intensity R (R 1.042 ) and so were modeled unit NO 3 ‐N release rates in the paddy fields (R 1.095 ). This result suggests that raindrop‐induced pumping is an important if not crucial process that enhances NO 3 ‐N runoff from rice paddy fields. An implication is that the nutrient loading of surface water bodies may grow as the occurrence of extreme rainfall events increases with climate change.