Long-term evaluation of the groundwater recharge function of paddy fields accompanying urbanization in the Nobi Plain, Japan

The groundwater recharge function, one example of the multifunctionality of agriculture, is closely related to hydrogeological phenomena and socioeconomic factors such as pumpage and land use. A long-term evaluation of the groundwater recharge function is necessary to understand its role among the multiple functions of agriculture. The Nobi Plain, one of the largest coastal plains in Japan, was selected as the study area because it has experienced typical socioeconomic changes. We conducted a long-term evaluation of the groundwater recharge function based on simple water-balance equations using long-term data on groundwater levels and river flows. Leakage recharge from paddy fields in 1975 was about 2.8×10⁸ m³/year, decreased to 0.6×10⁸ m³/year in 1984, and ceased in 1985. Its monetary value of the function in 1975 was estimated by the replacement cost method, was about $56 million for 20 ha×10³ ha of paddy fields. The value per unit area of paddy fields was calculated as $2820/ha and that in Japan was calculated as $400/ha. Paddy fields in suburbs of big cities have a higher value than the average paddy field in Japan. However, this recharge value is no longer produced because no leakage recharge occurs at present owing to socioeconomic changes.