Impact on groundwater level and water quality due to the crop conversion －Case of the middle region of Inabe river basin, Mie Prefecture, central Japan－

It has been reported in several areas in Japan that the decrease of paddy fields in the recharge area in recent years has a great influence on the lowering of the groundwater level in the basin. In Mie Prefecture, located in the center of Japan, the area is expanding where the rice cropping has been changed to wheat or soybeans. This situation, like other areas in Japan, indicates that the crop conversion affects groundwater levels and water quality. In order to manage groundwater resources in the basin, it is necessary to clarify the actual condition of groundwater flow in the study area, and the relationship between the situation of the crop conversion from the past to the present and the change of physicochemical characteristics of groundwater. The purpose of this study is to clarify the status of recent cropping in the recharge area, and the influence of the crop conversion on the physicochemical characteristics of groundwater from the recharge area to the runoff area in the middle of the Inabe river basin in the northern area of Mie Prefecture. Water survey and sampling were conducted in the study area (32 wells and 4points in rivers). The survey was conducted in 2012, 2013 and 2018. In the study area, groundwater level, electrical conductivity, water temperature and pH were measured, and using ion chromatography analyzer at Mie University laboratory, eight major elements (Ca^{2 +}, Mg^{2 +}, Na ⁺, K ⁺, SO₄^2-, NO₃^-, Cl^- , HCO₃^-) analyzing. Water level, water temperature, electric conductivity sensors were installed for the wells at six points, and continuous observation was conducted. We used the data about the crop conversion from local governments in the study area. During the period of 2008-2017, the crop was converted to mostly wheat. In each year of 2008-2017, about 5 to 60% of the paddy field was converted to wheat or soybeans. Since annual precipitation in the study area has not changed substantially in the last 10 years, it was shown that the decrease in the infiltration into the underground due to cropping in the recharge area is one of the factors of the lowering of the groundwater level at the downstream of the groundwater flow.