Characteristics Of Groundwater Recharge In Piedmont Plains And Its Potential Impact Factors

Making clear the groundwater recharge amount and distribution characteristics of groundwater recharge is the key for groundwater resources evaluation. It also could provide a theoretical basis for sustainable exploitation and utilization of groundwater resources. A piedmont plain, where is the transitional landscape from mountains to plains, is an important recharge region for groundwater. The area is usually characterized by the following features:the topography is changed from steep to gentle; the aquifers are varied from fissure aquifers to pore aquifers; the land use altered from forest to agricultural land and so on.This study took two piedmont plains with different hydrogeological backgrounds, where are Nasunogahara area and Ashikaga area of Japan, as examples and used both a tracer approach and a numerical simulation to clary recharge sources of groundwater in the piedmont plains and to estimate the contribution ratios of each recharge source of groundwater. This study presented the spatial distribution characteristics of contribution ratios of recharge sources to groundwater and analyzed the impact factors for the characteristics. Particularly, this study discussed the characteristics of river leakage and their controlling factors. Moreover, this study also proposed a method that using 8 values of stable isotopes as additional simulation and calibration terms except for groundwater tables to improve the outputs of numerical models. The improved model was further checked whether effective for analyzing the groundwater recharge characteristics in piedmont plains and was also checked whether effective for an area with sparse observed hydrometric data.The results indicate that the main recharge sources to piedmont plain aquifers are local precipitation, river water, paddy field water and mountain-block recharged groundwater. Among them, the precipitation is the most important recharge source and the regional mean contribution ratio of precipitation to aquifers is≥60%. Besides, the contribution ratio of precipitation in the most area of piedmont plain is more than 50%. River is an important recharge source to aquifers and the extent of river water-recharged groundwater with more than 50% contribution ratio is up to 1-4 km far from the channel. The large-scale topographic settings are the main reasons caused different distribution characteristics of river-recharged groundwater, while the geological settings and locations are important factors. The contribution ratios of paddy field water and mountain-block recharge are far smaller than that of precipitation and river water. The characteristic of paddy field water infiltration is highly controlled by the distribution of paddy field and the irrigation period. The occurrence of mountain-block recharge is restricted by regional geological settings. The mountain-block recharge is remarkable in a small area in front of mountains and the occurrence of mountain-block recharge diminishes the river water flow from channels to mountain front aquifers. It is effective to improve the outputs of groundwater numerical models through using stable isotopes as additional calibration terms except for groundwater tables. The calibration method is especially useful for numerical models in areas with spare hydrometric data. Combined use of numerical simulation and tracer approach could mitigate or make up deficiency of each approach.