Combined use of tracer approach and numerical simulation to estimate groundwater recharge in an alluvial aquifer system: A case study of Nasunogahara area, central Japan

•Distribution of groundwater recharge is mapped in an alluvial aquifer system.•Isotopes (18O, D) and hydraulic head are used to improve model calibration.•The results from the model and a tracer approach were compared to validate the model.

SummaryIn this study, we simulate the spatial and temporal distribution of groundwater recharge in an alluvial aquifer system in the Nasunogahara area of Japan. Natural stable isotopes (18O, D) were considered as additional calibration targets in a numerical model. The reliability of the model outputs was further validated by comparing the results from the numerical simulation and an independent tracer approach. The results indicated that the calibrated model can effectively simulate the spatial and temporal characteristics of the contribution ratios of recharge sources to groundwater in the Nasunogahara area. However, the tracer approach (i.e., end member mixing analysis) provided more reliable results at point scale, particularly for the estimated contribution ratios of paddy field water. The precipitation in the Nasunogahara area is the major recharge source; its mean contribution ratio is 58% for a one-year period over the entire alluvial fan. River seepage is significant in the upstream area of the alluvial fan, and the contribution ratio of river waters along the river channels in the upstream area increases during the wet season. Paddy field water is a highly important recharge source in the midstream and downstream areas of the alluvial fan, and the contribution ratio of paddy field water obviously increases from dry season to wet season because of irrigation. This study demonstrates that combined use of the tracer approach and numerical simulation with stable isotopes as additional calibration targets can eliminate their respective limitations and can assist in better understanding the groundwater recharge mechanism in alluvial aquifer systems.