Multicell Model for Ground Water Recharge Analysis: The Ibi River Basin, Japan

Abstract

A coupled water quantity/quality model was developed and applied to estimate ground water recharge and simulate tracer concentrations in the Ibi River Basin of Japan. The ground water basin was divided into five sub-basins, each of which contains a surface reservoir and up to three subsurface reservoirs. The reservoirs exchange water and solute mass among themselves and contribute direct runoff and base flow to adjacent streams. The simulation model is parameterized in terms of 14 water quantity and three water quality parameters. A parameter calibration procedure was implemented wherein streamflow and tracer concentration data were used in parameter estimation. Prior information on the parameters can be used to initialize and constrain calibrated parameters. The calibrated model was validated successfully and used to simulate Na+ and Ca2+ and to estimate ground water recharge in the sub-basins and the Ibi River Basin as a whole. Ground water recharge varied substantially among sub-drainage basins, reflecting the predominant role of precipitation variability in determining deep percolation to the first confined aquifer. It was determined that about 10% of the gross precipitation becomes ground water recharge over the entire Ibi River Basin. Analysis of ground water recharge to a large pulse of precipitation demonstrated the important effect of impermeable clay layers in controlling the magnitude and timing of deep ground water recharge. The validation results indicate that the equilibrium concentrations of Na− and Ca2+ in the first confined aquifer have negligible sensitivity to their initial concentrations in the simulation model. Similar behavior was observed for the equilibrium concentrations of these two tracers in reservoir 3, and to a lesser extent in reservoir 2. The top reservoir exhibited large fluctuations in tracer concentrations due to the effect of direct rainfall input.