In this study, the conservation of a spring in groundwater recharge by rainwater is investigated using a hydrological model which includes the direct runoff, infiltration, evapotranspiration, groundwater recharge, and groundwater runoff for simulating the total runoff from the rainfall. The Kokubunji basin is chosen the study area. In the basin, the direct runoff from impervious areas discharges to one main stream, and the groundwater runoff originates from the unconfined aquifer. We assume that the groundwater runoff rate is proportional to the second power of the amount of stored groundwater, and the groundwater recharge is proportional to the soil moisture excess. The total runoff is simply obtained from the sum of the direct runoff and the groundwater runoff. The groundwater recharge in the Kokubunji basin is restricted because of permeable inlets through which the rainwater from roofs penetrates into the soil. Hydrological data from a rain gauge and a stream flow gauge in the area are used for analyses of the rainfall-runoff relationship. Computations were carried out by using the 1995's hydrological data for 1000, 2000, 3000, and 4424 permeable inlets. The effects of the conservation of the spring in groundwater recharge by rainwater increase 26%, 52%, 78%, and 116% to the total runoff for 1000, 2000, 3000, and 4424 permeable inlets, respectively. The hydrological model has good applicability when calculated at intervals of one hour.
