Runoff analyses are important to efficient l y manage the watersheds such as precise prediction of discharge. Runoff is mainly composed of surface and groundwater flow components, therefore hydrological conditions should be well described before any runoff analysis. In this study, runoff analyses were performed for two small sub-basins of a mountainous catchment of Tono area Japan with the aim to forecast runoff after 1 and 1/2 hours by using 3 different numerical models and performances of these models were compared to each other. The runoff and other meteorological data have been collected in these sub-basins over the last 14 years. The effect of the basin area on the prediction time of runoff and the seasonal data impacts were also investigated. For the analyses, a new approach of training artificial neural network model (ANN) with real coded genetic algorithm (GA) named as GAANN model is proposed. The results of this model were compared with famous back propagation artificial neural network (BPANN) model and with multivariate autoregressive moving average model (MARMA). It was found that for very small catchments seasonal effect on the runoff is dominant and this effect should be considered for obtaining better forecasting estimates. It was also found that estimation by ANN models was better than MARMA model for analyzing the responses to intense rainfalls in summer, whereas the results were almost similar for the light rains of winter season. The accuracy of the forecasts after several time periods in future was also investigated and found to decrease as the time period is increased. The results showed that GAANN and BPANN models almost provided similar prediction estimates in a very small mountainous watershed when precisely measured dataset was used. Modelling advantages of using genetic algorithm instead of back propagation for the training of ANN models are also highlighted.

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It has been pointed out that groundwate r in the Nasu Fan is recharged by various processes including influent stream, irrigation at paddy field, and rainwater infiltration. We evaluated quantitatively the contribution from different sources of groundwater using isotopic tracers and a 3component mixing model, with special focus on the central part of the fan. The result showed that the Sabi River recharges great portion of groundwater (approx.60%) in its adjacent area. In contrast, less contribution from the Houki River and no contribution from the Naka River and Kuma River were found. Except for the area adjacent to the Sabi River, the principal source of groundwater was estimated to be rainwater fallen onto the fun. Unexpectedly, contribution from irrigated water at paddy field was generally less than 15% because there are few paddy fields in the fan apex area.

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Intensive observation of spring and stream flow was performed under the base flow condition during the wet and dry seasons to clarify the characteristic of the groundwater flow in Ontake volcano, central Japan. The springs with discharge rate of more than 50 L/s were located only in the younger volcano zone which constitutes the upper part of the volcano body. This indicates that the groundwater flows at shallow depth along the lava flow. In the younger volcano zone, the groundwater flow depends on the erosion condition of the volcano body. Thus the groundwater flow system with longer residence time may exist in the north area of the younger volcano zone with conical topography. In the older volcano zone in the foot of the volcano body, the discharge rate of springs was extremely low and its water chemistry was much different from that of the younger volcano zone, suggesting existence of the regional groundwater flow system. The mean discharge rate of base flow during the dry season were 5.7 mm/day in the north area of the younger volcano zone,4.5 mm/day in the south area of the younger volcano zone, and 2.1 mm/day in the older volcano zone. This suggests that storage capacity of the groundwater decreases with increasing age of the volcano body. In the younger volcano zone, the erosion of the volcano body might cause the change in the groundwater flow path and the decrease in groundwater storage capacity.

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