[O-08] Poster presentations by senior high school students

[O08-P70] Development of a new groundwater level prediction model based on tank model : An example for Otomeyama Park in Shinjuku City

*Taichi Shimokobe¹ (1.Kaijo Senior High School)

Keywords:spring, groundwater level, tank model

Located on the Ochiai cliff line, the northern cliff of the Kanda River, Shinjuku City' s Otomeyama Park is the only existing spring in Shinjuku City. The spring in this park is valuable in the current situation where many springs have disappeared in Tokyo due to the increase in paved areas and the decline in groundwater level caused by groundwater pumping, and in order to conserve it, it is effective to understand the groundwater recharge and flow processes in the area, and to develop a model to predict discharge and groundwater levels. For this purpose, we have been monitoring the discharge from the spring almost daily for more than 10 years, and have been monitoring the groundwater level in two wells (Wells A and B) in the recharge area. As a result, in Well A, the water level was observed to rise during rainfall and then fall, approaching a certain level, which is typical of unconfined groundwater, while in Well B, a unique fluctuation was observed in which the water level fell rapidly after falling below a certain level (T.P.28.9m). The geology of the area consists of Musashino Gravel Layer, Tuffaceous Clay Layer, and Musashino Loam Layer from the lower part, and the Musashino Gravel Layer and Musashino Loam Layer are the aquifer layers. Since T.P. 28.9m is the lower end of the Musashino Loam layer and of the two wells, only Well B reaches the depth of the Musashino Gravel layer, we infer that the unique water level fluctuation at Well B is caused by the influence of the two aquifers on its water level. That is, during periods when the water table exists within the Musashino Loam Layer, the water level in Well B simply reflects the water table, but during periods when the water table does not exist within the Musashino Loam Layer, the water level in Well B reflects the hydraulic head of groundwater in the Musashino Gravel Layer. In order to examine this mechanism of the unique water level fluctuation more quantitatively, we designed a model to simulate the discharge from the spring and the groundwater level from precipitation. In the past, a study has been conducted to examine the causes of long-term changes in the discharge by relating groundwater levels in Well A and discharge from the spring to the lower tank water level and discharge from the tanks, respectively, in a two-stage tank model in series (Takano, 2015). However, since the unique water level fluctuation in Well B cannot be expressed simply by using the same way, we devised a new model by applying the tank model. In the new model, Another tank representing a well was added in parallel to the regular in-line tank model, so that the water level in the tank representing the well was affected by the water levels in both of the two tanks. The groundwater levels of Wells A and B and the discharge at Otomeyama Park were related to the model, and the parameters of the model were calibrated from the observed data using the SCE-UA method. The results showed that the model was able to represent groundwater levels and discharge from the spring with high accuracy, and verified that the estimated water level fluctuation mechanism was valid. It is also expected that the model can be used to predict future groundwater inflow and groundwater levels.