On the Relationship between the Water Level in the River and the Water Table Depth of Shallow Groundwater in Paddy Fields

Introduction In paddy fields, the water table depth of shallow groundwater fluctuates through irrigation. Irrigation water, conveyed usually by the river, is distributed into paddy fields through irrigation canals. Therefore, by using data obtained from continuously measuring the water depth of the river, the water table depth of shallow groundwater in paddy fields can be predicted. Using data showing the water depth of the Aya River in Kagawa Prefecture, a prediction of the water table depth of shallow groundwater in paddy fields in the lower part of the river was attempted. Method The water table depth of the shallow groundwater in the study area was daily measured by using observation wells (Fig. 1). Data showing the water table depth during the six years from July 1964 to June 1970 was used for this analysis. The water level in the Aya River was observed one time daily at the Kamogawa Measuring Point of the Kamogawa Water Purifying Plant of the Bureau of Water Supply of the Sakaide Municipal Office. The Measuring Point is located about three km up the river from AR-5. Data, from July 1964 to June 1970, showing the water level of the Aya River, was obtained at this Measuring Point and used for this study. Results and Discussions As shown in Fig. 1, a low dam to protect the paddy fields from seawater intrusion at high tide has been constructed near the Kumoi Bridge in the lower part of the Aya River. Below this low dam, fluctuation of the water level in the Aya River is definitely affected by tidal motin. It is known that the groundwater fluctuates in response to ocean tides on the coast (1, 4). And as reported in the previous paper (2) , data (obtained from observation wells located in the lower part of the study area below the low dam) showed that the fluctuation of the water table of the shallow groundwater was more affected by tidal motion than by precipitation. Therefore, data obtained from the ten observation wells, such as ER-1 and ER 2, FR-1 to FR-5, EL-1 and EL-2, and FL-1 was not used for this analysis. In order to see the relationship between D and H, a figure, as shown in Fig. 2, was