Abstract
When the crop rotation is carried out in sloping paddy field areas, the groundwater table often rises with the seepage from the surrounding paddy fields. Consequently, it becomes one of the most important problems to lower the groundwater table and to improve the drainage conditions of such a rotational field. In this study, we have considered the groundwater table in the rotational field in sloping paddy field areas, when drain pipes were buried at the toe of the levee.
At first, the flow region bounded by an impervious boundary at a finite depth was considered. After the groundwater table was transformed into a straight line using the Zhukovsky's function as shown in Fig. 1, the flow problem was analyzed to obtain a theoretical equation of groundwater table by the theory of conformal mapping. And as a particular solution, a theoretical equation when the impervious boundary was infinitely deep was also derived.
Next, the results calculated from these theoretical equations were compared with the observed data at the Higanden test field in Okayama Prefecture, and the results have shown a good agreement with each other (Fig.3).
From the above calculations, it was found that the groundwater table is affected by the depth of the impervious boundary, namely, the groundwater table would fall deeper as the impervious boundary became shallower. However, the effect was not so large that the results obtained from the assumption that the impervious boundary was infinitely deep were applicable well for practical purposes.
The results of discussion are shown in Figs.4 and 5. In order to lower the groundwater table of the rotational field, the drain pipes were to be buried along the toe of the levee of upper paddy fields as near as possible. Moreover, they must be buried as deep as 1 m, which is rather deeper than in the case of level land.
The groundwater table was much affected by the slope as shown in Fig.6. In the sloping areas steeper than 1/20, the groundwater table could be lowered by only one drain pipe at the toe of the levee. In a more moderate slope, mpre than two drain pipes were necessary to lower the groundwater table.
