Abstract
For the normal growing and high yield of field crops, not only the lowering of ground water table but also the reasonable control of soil moisture in the root zone cannot be disregarded, especially in the field rotated from the paddy field, because of the less developed structureandhighly compacted profile of the soil. In this paper, therefore, the soil moisture profilesabovethegroundwater table are discussed in order to make clear the necessary amount of the drawdown of ground water table in subsurface drainage.
From the laboratory study using a polyvinyl-chloride cylinder with 100 cm in length and 30 cm in diameter, in which homogeneous soils were filled as shown in Fig. 1, it was found that thesoilmoisture profiles above the ground water table are different in the lowering and rising processes ofgroundwater table, i. e., it has a hysteresis. The soil moisture profile in the lowering processshowsacertainstate based on only the height from the ground water table as shown in Fig. 3, but it does not depend on the depth of ground water table.
Fig. 5 shows that the soil moisture profile above the ground water table in the lowering process obtained from the laboratory study has a good agreement with the results of numerical integration of equation (6), which is derived from the potential flow theory. And as a result, it follows thattheevaporation from the soil surface is more affacted by the atmospheric circumstances than the depth of ground water table, when it is shallow such as the case discussed in this study.
It was found from the field observations shown in Fig. 6, which were carried on in three fields of different soil types and topographical features, that the soil moisture movement from the ground watef table to the soil surface caused by the evapo-transpiration becomes a steady-state flow after between5 and 10 days from a rainfall. And then the soil moisture profile was approaching a certain state which did not depend on the depth of ground water table. The profile of this state could be also shown bythe same equation as that of the laboratory study, as shown in Fig. 7.
As the result of this study, therefore, the necessary amount of the drawdown of ground water table for the control of soil moisture in the root zone can be cleared-up. For example, in order to keep the tension of soil moisture in the root zone of the investigated fields more than pF 1.5, which is the critical point for normal growing of crops, the ground water table must be lowered more than 50-60 cm from the soil surface. This value is, however, affected not only by the depth of root zone but also by some factors such as rainfall and evapo-transpiration. The result of this study may be usable for determination of the above-mentioned value in any soil.
