In measuring the groundwater level with a perforated pipe, the water surface may be formed in the pipe even if the groundwater table does not exist under the ground. In the case with the existence of the groundwater table, the groundwater flow is disturbed due to the flow in the pipe. And inversely the water level in the pipe belongs to this disturbed groundwater flow. Thus, there is a possibility that the water level in the pipe may not be regarded as the true level of groundwater. In this paper, the variations of the flow and level of the groundwater were analyzed in the case of a perforated pipe which is installed in a three-dimensional groundwater flow. At first, the physical meaning of the water level in a perforated pipe installed in the paddy field was studied and it was confirmed that the water surface in the pipe is located at the level where the inflow rate into the pipe equals to the outflow rate out of it. This mechanism may be valid for the pipe installed in various types of groundwater flow. In consideration of this mechanism, the influences of the radius and length of the pipe upon the variations of the groundwater flow were investigated. It was concluded that the variations become greater with the larger radius of the pipe, with the deeper depth of the pipe below the groundwater and in the case of the steeper slope of the groundwater table.
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