PROBLEM TO BE SOLVED: To provide a method of controlling the underground water level of a field for cultivating the crops, to the predetermined water level so as to be horizontal over the whole field.
SOLUTION: The underground of a boundary part of the field 1 is surrounded by a water-barrier curtain 2 made of soft synthetic resin, vertically to the ground along a ridge 8, and the tip part of a culvert near a drainage canal 6 is provided with a drainage water level regulating gate 5 to control the underground water level of the field 1. The underground water level in the field 1 is maintained to the water level set by a covering cylinder 9 of the water level regulating gate 5, by a combination of the water-barrier curtain 2 and water level regulating gate 5. Moreover, the water level of the whole field 1 is maintained to be horizontal. In the case of draining from an upper overflow port 7 by lowering a drainage cylinder 8 of the water level regulating gate 5 to close a side cock, the supply of air to marl from a drainage hole 11 through the culvert is avoided. Since the underground water level on the drainage canal 6 side is not lowered, there is no supply of excessive air from the upper side, and air supply from the underground through the culvert is cut off, so that the decomposition of the marl on the drainage canal 6 side does not particularly advance, nor does the ground above subside to cause the inclination of a field surface.