Abstract
Efficiency of subsurface drainage in paddy fields depends on the permeability of soil above filler layer. The objective of present study is to clarify the variability of the percolation rate along subsurface drainage. In eight paddy fields, six of which are used for rice cultivation and the others used for upland crops, cylinders having a diameter of 27.8 to 29.0cm were inserted to 18cm deep at intervals between 1 and 5m along the subsurface drainage pipe. The percolation rate in stable state (5 minutes after the start) was evaluated from the measurement of dropping water level in the cylinders. The percolation rate varied considerably with the location in the fields, and spatial correlation was also revealed. The influential ranges were estimated between 2 and 12m depending on the experimental plots. The percolation rate decreased as the filler depth increased when the filler depth was less than 20cm. The influential ranges of the filler depth were estimated between 4 and 10m, which had good correspondences with those of percolation rate. However in the fields where the measured filler depths exceed 20cm, no correlation was observed between the percolation rate and the filler depth. The observations of the crack depths around the location for the measurement revealed that the balance between the crack depth and filler depth determined the accessibility of surface water to the filler layer.
