Functional Evaluation of Groundwater Level Decrease in Non-sloped Subsurface Drainage Systems in Upland Field on Peatland

抄録

In lowland farmland, the maintenance cost of conventional inclined subsurface drainage systems is high because the drainage pipes must be laid at depth, or the tile deepens as the farm field enlarges. To reduce the maintenance cost, the Hokkaido Provincial Government of Japan has introduced a subsurface drainage system with a smaller inclination angle than the conventional method. However, the drainage effects of non-sloped subsurface drainage systems have been rarely reported, and insufficient evidence has prevented their widespread acceptance. The present study investigates how the slope of the drainage pipe influences the groundwater-level decrease in poorly drained fields. The survey was conducted in the upland field on peatland located in the Ishikari River Basin, Hokkaido. In 2015, two types of subsurface drainage systems were constructed in the same lot: a conventional subsurface drainage (sloped area) system, and a subsurface drainage system with low inclination (non-sloped area). After measuring the groundwater level at 12 points and the precipitation from 2016 to 2017, we found that: i) the groundwater level was higher in the non-sloped area than in the sloped area, ii) between 2016 and 2017, the groundwater level decreased in the sloped area and rose in the non-sloped area. The above results suggest that the efficiency of decreasing the groundwater level during 3 years after the construction was lower in non-sloped subsurface drainage than in sloped subsurface drainage, which is different from that observed in the previous study.