IMPACTS OF CHANGING RICE IRRIGATION PRACTICES ON THE SHALLOW AQUIFER OF NASUNOGAHARA BASIN, JAPAN

摘要

The challenge for water resources planners is how best to manage the surface water and groundwater resources of a region to prolong their usefulness for present and future generations. This study was carried out in Nasunogahara Basin Tochigi Prefecture, Japan to evaluate the effect of changing rice crop irrigation practices on managing the use of water from the shallow aquifer of the area. The basin is an alluvial fan where paddy fields occupy 40% of the area. 66% of these paddy fields are irrigated by groundwater and are located in the southern part of the basin. The current practice is to carry out rice transplanting and land preparation, which is a high demand period for water, in a short period of 15 days. This short period usually starts in the middle of April and lasts until the beginning of May with the highest water demand is at the end of April and beginning of May. In the early 1990s a trend of lowering in the groundwater table and drying up of the natural springs was observed in the basin. The reasons for this trend are suspected to be low precipitation and the influences of various new developments for residential or industrial land use in the northern part of the basin. This trend and the relatively short duration of the high water demand period caused severe groundwater shortages during the transplanting period in the basin. An integrated surface water- groundwater model was developed as a tool for management of regional groundwater resources and used to examine the effect of prolonging the high demand period. The model simulated the effects of two longer periods of transplanting and land preparation, 30 and 60 days. The simulation results showed that extending the high demand period eased the groundwater shortage significantly. There are no major differences between the effects of the two periods, 30 and 60 days, in improving the water supply. Present institutional and economic constraints make the 60-day alternative impractical.