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