2013 Volume 26 Issue 2 Pages 99-113
Quantitative assessment of groundwater-level influencing factors such as land use and pumping schemes is important to achieve sustainable groundwater use. This study examined the Tedori River Alluvial Fan in Ishikawa Prefecture,Japan, where groundwater levels have long been declining. Observations and numerical simulations using MODFLOW reveal some remarkable results. Comparisons of groundwater level observations during the irrigation and the non-irrigation periods revealed that groundwater levels during the irrigation period are about 5 m higher than during the non-irrigation period, which implies that paddy irrigation water contributes significantly to groundwater recharge.Furthermore, groundwater levels during the non-irrigation period have declined 5 m during 1993-2009. Discharge observations of the Tedori River confirm that much river water infiltrates into shallow groundwater. A groundwater simulation model was developed considering the observed data. Steady state groundwater flows in the irrigation period for several scenarios of land use and pumping schemes were set based on past, present, and predicted future human activities. Simulated results indicate that groundwater flows in the upper and middle parts of the fan are affected considerably by paddy area and pumping discharge. Especially, in cases where the paddy field area is smaller by more than 30 % from the present condition, the groundwater levels were not maintainable by pumping regulations. Drawdown of the water level of the Tedori River also causes a slight groundwater level decline along the river.