Abstract
This study was conducted to clarify a change of shallow groundwater utilization after the establishment of large scale irrigation system built in 1975 in the Doki-river alluvial fan, Kagawa Prefecture. As the shallow groundwater is used for the irrigation of rice cropping from springs named as "Desui", the water level and discharge of springs and water balance in a research field were surveyed in a severe drought year, 1994 and in a normal-weather year, 1995. The results were: 1) Most of the springs are distributed in the wash trench of alluvial fan. 2) Water level of springs kept high from June to October and low from November to May relating to the supply of irrigation water. 3) Water discharge from springs increased in the irrigation period, and decreased or disappeared in the non-irrigation period. The flow velocity of the shallow-groundwater was calculated to be 11m d^<-1>. 4) The mean discharge of spring water per unit area of research field was estimated at 6.1mm d^<-1>, from which the repeated use ratio of spring water to the percolation water was calculated at 54% in the drought summer 1994. The ratio in the summer of normal-weather 1995 became as large as 96%, calculated from mean discharge of spring water 13.3mm d^<-1>. 5) The repeated use ratio of channel water to the percolation water was 31% in the summer 1994 and 12% in the summer 1995, whish showed a decrease compared with the value of summer 1956. 6) These results could be considered that the shallow ground water utilization through springs increased after the construction of large scale irrigation system.
