Effective use of limited water resources is often difficult in Taiwan due to the steep topography, rapid steam flows and uneven spatial-temporal distribution of precipitation. In addition, the climate changes alter the precipitation pattern of the local hydrologic condition, which creates significant impacts on the groundwater recharge. Therefore, it is critical to accurately assess the interactive recharge from the upstream mountain blocks to the downstream alluvial fan for groundwater resources management. In-situ measuring of stable oxygen and hydrogen isotopes, chemical ion tracer method and numerical groundwater model were applied to determine the relationship between the spatial-temporal variation in rainfall, river water, and groundwater in the upstream mountain blocks and proximal fan area of the Choushui river and the relative influence mechanisms. The results showed that the groundwater mainly recharging from river water both in high and low flow periods and more significant in low flow period. Lighter δ^(18)O discovered in groundwater at certain local regions indicated that recharge sources may come from the lateral recharge of the Choushui river and the upstream mountain blocks. Annual groundwater recharge amount estimated from stable base flow analysis and numerical simulation of Modflow model were 1.095 billion tons/year and 249 million tons/year for the upstream mountain blocks and proximal fan, respectively. Moreover, based on the oxygen isotope material balance result, the percentages of annual groundwater recharge from rainwater and river water in the upstream mountain blocks were 25.9 % and 74.1 %, respectively, and those in the proximal fan area were 37.3 % and 62.7 %, respectively. The quantity of groundwater recharge in the upstream mountain blocks was estimated to be 283 and 812 million tons/year from rainwater and river water, respectively, and that in the proximal fan area was 93 and 156 million tons/year, respectively. The results may serve as a reference for formulating a regional groundwater resources management plan.
臺灣因地形及降雨等分佈不均導致區域水文歷程和地面水/地下水交互補注特性有所變化,因此瞭解其交補關係乃為水資源管理重要課題。本研究藉由現地採樣、氫氧同位素/化學離子示蹤和數理模式,探討濁水溪沖積扇上游山區及扇頂區域雨水、河水與地下水之時空變異關係,並研判地下水補注來源及補注量。根據雨水、河水與地下水同位素/化學離子組成,以及質量平衡法估算地下水補注來源,顯示皆以河水為主;應用天水線及化學離子多變量分析,將補注水源分成山區北區、山區南區及扇頂區域,局部區域地下水有較輕δ^(18)O值,應受濁水溪側向補注和上游山前補注影響;另以穩定基流分析法及Modflow模式估算沖積扇上游山區及扇頂區域地下水年補注量分別為10.95及2.49億噸/年,結合素質量平衡法推估沖積扇上游山區雨水/河水之年補注地下水比例為25.9%及74.1%、扇頂區域為37.3%及62.7%,量化估算上游山區雨水/河水年補注量分別為2.83及8.12億噸/年、扇頂區域為0.93及1.56億噸/年。本研究綜合應用現地採樣及水文定性/定量評估,探討時空地面水/地下水交互補注機制並量化地下水補注量,研究成果可作為擬定地下水資源管理計畫之參考。
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