地下水補注量推估之研究-以濁水溪沖積扇為例

Abstract

全台灣地區蘊有豐富之地下水資源，要如何適當地開發地下水資源是為一相當重要之議題；若能合理推估出地下水資源之蘊藏量及流動方式，便能妥善規劃利用地下水資源。濁水溪沖積扇以農、養殖業為主，未登記私井大量超抽地下水，造成地層下陷、土壤鹽化等問題。隨著高鐵的興建，在高鐵通過之土庫、元長一帶，地層有著嚴重下陷情形。本研究建立濁水溪地區地下水文數值模式，使用MODFLOW模式推估其淨補注量、側流補注量，並使用SWAT模式推估地表補注量。聯合MODFLOW與SWAT之結果推求抽水量，並且與水利署登記之地下水水權量做比較，推估該地區未登記抽水量。本研究亦針對過去研究建議之補注區設立地點，使用SWAT模式，以人工湖的形態，模擬各不同補注區之地表補注效益。最後，將高鐵沿線行經土庫、元長一帶之地點設立為補注區，討論沿線兩側3公里及1.5公里大小之人工湖補注效益。本研究推估出濁水溪地區於1999~2002年之地下水水文資訊。平均每年地表補注量為12.378億噸、側流補注量為8.957億噸、非法抽水量為11.941億噸。在補注區案例模擬中，因補注區設立位置不同，其高程、附近土壤類型及土地利用型態亦不相同，造成在補注量之模擬上有明顯之差異。在扇頂區補注效益最高，平均每年每平方公里有100.88萬噸之補注量，補注效益最佳；同屬扇頂之古坑、斗六區補注效益亦佳，每年每平方公里約有94.09萬噸之補注量；而附近多為細質土壤之北港溪流域補注效益最差，每年每平方公里僅有53.28萬噸之補注量。高鐵補注區模擬中，沿線兩側3公里補注區平均每公里總補注量為139.38萬噸，地表補注效益為每年每平方公里80.78萬噸；沿線兩側1.5平方公里之補注區平均每平方公里總補注量為141.29萬噸，地表補注效益為每年每平方公里79.72萬噸。因其位置接近，故其單位面積補注量差異不大。

The groundwater resource is abundant in Taiwan and it is making a critical issue to exploit groundwater resource properly. Groundwater resources could be well managed and utilized if the storage and movement of groundwater is rationally evaluated. The continually declining groundwater level in Chou-shui river alluvial fan is mainly caused by the unregistered pumping by private wells due to enormous water demand from agriculture and aquaculture, in turn leading to land subsidence and soil salination. In addition, the construction of Taiwan High Speed Rail (THRS) has caused serious land subsidence in Tuku and Yuan-Chang township. This research estimated ground water storage, recharge, and withdraw rate in Chou-Shui river alluvial fan by developing numerical model with MODFLOW and SWAT. Unregistered withdraw by private wells in this area are investigated through comparing registered groundwater rights from Water Resources Agency with simulated discharge. Finally, the recharge efficiency of artificial lakes with radii of 3 km and 1.5 km along the THRS were addressed. Results show that, during 1999 to 2002, averaged recharge in the Chou-Shui river alluvial fan is 1.24 billion tons/year, side-stream recharge is 895.7 million tons/year, and unregistered pumping from private wells is 1.1941 billion tons/year. Scenarios with different elevation, soil, and land use type lead to diverse recharge rate. Highest recharge rate is found at the proximal fan, which is 1 million tons/year/km2. Moreover, recharge rate is 940 thousand tons/year/km2 at Gu-Keng and Dou-Liu, and 532 thousand tons/year/km2 near the Pei-Kang river, the worst case. The locations of two THSR recharge scenarios are close, result in similar recharge rates, which are 807.8 thousand tons/year/km2 for the 3 km lake and 797.2 thousand tons/year/km2 for the 1.5 km lake.