濁水溪沖積扇補注量與抽水量空間分佈模式建立

Abstract

濁水溪沖積扇係台灣農業重鎮又是新興工業發展區域，地面水源已無法滿足需求，地下水的供給為一重要供水來源。然為避免地下水不當抽用而造成地層下陷等不良後果，地下水資源之抽用應進行適當之管理，而各種管理策略是否有效，必須利用率定良好（Well calibrated）之地下水數值模式進行評估。欲建立率定良好之地下水數值模式，必須完成以下兩項工作：一是利用資料融合（Data fusion）技術，進行系統辨識；二為建立水文地質參數、補注量與抽水量之空間分布。 本研究首先蒐集研究範圍之現地基本資料，進行濁水溪沖積扇之地下水平衡分析，以推估區域性地下水之補注量與抽水量，接著利用MODFLOW建立濁水溪沖積扇補注量與抽水量空間分佈之模式。模式建立的過程，包括模式的架構，水文地質參數及邊界條件之設定，補注量與抽水量之給定，最後比較模式模擬水位與實際觀測水位進行模式的率定。補注量分為河床入滲補注、降雨補注、灌溉用水補注三部份設定之，並考慮研究區域內河床質地資料、土壤質地資料、耕作制度等因素，將補注量合理分配至模式網格中；抽水量分為公井抽水與民井抽水兩大部份設定之，公井抽水量分別依據自來水公司及農田水利會紀錄資料給定，民井抽水量則以地下水平衡分析結果配合水井清查資料設定之。 研究結果顯示，本研究於補注量及抽水量設定部分，詳細考慮諸多因素，對於濁水溪沖積扇地下水流的情形能更為真確的反應，具有相當之參考價值。模式經率定後，各含水層抽水比例有所變動，並發現第一含水層之民井抽水量可能有超估的情形，而第三、四含水層深層抽水情形較難掌握，需增設觀測井並進行更詳細之水井清查。

Chou-Shui River alluvial fan is the agricultural center and emerging industrial area of Taiwan. Surface water has been unable to meet the rising demand, and groundwater has become an important source of water supply. However, groundwater resources should be appropriately managed to avoid land subsidence and other adverse consequences caused by improper pumping. To achieve this goal, we need a well calibrated groundwater numerical model to assess the effectiveness of various management strategies. To establish the numerical model, we must complete the following two tasks: First, utilizing the data fusion technology for groundwater system identification; Second, integrating site survey for clarification of spatial distribution of hydrogeological parameters, recharging, and pumping. Thus, in this study, we present a two-stage procedure to establish a representative groundwater model. In the first stage, groundwater-hydrograph analysis is applied to evaluate the water balance of Chou-Shui River alluvial fan, and the total quantity of pumping and recharging is obtained. In the second stage, we used MODFLOW as the groundwater flow simulation model to correctly distribute the quantity of pumping and recharging into different space and time. The model establishment process includes aquifer stratification, hydrogeological parameters and boundary conditions setting, pmping and recharging distribution, and groundwater level observation fitting. The recharging is subdivided into three hydrologic components including rainfall infiltration, river leakage, and groundwater irrigation seepage. The subdivision is implemented according to riverbed materials, soil textures, and cropping systems. The pumping is subdivided into two components including public water supply pumping (involving pumping of both Water Corporation and Irrigation Association) and private pumping. The setting of each pumping is based on official pumping records and pumping well census results. The results show that, the groundwater resources of Chou-Shui River alluvial fan can be more properly distributed with the detailed settings of pumping and recharging. Furthermore, the quantity of pumping from first shallow unconfined aquifer might be overestimated before; the local pumping behavior in F3 and F4 are not clear, having to add the observation wells and more detailed pumping well census.