溪頭森林集水區地下水文特性之研究

Abstract

山坡地水文研究已將近有六十年的歷史，但水文收支的演算因時代背景，地理環境等時空上的差異而有極大的變化，其數據始終無法獲得合理的共識。其原因除了降雨時空分佈的不均勻，蒸發散量的不確定性之外，最主要的因素是在地下水文機制的不明，尤其是地表下未飽和帶及土壤水分的動向，是今後山地水文研究上的重要課題。 因此本研究以台灣中部台大實驗林集水區為研究對象，針對溪頭集水區山腹崩積層，有效孔隙的水文特性進行調查分析，從降雨時空分佈的觀測到水位變化的測計，尤其在地震、颱風及土石流以後，水脈變動的追蹤，以及定量上的把握進行研究，對今後山地水資源的確保，將有極大之助益。 除地理環境的空間差異特性之外，山地集水區受到降雨時空分佈的影響，有不同的水文反應特性。以溪頭為例，濕季降水補注淺層伏流水稽延時間，在土石流坑溝觀測井最高可延續6天；山麓崩積層則經常延續7天以上。乾季時無明顯補注相關，主要受到調查時間點及水井深度的限制。 崩積層相對於土石流坑溝有較長的蓄水稽延時間，顯示崩積層蓄水機制較土石流坑溝連續。由於土石流坑溝內的水文補注與退水都較快速，利用檢層或抽水回復試驗未發現飽和層流的現象，顯示土石流坑溝地下伏流水呈現局部飽和而分散竄流，既有水井調查法應用困難。因此本研究以水位變動觀測法進行山區地下水調查。結果顯示，溪頭屬於二次堆積的山地集水區水文地質環境，本研究以地下水位觀測法輔助量水堰逕流量測計，反映逕流與伏流無論在補注量與退水延時均有重要差異，是補足山地集水區水文收支演算對地下水文動向不明朗的突破關鍵。

Even though Slope Hydrology has been developed for 60 years, Hydrologic budgets are still not very clear because of some environmental characteristic variations such as erratic rainfall distribution, evapotranspiration rates and so on. The mainly reason of the uncertainty is unknown subsurface water transportation mechanisms. Those mechanisms include water through vadose zones, unsaturated water flow, and changes in soil moisture etc. All these parameters are important subjects of Slope Hydrology now and in future. This study proposes to take Chi-Tou experimental forestry watershed which belongs to National Taiwan University in central Taiwan as the research site. Aim to investigate groundwater moving through effective porosities of debris flow valleys and colluviums slope lands. The precipitation space-time distributions, water levels change in wells, and soil moisture rates are recorded. All these works will be done for water conduit variations tracing and groundwater quantitative analysis which are meaningful to water resources preservation. Different rainfall distributions will cause various hydrology processes in mountainous watersheds like Chi-Tou watershed. During the wet season, subsurface water recharge from precipitations immediately, the water levels in wells grow higher in wet season, and the subsurface detention time takes longer than that in dry season. The recession times of water level change in wells take up to 6 days in debris flow valleys while they take more than 7 days in wells at colluviums slopes. It is difficult to investigate well water recharge from rainfall during dry season, because wells designs are not deep enough and lack of auto data log systems in site. Owing to the longer subsurface detention time, we can see hydrology processes at colluviums slopes and debris flow valleys are vary. The subsurface water storage system might be more continuous in colluviums slopes than in debris flow valleys. Groundwater logging and pumping tests are done to figure out the properties of through flow in debris flow valleys, but no layer flow has been observed. It means there are partially saturated flow through porous environment and have no regular path way. We try to invest the hydrologic properties by investigations of water level changes in wells to realize subsurface flow characteristics in mountainous watersheds. Consequently, the hydrogeology of Chi-Tou watershed is complex deposition. Water level change data in different wells provide well assistant to Weir runoff gauge. To understand the relationship between run off and subsurface flow after rainfall recharge events is an important key to uncover the uncertainty of unknown subsurface water transportation mechanisms in mountainous watershed.