天然林沖蝕溝的地表逕流及淺層地下水特性

臺灣山高流急且降雨時空分布不均，水資源的永續利用是一個重大的議題，即便山區地下蘊含大量水資源，但國內有關山區地下水的研究侷限於深層地下水，且受制於傳統大型鑽鑿機具的低機動性，使得對於山區淺層地下水的特性仍缺乏高空間解析度的資訊。若缺乏集水區上游的地下水相關研究，則無法釐清源頭的逕流產生機制，因此為了釐清臺灣集水區源頭部淺層地下水的反應特性，本研究利用攜帶式鑽探設備於天然林沖蝕溝鑽鑿7口水井以觀測淺層地下水位，並建置簡易量水堰以利觀測地表逕流量。沖蝕溝的崩積層由土石堆積交雜排列而成，地層結構複雜，無法單憑地層結構判釋淺層地下水深度，需透過攜帶式鑽探設備的直接鑽探才可掌握地下水位深度。地下水動態依地而異，而本樣區呈現淺層地下水位最深與水位變動幅度最大的位置皆位於沖蝕溝中段。中上段為水脈分散處，地表與地下的水文連結性弱；中下段為水脈匯集處，地表與地下的水文連結性強。各測點的淺層地下水補注量不同，中段淺層地下水的流動深度較深，非單純由雨水供給淺層地下水，而上段的雨水則無法有效補注至淺層地下水，使得水位上升量小於降雨供給量。多數測點於淺層地下水位較高時的退水速率較快，且退水期水位較淺之處的淺層地下水位可於短時間內大幅下降。中段的地表逕流與淺層地下水之平均延遲時間差異最大，地表與地下的水文反應速率差異大；其他水位較淺之處的淺層地下水與地表逕流有相近的水文反應。此外，地形指標僅與水位較淺之處的平均淺層地下水位深度有顯著相關，表示地表逕流以伏流水型態流動於淺層地層，地表逕流於枯水期隱沒至地表之下而形成淺層地下水，故水位深度淺之處的淺層地下水反應受地表地形特性控制。本研究證實攜帶式鑽探設備適用於臺灣山區的淺層地下水觀測，有利於提高地下水觀測井的設置密度，以利取得更高空間解析度的水文資料，除了可以釐清源頭集水區的降雨逕流機制外，也有助於山區水資源的評估。

關鍵字

崩積層；伏流水；攜帶式鑽探設備；淺層地下水；地表逕流

並列摘要

Sustainable utilization of water resources is a critical concern in Taiwan because of the island’s environmental features, including its mountainous topography, rapid surface flow, and uneven distribution of rainfall in time and space. Although groundwater resources are abundant in mountainous areas, most studies on groundwater monitoring in the mountainous environments of Taiwan have focused on deep groundwater owing to the low mobility of the traditional drilling equipment. By contrast, high spatial resolution information on shallow groundwater characteristics remains lacking, which hinders the clarification of runoff generation mechanism. To determine the dynamics of shallow groundwater in headwater catchments of Taiwan, this study installed seven groundwater wells using portable drilling equipment and constructed simple weirs to monitor surface runoff at several locations along a gully in a natural forest. The geological structure of the gully classified as colluvium comprised piles of soil and gravel. The depth of the groundwater table could not be estimated using geological structure information but detected directly via boring using the portable drilling equipment. The dynamics of the groundwater varied with the locations along the gully. As for this study site, both the positions of the deepest point and the largest variations of the shallow groundwater table were at the middle slope. Besides, shallow groundwater pathways were divergent at the upper-middle slope where the hydrological connectivity between surface runoff and shallow groundwater was weak. By contrast, shallow groundwater pathways were convergent at the lower-middle slope where the hydrological connectivity between surface runoff and shallow groundwater was strong. The amount of recharge also varied with the locations along the gully. Apart from rainfall, other sources also recharged shallow groundwater at the middle slope where the flowing depth of shallow groundwater was deeper. Rainfall could not recharge shallow groundwater at the upper slope effectively, so the rise of the water table was less than the amount of rainfall supply. A higher recession rate occurred at the higher shallow groundwater table at most positions, and the shallow groundwater table decreased drastically in a short period of time at the positions where the depth of the water table was shallower. The difference between the lag time of surface runoff and shallow groundwater was highest at the middle slope where the difference between the speed of the hydrological response on the surface and in the subsurface was the largest. By contrast, the hydrological response of surface runoff and shallow groundwater was similar at the positions where the depth of the water table was shallower. Moreover, the topographic index was only significantly correlated to the average shallow groundwater table at the points where the water table was shallower, implying that surface runoff flowed in the shallow layers with the hyporheic flow pattern. During the dry period, surface runoff in the gully flowed below the ground surface and became shallow groundwater. Therefore, the hydrological response of shallow groundwater with the shallower water table was controlled by surface topography characteristics. This study demonstrated that the portable drilling equipment can be effectively employed for shallow groundwater observations in the mountainous areas of Taiwan for increasing the density of observation wells for higher spatial resolution hydrological data. With the help of this approach, we can not only clarify the mechanism of rainfall-runoff processes in headwater catchments but also evaluate the water resources in mountainous environments.