集水區源頭部基岩面上飽和帶在降水中的時空變動

Abstract

森林源頭集水區是水文變動敏感、土砂移動盛行的區域，基岩面上飽和帶的分布和連通性與逕流量有很高的相關性。前人研究中指出基岩面上的飽和帶有由下而上和由上而下兩種擴張類型，而飽和帶的擴張受土壤厚度、基岩面地形所控制。本研究透過高密度觀測集水區源頭部基岩面上孔隙水壓的時空變化，探討飽和帶在降雨期間的擴張模式，並推測飽和帶擴張的機制。研究結果顯示土壤厚度具有很大的空間變異，基岩面地形相較於地表面有很大的起伏，特別在坡面中、下段。推測坡面中段岩層曾向下滑動並堆積於坡面下段，因此坡面中、下段基岩面應較坡面上段還破碎。飽和帶的時空變動，在降雨發生後，除了由坡面下段沖蝕溝及左側凹地向上擴張，亦於坡面上段有獨立的飽和帶發生並向下擴張，當累積雨量更大時，上下坡段的飽和帶會透過坡面中段的飽和帶通道互相連通。土壤厚度是主導飽和帶發生先後順序的重要因子，飽和帶擴張的主要機制是水分垂直滲透及沿基岩面向下移動的飽和側向流。坡面中段除飽和帶較晚發生，多數測點在降雨中未達飽和，推測是坡面中段基岩面較破碎，水分會往岩層中滲漏，降低坡面上下段飽和帶的連通性。本研究提出在探討源頭集水區地表下飽和帶的變動情形，除了土壤厚度、基岩面地形外，基岩面的水文特性如破碎程度也應納入考量。

A headwater catchment is an area with variable hydrological characteristics and sediment transportation. Many previous studies indicated that stream flow is highly correlated with the distribution and connectivity of the saturated zones at the soil–bedrock interface. Subsurface saturation would expand downslope or upslope during rainfall events, which is controlled by soil depth and bedrock topography. In this study, we measured the pressure head at the soil–bedrock interface with a high resolution during rainfall events to clarify the pattern and mechanism of the expansion of subsurface saturation. The result showed that there are great spatial variations in soil depth, and the bedrock topography is more uneven than surface topography, especially at the middle and the lower parts of the slope. Based on surface and bedrock topography, we presume that shallow landslide occurred at the middle slope and sediment moved from the middle slope toward the lower slope. Consequently, the soil–bedrock interface at the middle and lower parts of the slope could be more broken than that at the upper part of the slope. The subsurface saturation at the upper slope would generate locally and expand downward, and subsurface saturation at the lower slope (i.e., gully) would expand to the middle slope in the early stage during the rainfall events. In the later stage when the rainfall amount increased, subsurface saturation at the upper and lower slopes would be connected at the middle slope. The sequence of the generation of subsurface saturation was much related to soil depth. Infiltration and lateral saturated flow at the soil–bedrock interface are the main mechanisms of the expansion of subsurface saturation. We presume that subsurface saturation at the middle slope would infiltrate into bedrock layers rapidly, which reduced the connectivity of subsurface saturation between the upper slope and the lower slope. We proposed that the hydrological properties of soil–bedrock interface, such as the fragmentation in the soil–bedrock interface, is an important factor in discussing the spatial and temporal variations in the subsurface saturation in a headwater catchment.