Groundwater flow direction within the critical zone of headwater catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced by subsurface permeability contrasts, which can result in variability in flow direction and magnitude. In this study, we investigated the relationship between shallow groundwater flow direction, surface topography, and the subsurface topography of low permeability units in a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), NH. We continuously monitored shallow groundwater levels in the solum throughout several seasons in a well network (20 wells of 0.18–1.1 m depth) within the upper hillslopes of Watershed 3 of the HBEF. Water levels were also monitored in four deeper wells, screened from 2.4 to 6.9 m depth within glacial drift of the C horizon. We conducted slug tests across the well network to determine the saturated hydraulic conductivity (K_sat) of the materials surrounding each well. Results showed that under higher water table regimes, groundwater flow direction mimics surface topography, but under lower water table regimes, flow direction can deviate as much as 56 degrees from surface topography. Under these lower water table conditions, groundwater flow direction instead followed the topography of the top of the C horizon. The interquartile range of K_sat within the C horizon was two orders of magnitude lower than within the solum. Overall, our results suggest that the land surface topography and the top of the C horizon acted as end members defining the upper and lower bounds of flow direction variability. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone and runoff generation studies of headwater catchments that are underlain by glacial drift.

中文翻译：

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地下渗透率对比控制冰川水源流域临界区的浅层地下水流动动力学

源头集水区临界区内的地下水流向通常被假定为模拟地表地形梯度。然而，地下水水力梯度也受到地下渗透率对比的影响，这可能导致流动方向和大小的变化。在这项研究中，我们调查了新罕布什尔州哈伯德布鲁克实验森林 (HBEF) 源头集水区低渗透单元的浅层地下水流向、地表地形和地下地形之间的关系。我们在 HBEF 分水岭 3 的上山坡上的一个井网（20 口井，深度 0.18-1.1 m）中连续监测了几个季节的浅层地下水位。还在四口较深的井中监测水位，从 2.4 到 6 进行筛选。C 地平线的冰川漂移内 9 m 深度。我们在整个井网进行了段塞测试以确定饱和水力传导率（K _sat ) 的每个孔周围的材料。结果表明，在地下水位较高的情况下，地下水流向模拟地表地形，但在地下水位较低的情况下，水流方向与地表地形的偏差可达 56 度。在这些较低的地下水位条件下，地下水的流动方向反而遵循 C 层顶部的地形。K _sat的四分位距在 C 范围内比在 solum 内低两个数量级。总体而言，我们的研究结果表明，地表地形和 C 地平线的顶部充当了定义流向变化的上限和下限的端部成员。这表明在计算临界区的水文通量和冰川漂移下的源头集水区的径流生成研究时，应考虑地下水流向的时间动态。