Monitoring the generation and transformation of subsurface stormflow from hillslopes through the riparian zone

项目D旨在更好地了解山坡地下暴雨流的产生、相关的阈值和控制，以及山坡地下暴雨流的物理和化学信号在通过河岸带时受到多大程度的改变。我们打算减少SSF作为跨越地形、气候和土地利用梯度的流生成过程的难以捉摸性。SSF信号虽然通常是在山坡上产生的，但在从脚坡经过河岸带进入溪流的过程中可能会发生变化。这对SSF研究来说是个问题，因为最常见的实验方法要么集中在山坡上（观察浅层地下水位波动和/或沟槽中的地下水流），要么集中在山坡上识别的SSF溶质示踪信号上，然后在溪流中测量。SSF信号的变换可以包括时间延迟、流道方向和流量大小的变化。这些物理效应主要增加了在河岸带的停留时间，因此可能通过混合和/或生物地球化学反应导致SSF信号的化学转化。当它到达流时，原始的SSF信号可能被严重修改，难以识别。最终，坡流连通性的时空变异性将决定坡面SSF如何以及何时影响水流。为了评估山坡SSF与水流数量、时间和质量的相关性，项目D将解决以下研究问题：(1)山坡上产生的SSF的阈值和动态是什么？(2)是否有SSF特异性示踪剂或示踪剂组合可用于识别溪流中的山坡SSF ？这些签名有多具体？(3)坡面SSF的理化信号在河岸带是如何变化的，是否随季节或流域湿润状况而变化？(4) SSF的产生和河岸带的转变在时间和空间上的主要控制因素是什么？我们将用一种创新的多地点实验方法来解决上述挑战。这种方法包括开发一个双重用途的沟渠，使我们能够测量沟渠上坡侧的SSF流出信号（数量、时间和天然示踪剂），同时作为沟渠下坡侧人工示踪剂的注入点。在海沟下坡安装了一系列压力表，用于水位观测和采样，从而可以追踪从脚坡穿过河岸带到河流的水流路径。我们将在四个测试集水区各安装三个沟槽。下一步，我们可以确定流动路径方向，与河岸带地下水的混合以及河岸带化学信号的转化程度。基于此，我们将能够在一系列景观配置中识别ssf特异性示踪剂。

Project D aims at better understanding the generation of subsurface stormflow (SSF) on hillslopes, the associated thresholds and controls, and to what extent the hillslope SSF physical and chemical signals are modified on the passage through the riparian zone. We intend to reduce the elusiveness of SSF as a streamflow generation process across topographical, climatic and land use gradients. The SSF signal, although most often generated on the hillslopes, is likely to become transformed on its path from the footslope through the riparian zone into the stream. This is problematic for SSF research as the most common experimental methods focus either on the hillslopes (observing shallow groundwater table fluctuations and/or subsurface flow in trenches) or on the SSF solute tracer signal identified on the hillslope and then measured in the stream. The transformation of the SSF signal can include time delays, changes in flow path direction and in the magnitude of flow. These physical effects mostly increase the residence time in the riparian zone and may thus lead to a chemical transformation of the SSF signal by mixing and/or biogeochemical reactions. By the time it reaches the stream, the original SSF signal might be heavily modified and hard to identify. Ultimately, the spatio-temporal variability of hillslope-stream connectivity will govern how and when hillslope SSF affects streamflow. To evaluate the relevance of hillslope SSF for streamflow quantity, timing and quality, Project D will address the following research questions:(1) What are the thresholds and dynamics of SSF generated on the hillslopes?(2) Are there SSF-specific tracers or combinations of tracers that could be used to identify hillslope SSF in streamflow? How site specific are these signatures?(3) How is the hillslope SSF physical and chemical signal transformed in the riparian zone and does this change seasonally or with catchment wetness state?(4) What are the major controls in time and space for SSF generation, as well as riparian zone transformation?We will tackle the challenges described above with an innovative multi-site experimental approach. This approach includes the development of a dual-use trench, allowing us to measure SSF outflow signals (quantity, timing, and natural tracers) at the upslope side of the trench, while at the same time serving as an injection site for artificial tracers at the downslope side of the trench. An array of piezometers installed downslope of the trench for water level observation and sampling will make it possible to trace the flowpaths from the footslope across the riparian zone to the stream. We will install three trenches in each of our four test catchments. In a next step, we can identify flow path directions, mixing with riparian zone groundwater and the extent of transformation of the chemical signal in the riparian zone. Based on this, we will be able to identify SSF-specific tracers across a range of landscape configurations.