Fast and invisible: Conquering Subsurface Stormflow
through an Interdisciplinary Multi-Site Approach
(DFG RU 5288)

Abstract:

Subsurface stormflow (SSF) is a streamflow generation process that is difficult to observe. It has therefore been challenging to evaluate the relevance and magnitude of SSF contributions to streamflow quantity and quality. However, some earlier studies have shown that it can deliver substantial amounts of water to the stream at the time scale of an event. One possible approach for detecting SSF in streamflow has been to sample SSF on hillslopes, characterize it by analyzing various tracers and search for this SSF fingerprint in stream water samples. In this study, we ask the following questions: Does subsurface stormflow generated on hillslopes and moving downslope towards the stream have a typical chemical fingerprint or signature by which we could recognize it in the stream? And does this signature vary over time? Here, we present data from a headwater catchment near Freiburg, Germany, where we installed three trenches to measure SSF flow rates and to obtain SSF samples for chemical analysis. We collected SSF samples from the three trenches over multiple events during spring 2023, fall 2023 and spring 2024 and analyzed them for dissolved organic carbon and major ions. We compared chemical SSF signatures through the events, across seasons and between the three trenches. Preliminary analyses indicate that the SSF signature changed during events, with SSF signatures at the beginning and at the end of events being remarkably similar to each other. Results also hint at a seasonal stability of SSF signatures. In our presentation, we are going to present a detailed analysis of the dynamics of the chemical SSF signature. This dataset provides a unique opportunity to evaluate the chemical composition of subsurface stormflow in sub-daily resolution at three different hillslopes and to improve our capability to recognize contributions of SSF to streamflow.

Abstract:

Subsurface stormflow (SSF) can be a major streamflow generation process in small catchments. It is known that SSF generated on the hillslopes of the catchment may change both in its chemical and quantitative composition on the way to the stream. This occurs primarily due to processes in the riparian zone. The riparian zone plays the role of a "reactor" where mixing, storage, and biogeochemical transformation of the hillslope SSF composition occurs. However, we still lack a comprehensive understanding of this “gatekeeper function” of the riparian zone, controlling the timing and spatial patterns of connectivity and the chemistry of the water being transferred from the hillslopes into the stream. In our study we aim to investigate the SSF signal transformation in the riparian zone. We installed three “dual-use trenches” per catchment in four different catchments located in Germany and Austria. With this novel dual-use trench approach we are able to measure hillslope SSF as well as inject tracer into the riparian zone. We measure response dynamics, timing, flow volumes and chemistry at the upslope side of the trench. We will identify tracers or tracer combinations that characterize SSF and can be used to identify hillslope SSF in riparian zone groundwater and stream flow. The inter-comparison of the four different catchments allows us to evaluate the influence of landscape and climate characteristics. We then use tracer injections at the downslope side of the dual-use trench in combination with an array of shallow groundwater observation wells downslope of the trench to investigate the physical and chemical transformation of hillslope SSF in the riparian zone. This array of wells extends both upstream and downstream of the trench, enabling us to trace the transformation of the uninterrupted physical and chemical signal of SSF on the adjacent hillslopes on its passage to the stream and to evaluate the influence of parafluvial flow. Here, we present first data on tracer concentrations in hillslope SSF and riparian zone groundwater from our test catchments. Ultimately, we aim to develop a conceptual matrix, by which it will be possible to estimate the degree of SSF transformation in the riparian zone, depending on watershed characteristics (topography, soil depth and soil hydraulic properties) and hydrological conditions (antecedent wetness of the watershed and seasonal dynamics).

Abstract:

Subsurface hillslope-stream connectivity is a major control on runoff-generation and catchment storage dynamics. However, detecting this connectivity is challenging, as processes in the subsurface are not easily observable. Furthermore, we are faced with a high spatial variability as well as pronounced temporal dynamics. In this context, we are investigating three catchments in German mid-mountain ranges: Black Forest, Ore Mountains and Sauerland. The experimental design consists of three trenched hillslopes per catchment as well as numerous observation wells and stream gauges along the stream. Water samples are taken at all locations during snapshot campaigns and are analyzed for major cations and anions to complement event-based sampling at the trenches and in the stream. This comparative design aims at moving beyond single-site insights to gaining a broader view of the process and its spatio-temporal patterns. First observations of these patterns based on physical and chemical signals of subsurface connectivity are presented.

Abstract:

Subsurface stormflow (SSF) is a key runoff generation mechanism in small catchments, yet its dynamics and thresholds remain poorly understood due to the challenges of observing and measuring subsurface processes. To address this, we conducted a study in a headwater catchment in the Black Forest, Germany, where we investigated the depth of SSF flow paths, if SSF is dominated by event water and if all SSF events have similar tracer behavior. Three trenches were installed on slopes with different landuse and topography. They were excavated down to bedrock ( 15 m wide, 2.5 m deep), collecting SSF separately from upper and lower soil layers. Continuous measurements of SSF volume and temperature were combined with water sampling for natural tracers: stable water isotopes, dissolved organic carbon, electric conductivity, and major ions. During the study period, 6 large SSF events were recorded in each trench. Using heat as a tracer we found stable SSF flowpath depths across events. Multitracer analysis suggested that SSF consisted mostly of pre-event water, with antecedent wetness governing event water contributions. Our results also show that C–Q relationships for solutes varied considerably between events and trenches, with some (e.g., Si) remaining chemostatic while others (e.g. DOC, SO24− and NO−3 ) behaved chemodynamically. This indicates that the chemical composition of SSF depends on both the antecedent hydrometeorological conditions and local factors such as landuse, reinforcing the complexity of these processes. Our results highlight the dynamic nature of subsurface stormflow and the challenges involved in its characterization.

Abstract:

Headwater streams account for 70% or more of total stream length in most catchments, making it crucial to better understand the processes and controlling factors governing streamflow generation as well as water quality. In this context, stream water chemistry longitudinal profiles can provide valuable insights. This study examines longitudinal stream chemistry profiles across six headwater catchments in three mid-mountain ranges in Germany: The Ore Mountains (catchments OM 1 and OM 2), Black Forest (BF 1 and BF 2), and Sauerland (SL 1 and SL 2). Three to four snapshot sampling campaigns were conducted per catchment across different seasons and catchment wetness conditions. During the campaigns, water samples were collected from 22 stream monitoring points in the Ore Mountains catchments, 14 in the Black Forest, and 14 in Sauerland, and the samples were analyzed for major cations, anions, and dissolved organic carbon. Subsequently, the longitudinal profiles observed were grouped into spatial and temporal patterns. In the Ore Mountains, solute concentrations were generally stable over time. However, the spatial patterns varied between the two neighbouring catchments (OM 1 and OM 2). OM 2 exhibited chemostatic longitudinal profiles for most solutes, while OM 1 showed pronounced spatial variability in solutes such as nitrate, dissolved organic carbon (DOC), chloride, and sodium. This variability is usually linked to monitoring points located near springs, tributaries, and drainage systems. However, some spikes in ion concentrations along the stream were not linked to these obvious inflows, thus potentially indicating hotspots for groundwater inflow. The Sauerland catchments showed elevated concentrations of DOC, magnesium, calcium, and sodium in July 2023, a period associated with lower streamflow. An increase in concentration from upstream to downstream was here seen in both streams for solutes like calcium and sodium, during all snapshot campaigns. However, other solutes, like nitrate and sulfate, showed different longitudinal patterns and notable shifts in solute concentration during the snapshot campaigns in SL 2. The shifts in patterns indicate a dependency on time-variant factors like seasonal changes in water input, and land use practices. BF 1 catchment in the Black Forest showed a decreasing pattern in DOC, from upstream to downstream, while the neighbouring catchment BF 2 showed a chemostatic trend. These trends could be influenced by the land use changes within the catchments. Notable increased nitrate concentrations were seen along reaches adjacent to grassland areas and at sampling points near tile drains in OM 1, BF 1, SL 1, and SL 2. Overall, solute spatial and temporal patterns were stream-specific, with no universal behaviour observed across all catchments. This variability likely results from the interplay of factors such as geology, soils, land use, stream morphology, and climate. High-resolution spatial sampling enabled the identification of point sources and hotspots of groundwater inflow which could be missed by sparse sampling. These findings enhance our understanding of the processes regulating water quality and flow in headwater systems, providing a basis for better management of these systems.

Abstract:

Environmental tracers have been used to separate streamflow components for many years. They allow us to quantify the contribution of water originating from different sources, such as direct runoff from precipitation, subsurface storm flow, or groundwater to total streamflow at variable flow conditions. Although previous studies have explored the value of incorporating experimentally derived fractions of event and pre-event water into hydrological models, a thorough analysis of the value of incorporating hydrographseparation-derived information on multiple streamflow components at varying flow conditions into model parameter estimation has not yet been performed. This study explores the value of such information to achieve more realistic simulations of catchment discharge. We use a modified version of the process-oriented HBV model that simulates catchment discharge through the interplay of hillslope, riparian-zone discharge, and groundwater discharge at a small forested catchment which is located in the mountainous north of South Korea, subject to a monsoon season between June and August. Applying a Monte-Carlo-based parameter estimation scheme and the Kling–Gupta efficiency (KGE) to compare discharge observations and simulations across two seasons (2013 and 2014), we show that the model is able to provide accurate simulations of catchment discharge (KGE 0.8) but fails to provide robust predictions and realistic estimates of the contribution of the different streamflow components. Using a simple framework that compares simulated and observed contributions of hillslope, riparian zone, and groundwater to total discharge during two sub-periods, we show that the precision of simulated streamflow components can be increased, while remaining with accurate discharge simulations.We further show that the additional information increases the identifiability of all model parameters and results in more robust predictions. Our study shows how tracer-derived information on streamflow contributions can be used to improve the simulation and predictions of streamflow at the catchment scale without adding additional complexity to the model. The complementary use of temporally resolved observations of streamflow components and modeling provides a promising direction to improve discharge prediction by representing model internal dynamics more realistically.

Abstract:

Hydrologic connectivity is the linkage of separate regions of a catchment via water flow. Knowledge of hillslope–stream connectivity (both at the surface and in the subsurface) is essential for understanding and predicting runoff responses and streamwater quality. Connectivity can be very dynamic: hillslopes may connect to the stream only during certain events or seasons. While surface connectivity is often discussed, particularly in the context of sediment transport, subsurface connectivity is more difficult to describe and assess. This difficulty has led to a wide variety in methodologies that are used in various contexts. Field approaches have focused on intensive monitoring of processes on the hillslope or the fingerprint of connectivity in the stream. Combining experimental studies with modeling allows for testing of hypotheses with respect to thresholds and controls on connectivity, and extrapolation from the hillslope scale to the catchment scale. However, as most modeling approaches are based on datasets from a few intensively studied hillslopes, this carries the inherent risk of oversimplification because it assumes that the observed hillslope responses are representative for the catchment or even the region. Focussed efforts on catchment scale assessment of hillslope–stream connectivity, as well as site intercomparisons and the search for similarity measures may allow us to capture the wider picture of the mechanisms and factors that control hillslope–stream connectivity, and its effects on flow and transport at the catchment scale. This overview focuses on how hillslope–stream connectivity has been studied and describes the advantages, disadvantages, and challenges of the different methods. WIREs Water 2015, 2:177–198. doi: 10.1002/wat2.1071 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality