FOR 5288: Fast and invisible: Conquering Subsurface Stormflow
through an Interdisciplinary Multi-Site Approach

Abstract:

Extreme rainfall events are likely to increase in intensity and frequency due to climatic changes and therefore the forecast of flooding events will become more important in the following decades. The flow properties of rainwater in the subsurface play a critical role in the flood formation process, but the underlying mechanism of this subsurface stormflow (SSF) formation has not been fully understood so far. Here, we explore the viability of environmental DNA (eDNA) as an indicator for small-scale flow pathway reconstruction. eDNA comprises genetic signatures from organisms across the Tree of Life (ToL), from whole microorganisms to molecular traces of higher taxa, such as plants or animals. The degree of similarity of biodiversity patterns indicates biological and therefore, in principle, also hydrological connectivity. As part of the SSF Research Unit we characterised 3 trenched hillslopes in 4 catchment areas in Germany and Austria through eDNA ToL-metabarcoding. With this broad-range approach, we aim to understand whether and how eDNA diversity patterns can inform subsurface flow pathways. We found three-dimensional connectivity patterns of biodiversity indicating systematic barriers as well as pathways of hydrological connectivity within each hillslope. Variation between catchments reflects their geographic differences as well as geological peculiarities. Although our results support the potential of eDNA to identify flow pathways and enhance our understanding of SSF, we are still at the beginning of understanding the viability of eDNA as a tracer in hydrological research. Nonetheless, making use of such natural occurring tracers can extend our understanding of hydrological phenomena and can contribute to a more accurate flood prediction.

Abstract:

Rainfall runoff contributes to a large proportion of the discharge in streams and therefore, heavily influences stream water quality but also flood generation. Rainfall runoff generation is usually a combination of overland and subsurface flow processes, the latter of which being especially difficult to trace. Here, we explored the viability of environmental DNA (eDNA) for subsurface water flow pathway reconstruction and simultaneous biodiversity assessment. The degree of similarity of community patterns indicates biological and therefore, in principle, also hydrological connectivity. We applied eDNA metabarcoding to characterise 10 drilling cores (0.7-3.2 m depth) on 3 hillslopes (10x50 m) in 4 catchment areas in Germany and Austria. In total, more than 2000 species across taxonomic groups could be identified down to species level. Analysis of alpha and beta diversity in the different catchments showed significant differences in spatial clustering patterns between taxonomic groups, but also between geomorphological and geochemical properties, such as the composition of dissolved organic carbon, of the respective catchment. We could assign indicator species sets in all taxonomic groups to various depth layers and identify habitat-specific communities that can be used as hydrological tracers. Although our results support the potential of eDNA to identify flow pathways and enhance our understanding of subsurface flow processes, we are still at the beginning of understanding the viability of eDNA as a tracer in hydrological research. However, our results show that making use of such naturally occurring tracers can expand our understanding of hydrological phenomena, especially those hidden in the subsurface.

Abstract:

The transport of water-soluble organic matter (WSOM) during stormflow events is an important link between hillslope hydrology and biogeochemical cycling, as it determines the movement of organic carbon from soils to streams. Hydrological dynamics in hillslopes, particularly subsurface stormflow (SSF), exhibit substantial spatial and temporal variability, making quantification and generalization challenging. SSF can account for up to 90% of rainfall input to stream discharge during storm events, highlighting its importance in catchment hydrology. Despite its significance, current research frequently overlooks WSOM dynamics during SSF, which are not only key components of carbon cycling but may also serve as tracers for identifying potential critical source areas. This emphasizes the importance of studying hillslope hydrological dynamics and determining the factors that contribute to SSF spatial and temporal variability. Furthermore, the specific flow paths within hillslopes remain poorly understood, which complicates the identification of spatial sources and transport mechanisms for organic carbon. To fill these knowledge gaps, we conducted a field study in the Black Forest, Germany, using a trench system to collect lateral subsurface flows at two depths (0-100 cm and 100-200 cm) over several rain events. We analysed WSOM concentration and quality using absorbance and fluorescence properties to assess the variability in critical source areas. We also conducted isotopic analyses of oxygen (δ¹⁸O) and hydrogen (δ²H) of the same water samples to infer flow pathways with a conservative tracer. This approach provides valuable insights into the temporal dynamics and spatial heterogeneity of SSF. Our findings will contribute to our understanding of flow paths, transit times, and the characteristics of WSOM export, offering a deeper understanding of subsurface flow processes in catchments. Finally, the findings of this study can help to improve biogeochemical models and improve scaling of hillslope processes models, particularly in understanding their contribution to organic carbon transport via SSF.

Abstract:

Hydrological dynamics of hillslopes, particularly subsurface stormflow (SSF), are highly complex and variable in space and time. Frequently, available studies are often limited to single slopes or few storm events. As a result, the transfer of these findings to other slopes or catchments is associated with great uncertainties. Thus, for upscaling and model validation, a quantification of the hydrological dynamics of hillslopes and the factors influencing the spatial and temporal patterns of subsurface stormflow is urgently needed. Closely related to the hydrological dynamics of hillslopes is the export of organic carbon from the soils to the adjacent stream. However, the spatial sources of carbon are still largely unclear because the exact flow paths of SSF within the slope are not well known. In order to address this knowledge gap, we took a hydro-biogeochemical approach, that measures the water-soluble organic matter (WSOM; concentration, absorbance and fluorescence) along 480 locations on 100 hillslopes, in four contrasting catchments – varying from low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps). This enables us to derive empirical relations among different landforms (i.e., convergent, divergent and planar slope shapes, flow path lengths and valley shapes), bedrock and soil properties, and to quantify the spatial variability and stability of subsurface hydrological process patterns (e.g., flow directions, transit times, hydrochemical and biochemical composition). Distributed sampling of WSOM along the soil profile (6 WSOM samples per profile; both during wet and dry conditions) will help to assess the vertical and lateral subsurface flowpaths of water in the unsaturated and saturated zone, and the spatial discretization of source areas for SSF. We will use an array of state-of-the-art laboratory equipment and methods (TOC-Analyzer, Fluorescence Spectrometry) to analyze WSOM. First results will show depth profiles of WSOM in the four contrasting catchments from the low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps). By these depth profiles source areas of SSF can be detected.

Abstract:

Where does water go when it rains? Where are floods generated and how? What controls stream water quality during events? These questions are important to many fields from engineering and flood protection to water and ecosystem management and prediction of impacts of global change. The most elusive processes in the process-ensemble underlying these questions is subsurface stormflow (SSF), the fast event response triggered by lateral subsurface flow. SSF is prevalent and a more important process than generally accounted for because a basic understanding based on systematic studies across scales and sites is still lacking. However, only with systematic studies will it be possible to really advance our understanding by discovering general principles of SSF functioning and to provide protocols and best practices for its assessment, both experimentally and with respect to modelling. In many natural landscapes, SSF, i.e. any subsurface flow that occurs in response to a precipitation event, plays a major role in runoff generation: either by contributing directly to streamflow or by producing saturated areas or return flow, which then is the underlying cause of saturation excess overland flow. Therefore, much of what we see as event response in the hydrograph might be the direct or indirect result of SSF. It is likely that the discharge signal of SSF, including the indirectly triggered response in the stream, is larger than we generally assume. While its importance is probably largest in the headwaters, headwaters make up 70% of the stream network and greatly influence the supply and transport of water and solutes downstream. However, SSF is elusive and poorly accounted for as measurements are difficult for several reasons: the inaccessibility of the subsurface, the large spatial variability and heterogeneity, the variable sources and the fact that it is a threshold-driven process that only occurs during certain events. Thus, systematic studies of SSF are lacking, mainly due to difficulties of quantification. We suggest such a systematic study of SSF in different environments, across scales, and using a well-designed and replicated selection of approaches including novel approaches. This will be followed by a systematic evaluation of methods and possible proxies as well as model intercomparison, evaluation and improvement. Thereby, we will focus on 4 challenges: 1) Development of novel experimental methods,2) Spatial patterns of SSF, 3) Thresholds and cascading effects of SSF, 4) Impacts of SSF. Whereas standard single research projects investigate part of this puzzle at a specific location, this Research Unit provides the unique opportunity of fitting a large number of puzzle pieces together. This Research Unit will have a strong emphasis on experimental work in four contrasting catchments from the low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps) that then directly feeds into a collaborative modelling effort, which in turn influences experimental design in an iterative process.

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:

Abstract Environmental DNA (eDNA) has revolutionized ecological research, particularly for biodiversity assessment in various environments, most notably aquatic media. Environmental DNA analysis allows for non-invasive and rapid species detection across multiple taxonomic groups within a single sample, making it especially useful for identifying rare or invasive species. Due to dynamic hydrological processes, eDNA samples from running waters may represent biodiversity from broad contributing areas, which is convenient from a biomonitoring perspective but also challenging, as hydrological knowledge is required for meaningful biological interpretation. Hydrologists could also benefit from eDNA to address unsolved questions, particularly concerning water movement through catchments. While naturally occurring abiotic tracers have advanced our understanding of water age distribution in catchments, for example, current geochemical tracers cannot fully elucidate the timing and flow paths of water through landscapes. Conversely, biological tracers, owing to their immense diversity and interactions with the environment, could offer more detailed information on the sources and flow paths of water to the stream. The informational capacity of eDNA as a tracer, however, is determined by the ability to interpret the complex biological heterogeneity at a study site, which arguably requires both biological and hydrological expertise. As eDNA data has become increasingly available as part of biomonitoring campaigns, we argue that accompanying eDNA surveys with hydrological observations could enhance our understanding of both biological and hydrological processes; we identify opportunities, challenges, and needs for further interdisciplinary collaboration; and we highlight eDNA's potential as a bridge between hydrology and biology, which could foster both domains. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods Water and Life > Nature of Freshwater Ecosystems

Abstract:

Abstract DNA metabarcoding of freshwater communities typically relies on PCR amplification of a fragment of the mitochondrial cytochrome c oxidase I (COI) gene with degenerate primers. The advantage of COI is its taxonomic resolution and the availability of an extensive reference database. However, when universal primers are used on environmental DNA (eDNA) isolated from water, benthic invertebrate read and OTU numbers are typically “watered down,” that is, under represented, compared to whole specimen “bulk samples” due to greater co-amplification of abundant nontarget taxa (e.g., fungi, algae, and bacteria). Because benthic stream invertebrate taxa are of prime importance for regulatory biomonitoring, more effective ways to capture their diversity via eDNA isolated from water are important. In this study, we aimed to improve benthic invertebrate assessment from eDNA by minimizing nontarget amplification. Therefore, we generated eDNA data using universal primers BF2/BR2 on samples collected throughout 15 months from a German Long-Term Ecological Research site (Rhine-Main-Observatory, Kinzig River) to identify most abundant nontarget taxa. Based on these data, we designed a new reverse primer (EPTDr2n) with 3’-specificity toward benthic invertebrate taxa and validated its specificity in silico together with universal forward primer fwhF2 using available data from GenBank and BOLD. We then performed in situ tests using 20 Kinzig River eDNA samples. We found that the percentage of target reads was much higher for the new primer combination compared to two universal benthic invertebrate primer pairs, BF2/BR2 and fwhF2/fwhR2n (99.6% versus 25.89% and 39.04%, respectively). Likewise, the number of detected benthic invertebrate species was substantially higher (305 versus 113 and 185) and exceeded the number of 153 species identified by expert taxonomists at nearby sites across two decades of sampling. While few taxa, such as flatworms, were not detected, we show that the optimized primer avoids the nontarget amplification bias and thus significantly improves benthic invertebrate detection from eDNA.