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

Abstract:

Subsurface flow in preferential pathways in soils may transport water more rapidly than the soil matrix, which may be quickly activated during precipitation events and enhancing infiltration or interflow. Vertical pathways are particularly important for runoff generation. However, identifying these pathways is challenging because traditional methods such as piezometers, soil moisture sensors, or hillslope trenches do not adequately capture the spatial scale and frequency of prefer ential flow features, while other experimental techniques like dye tracing are labor-intensive and invasive. In this study, we introduce a novel method to identify the locations of preferential flow by analysing vertical soil profiles of stable water isotope. Across four catchments, we drilled 100 soil cores (1–3 m deep) per catchment and analyzed the stable isotope composition of the soil water in 10–20 cm depth intervals to construct depth profiles. We employed clustering techniques to group soil-water isotope profiles and selecting those that match to a seasonal sampling date to establish a reference profile for each catchment using LOESS regression, representing profiles influenced solely by matrix infiltration. Deviations from these reference profiles were then used as indicators of being influenced by vertical or lateral preferential flow. Our results revealed evidence of preferential flow in all studied catchments. Especially in the alpine catchment with highly heterogeneous soils many profiles showed distinct preferential flow features, including multiple, vertically independent pathways occurring at variable depths, even among adjacent profiles. These findings demonstrate the feasibility of using soil water isotope profiles to assess preferential flow pathways highlighting the substantial spatial and vertical variability of preferential flowpaths at hillslope and catchment scale.

Abstract:

In many natural landscapes, subsurface stormflow (SSF) is a runoff-producing mechanism which can substantially contribute to the storm hydrograph of a stream. Despite its importance, its complex and highly dynamic nature have hindered its conceptualization and integration in most hydrological models. The lack of general rules to describe SSF is partly linked to the fact that SSF studies are often conducted at only one specific site or analyze only a handful of storm events. In the quest to gain a better understanding of the processes governing SSF, multiple SSF-capturing trenches have been excavated on intensely instrumented hillslopes characterized by different land uses, geology, soils and climates. The trenches are 10-15 m wide and 2-3 m deep and are vertically divided into an upper and lower flow-capture zone, which allows to study SSF at different depths. At the sites, SSF was continuously recorded over a period of ca. 1.5 year, during which numerous rainfall events occurred. This study analyses how the different rainfall event characteristics (e.g. total rainfall, intensity, etc.) influence the SSF response and to what degree the relationships between rainfall and SSF event characteristics are affected by the initial subsurface conditions (i.e. initial trenchflow and initial water content).

Abstract:

In many natural landscapes, subsurface stormflow (SSF) is a runoff-producing mechanism which can substantially contribute to the storm hydrograph of a stream. Despite its importance, there is a lack of systematic studies exploring SSF across sites with different land uses and hydrogeological characteristics. Thus, we face limitations to properly conceptualize and parametrize hydrological models. In order to gain a better understanding of the processes governing SSF, multiple SSF-capturing trenches were excavated. The selected trench sites span over different land uses, geology, soils and climates in Germany and Austria. Depending on local boundaries, the trenches were designed with a width of 11–15 m allowing to collect water flowing laterally at depths of up to 1–3 m. Using separate drainage pipes, the trench’s face is divided into an upper and lower flow-capture zone. Combining the measurements of vertically separated SSF outflow with upstream monitored groundwater levels and soil moisture dynamics, allows to estimate flow propagations along the hillslope. Besides the continuous monitoring, these installations were used to measure SSF events triggered by artificial rainfall. In this study we investigated the SSF response at 11 different trench sites under controlled conditions using a large-scale (200 m²) experimental sprinkling system in combination with deuterated water, which served as an artificial tracer. The irrigation was applied at a rate of ca. 16 mm h-1 for about 3 hours. The analysis focuses on trenchflow dynamics (e.g., timing and magnitude of the peak flow, recession curve analysis) and their relationship with changes in soil moisture and groundwater level. The experiments highlighted the vastly different responses between sites; while some trenches remained dry, others were characterized by extremely high subsurface runoff coefficients and short response times.

Abstract:

Summary This study investigates how different tree species influence soil hydrological properties that are relevant for the rainfall–runoff response of a given soil type. We hypothesize that for the same soil type, tree species that differ in rooting system, water consumption and associated soil fauna and soil flora lead to different soil moisture dynamics and lateral flow processes during rainfall and hence to different runoff responses. To test this hypothesis, we compare soil moisture patterns and interflow at different soil depths in a Norway spruce (Picea abies (L.) Karst) forest and in a European beech (Fagus sylvatica L.) forest during sprinkling experiments on two 6×10m hillslope segments with the same soil type. Spruce with a shallow rooting system and sinkers that remain very shallow on poorly aerated soils and beech with a heart shaped, often deeper rooting system are two of the most important tree species in Central Europe. At each hillslope, volumetric soil water contents were measured in 6min intervals with 48 TDR waveguides during and after sprinkling with intensities of 100mm/h and 60mm/h (for 1h). The waveguides were installed in 12 soil pits, whereby a single soil pit consisted of four 20cm buriable waveguides installed in 10cm, 30cm, 50cm and 70cm soil depth. Surface and shallow interflow at 10 cm soil depth and interflow at soil depths of 30cm and 60cm was automatically recorded. Despite the high rainfall intensities, no surface flow was observed in any of the experiments and only small amounts of shallow interflow were measured. Soil moisture patterns of lateral cross sections during and after the sprinkling reveal how tree species can alter runoff dynamics: under spruce, coinciding with rooting patterns, a water table develops in approximately 30cm soil depth while the soil water content in 50 and 70cm depth remains low. At the beech site, where coarse roots are found in deeper soil horizons, more water is directed towards deeper, already wetter soil horizons, from where the water table raises into the topsoil with high lateral conductivity. Because the higher water content on top of the stagnic layer allows segments of macropores like old root channels to connect earlier under beech, the beech hillslope exhibits a faster runoff response than the spruce hillslope. A lower water table and a higher macro-porosity makes saturation excess overland flow unlikely under beech. With the shallow water table and a lower available soil volume for preferential flow, a site planted with spruce is prone to saturation excess overland flow under natural rainfall conditions with inflow from the top. The results suggest that different tree species can lead to different rainfall–runoff responses at the same soil type. Though the study site showed minimal variation in soil properties, we cannot exclude that some of the differences in runoff processes we observed are caused by factors other than tree species, because only one large hillslope segment in each forest stand was sprinkled.

Abstract:

The phrase form and function was established in architecture and biology and refers to the idea that form and functionality are closely correlated, influence each other, and co-evolve. We suggest transferring this idea to hydrological systems to separate and analyze their two main characteristics: their form, which is equivalent to the spatial structure and static properties, and their function, equivalent to internal responses and hydrological behavior. While this approach is not particularly new to hydrological field research, we want to employ this concept to explicitly pursue the question of what information is most advantageous to understand a hydrological system. We applied this concept to subsurface flow within a hillslope, with a methodological focus on function: we conducted observations during a natural storm event and followed this with a hillslope-scale irrigation experiment. The results are used to infer hydrological processes of the monitored system. Based on these findings, the explanatory power and conclusiveness of the data are discussed. The measurements included basic hydrological monitoring methods, like piezometers, soil moisture, and discharge measurements. These were accompanied by isotope sampling and a novel application of 2-D time-lapse GPR (ground-penetrating radar). The main finding regarding the processes in the hillslope was that preferential flow paths were established quickly, despite unsaturated conditions. These flow paths also caused a detectable signal in the catchment response following a natural rainfall event, showing that these processes are relevant also at the catchment scale. Thus, we conclude that response observations (dynamics and patterns, i.e., indicators of function) were well suited to describing processes at the observational scale. Especially the use of 2-D time-lapse GPR measurements, providing detailed subsurface response patterns, as well as the combination of stream-centered and hillslope-centered approaches, allowed us to link processes and put them in a larger context. Transfer to other scales beyond observational scale and generalizations, however, rely on the knowledge of structures (form) and remain speculative. The complementary approach with a methodological focus on form (i.e., structure exploration) is presented and discussed in the companion paper by Jackisch et al.(2017).

Abstract:

Generalizable process knowledge on hillslope hydrological dynamics is still very poor, yet indispensable for numerous theoretical and practical applications. To gain insight into the organization of hillslope hydrological dynamics we intercompared 90 observations of shallow water table dynamics at three neighboring large-scale (33 × 75 m) hillslopes with similar slope, aspect, curvature, geologic, and pedologic properties but differences in vegetation cover (grassland, coniferous forest, and mixed forest) over a time period of 9 months. High-resolution measurements of water table fluctuations, rainfall, and discharge in the creek at the foot of all hillslopes allowed a good system characterization. The aim of this study was to explore the spatio-temporal variability of water table fluctuations within and between hillslopes, the effect of event and antecedent characteristics on the observed dynamics, and how the hillslope subsurface flow (SSF) response is reflected in the runoff response. To intercompare the SSF behavior we conducted an event-based analysis of the percentage of well activation, several metrics characterizing the shape and timing of the water table response curves, rainfall characteristics, antecedent wetness conditions, and several runoff response metrics. The analysis reveals that there are distinct differences in SSF response between the grassland hillslope and the forested hillslopes, with a lower frequency of well activation and absolute water table rise at the grassland hillslope. Second, spatial patterns of water table dynamics differ between wet fall/winter/spring (predominantly saturation of the lower part of the hillslope, weaker water table response, and slower response times) and dry summer conditions (whole-hillslope activation but higher spatial variability, generally stronger water table dynamics, and quicker response times). The observed seasonally changing water table dynamics suggest the development of a preferential flow network during high-intensity rainstorms under dry summer conditions. Third, catchment runoff is strongly driven by hillslope dynamics, yet contrasting hydrographs during events with similar hillslope dynamics indicate the influence of additional processes. Overall, the observed high spatio-temporal variability of seemingly homogeneous hillslopes calls for rethinking of current monitoring strategies and developing and testing new conceptual models of hillslope hydrologic processes.