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

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.

Abstract:

Abstract The simulation of floods with conceptual rainfall-runoff models is a frequently used method for various ap - plications in flood risk management. In mountain areas, the identification of the optimum model parameters during the calibration is often difficult because of the complexity and variability of catchment properties and hydrological processes. Central European mountain ranges are typically covered by Pleistocene periglacial slope deposits. The hydraulic conductivity of the cover beds shows a high degree of anisotropy, so it is impor - tant to understand the role of this effect in flood models of mesoscale mountain watersheds. Based on previ - ous field work, the study analyses the sensitivity of the NASIM modeling system to a variation of vertical and lateral hydraulic conductivity for the Upper Flöha watershed (Ore Mountains, Germany). Depending on the objective function (Nash-Sutcliffe coefficient, peak discharge), two diametric parameter sets were identified both resulting in a high goodness-of-fit for total discharge of the flood events, but only one reflects the hydro- logical process knowledge. In a second step, the knowledge of the spatial distribution of the cover beds is used to investigate the potential for a simplification of the model parameterisation. The soil types commonly used for the spatial discretisation of rainfall-runoff models were aggregated to one main class (periglacial cover beds only). With such a simplified model, the total flood discharge and the runoff components were simulated with the same goodness of fit as with the original model. In general, the results point out that the anisotropy in the unsaturated zone, which is intensified by periglacial cover beds, is an important element of flood models. First, a parameter set corresponding to the hydraulic anisotropy in the cover beds is essential for the optimum reproduction of the flood dynamics. Second, a discretisation of soil types is not necessarily required for flood modeling in Central European mountain areas

Abstract:

Electromagnetic induction (EMI) measurements are widely used for soil mapping, as they allow fast and relatively low-cost surveys of soil apparent electrical conductivity (ECa). Although the use of non-invasive EMI for imaging spatial soil properties is very attractive, the dependence of ECa on several factors challenges any interpretation with respect to individual soil properties or states such as soil moisture (θ). The major aim of this study was to further investigate the potential of repeated EMI measurements to map θ, with particular focus on the temporal variability of the spatial patterns of ECa and θ. To this end, we compared repeated EMI measurements with high-resolution θ data from a wireless soil moisture and soil temperature monitoring network for an extensively managed hillslope area for which soil properties and θ dynamics are known. For the investigated site, (i) ECa showed small temporal variations whereas θ varied from very dry to almost saturation, (ii) temporal changes of the spatial pattern of ECa differed from those of the spatial pattern of θ, and (iii) the ECa–θ relationship varied with time. Results suggest that (i) depending upon site characteristics, stable soil properties can be the major control of ECa measured with EMI, and (ii) for soils with low clay content, the influence of θ on ECa may be confounded by changes of the electrical conductivity of the soil solution. Further, this study discusses the complex interplay between factors controlling ECa and θ, and the use of EMI-based ECa data with respect to hydrological applications.

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:

Summary The links between soil water movement at the plot scale and runoff generation at the hillslope scale are highly non-linear and still not well understood. As such, a framework for the general characterization of hillslopes is still lacking. Here we present a number of virtual experiments with a 3D physically-based finite element model to systematically investigate the interactions between some of the dominant controls on subsurface stormflow generation. We used the well-studied Panola experimental hillslope to test our base case simulation and used the surface and subsurface topography and the stormflow data of this site as a framework for a subsequent series of virtual experiments. The parameterization of the soil and bedrock properties was based on field measurements of soil moisture and saturated hydraulic conductivity. After calibration and testing against multiple evaluation criteria including distributed trench flow data and internal tensiometric response, we varied slope angle, soil depth, storm size and bedrock permeability across multiple ranges to establish a set of response surfaces for several hillslope flow metrics. We found that connectivity of subsurface saturation was a unifying descriptor of hillslope behavior across the many combinations of slope type. While much of the interplay between our four hillslope variables was intuitive, several interactions in variable combinations were found. Our analysis indicated that, e.g. interactions between slope angle, soil depth and storm size that caused unexpected behavior of hydrograph peak times were the result of the interplay between subsurface topography and the overlying soil mantle with its spatially varying soil depth distribution. Those interactions led to new understanding of process controls on connectivity .

Abstract:

Principles of optimality provide an interesting alternative to modeling hydrological processes in detail on small scales and have received growing interest in the last years. Inspired by the more than 20 years old concept of minimum energy dissipation in river networks, we present a corresponding theory for subsurface flow in order to obtain a better understanding of preferential flow patterns in the subsurface. The concept describes flow patterns which are optimal in the sense of minimizing the total energy dissipation at a given recharge under the constraint of a given total porosity. Results are illustrated using two examples: two-dimensional flow towards a spring with a radial symmetric distribution of the porosity and dendritic flow patterns. The latter are found to be similar to river networks in their structure and, as a main result, the model predicts a power-law distribution of the spring discharges. In combination with two data sets from the Austrian Alps, this result is used for validating the model. Both data sets reveal power-law-distributed spring discharges with similar scaling exponents. These are, however, slightly larger than the exponent predicted by the model. As a further result, the distributions of the residence times strongly differ between homogeneous porous media and optimized flow patterns, while the mean residence times are similar in both cases.

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:

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

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.