Surface runoff of horse grazed pasture – a disregarded hydrological response unit in low mountain ranges

Accurate prediction of surface runoff is of vital interest for flood prediction which in turn requires the process knowledge about key factors affecting its temporal and spatial variability. Antecedent soil moisture and grazing intensity have been detected as important factors, but there exists no explicit field study investigating the spatial and temporal variability of surface runoff generation on horse grazed pasture. In our study, for the first time the surface runoff generation on horse grazed pasture was analyzed using a rainfall simulator along with measurements of soil water content and soil physical properties. The results were compared with concurrent investigations on cattle grazed pasture land. The analyses of 8 rainfall simulations on 1 m² plots at a rate of 46.6 mm/h revealed mean runoff coefficients ranging from 0.9% to 50.5%. The most important findings of our study are that the antecedent soil moisture distinctly impacts the amount of surface runoff and the runoff coefficient is significantly higher on horse grazed pasture than on cattle grazed pasture. These results underline the importance of further experimental studies to obtain a broader process knowledge about this specific hydrological response unit, especially in regard to the increasing portion of horse grazing in the low mountain ranges

How can we model subsurface stormflow at the catchment scale if we cannot measure it?

Subsurface stormflow (SSF) can be a dominant run‐off generation process in humid mountainous catchments (e.g., Bachmair & Weiler, 2011; Blume & van Meerveld, 2015; Chifflard, Didszun, & Zepp, 2008). Generally, SSF develops in structured soils where bedrock or a less permeable soil layer is overlaid by a more permeable soil layer and vertically percolating water is deflected, at least partially, in a lateral downslope direction due to the slope inclination. SSF can also occur when groundwater levels rise into more permeable soil layers and water flows laterally through the more permeable layers to the stream (“transmissivity feedback mechanism”; Bishop, Grip, & O'Neill, 1990). The different existing terms for SSF in the hydrological literature such as shallow subsurface run‐off, interflow, lateral flow, or soil water flow reflects the different underlying process concepts developed in various experimental studies in different environments by using different experimental approaches at different spatial and temporal scales (Weiler, McDonnell, Tromp‐van Meerveld, & Uchida, 2005). Intersite comparisons and the extraction of general rules for SSF generation and its controlling factors are still lacking, which hampers the development of appropriate approaches for modelling SSF. But appropriate prediction of SSF is essential due to its clear influence on run‐off generation at the catchment scale (e.g., Chifflard et al., 2010; Zillgens, Merz, Kirnbauer, & Tilch, 2005), on the formation of floods (e.g., Markart et al., 2013, 2015) and on the transport of nutrients or pollutants from the hillslopes into surface water bodies (Zhao, Tang, Zhao, Wang, & Tang, 2013). However, a precise simulation of SSF in models requires an accurate process understanding including, knowledge about water pathways, residence times, magnitude of water fluxes, or the spatial origin of SSF within a given catchment because such factors determine the transport of subsurface water and solutes to the stream. But due to its occurrence in the subsurface and its spatial and temporal variability, determining and quantifying the processes generating SSF is a challenging task as they cannot be observed directly. Therefore, it is logical to ask whether we can really model SSF correctly if we cannot measure it well enough on the scale of interest (Figure 1). This commentary reflects critically on whether current experimental concepts and modelling approaches are sufficient to predict the contribution of SSF to the run‐off at the catchment scale. This applies in particular to the underlying processes, controlling factors, modelling approaches, research gaps, and innovative strategies to trace SSF across different scales

Spatial variability in heavy metal concentration in urban pavement joints – a case study

Heavy metals are known to be among one of the major environmental pollutants, especially in urban areas, and, as generally known, can pose environmental risks and direct risks to humans. This study deals with the spatial distribution of heavy metals in different pavement joints in the inner city area of Marburg (Hesse, Germany). Pavement joints, defined as the joint between paving stones and filled with different materials, have so far hardly been considered as anthropogenic materials and potential pollution sources in urban areas. Nevertheless, they have an important role as possible sites of infiltration for surface run-off accumulation areas and are therefore a key feature of urban water regimes. In order to investigate the spatial variability in heavy metals in pavement joints, a geospatial sampling approach was carried out on six inner city sampling sites, followed by heavy metal analyses via inductively coupled plasma–mass spectrometry (ICP–MS) and additional pH and organic matter analyses. A first risk assessment of heavy metal pollution from pavement joints was performed. Pavement joints examined consist mainly of basaltic gravel, sands, organic material and anthropogenic artefacts (e.g. glass and plastics), with an average joint size of 0.89 cm and a vertical depth of 2–10 cm. In general, the pavement joint material shows high organic matter loads (average 11.0 % by mass) and neutral to alkaline pH values. Besides high Al and Fe content, the heavy metals Cr, Ni, Cd and Pb are mainly responsible for the contamination of pavement joints. The identified spatial pattern of maximum heavy metal loads in pavement joints could not be attributed solely to traffic emissions, as commonly reported for urban areas. Higher concentrations were detected at run-off accumulation areas (e.g. drainage gutters) and at the lowest sampling points with high drainage accumulation tendencies. Additional Spearman correlation analyses show a clear positive correlation between the run-off accumulation value and calculated exposure factor (ExF; Spearman correlation coefficients (rSP) – 0.80; p<0.00). Further correlation analyses revealed different accumulation and mobility tendencies of heavy metals in pavement joints. Based on sorption processes with humic substances and an overall alkaline pH milieu, especially Cu, Cd and Pb showed a low potential mobility and strong adsorption tendency, which could lead to an accumulation and fixation of heavy metals in pavement joints. The presence of heavy metals in pavement joints poses a direct risk for urban environments and may also affect environments out of urban areas if drainage transports accumulated heavy metals. Finally, we encourage further research to give more attention to this special field of urban anthropogenic materials and potential risks for urban environments. Overall urban geochemical background values, and the consideration of run-off-related transport processes on pavements, are needed to develop effective management strategies of urban pavement soil pollution

Investigating the dispersal of macro- and microplastics on agricultural fields 30 years after sewage sludge application

Plastic contamination of terrestrial ecosystems and arable soils pose potentially negative impacts on several soil functions. Whereas substantial plastic contamination is now traceable in agro-landscapes, often internal-caused by the application of fertilizers such as sewage sludge, questions remain unanswered concerning what happens to the plastic after incorporation. Based on a combined surface and depth sampling approach, including density separation, fuorescence staining and ATR-FTIR or µFTIR analyses, we quantifed macro- and microplastic abundance on two agricultural felds—34 years after the last sewage sludge application. By sub-dividing the study area around sludge application sites, we were able to determine spatial distribution and spreading of plastics. Past sewage sludge application led to a still high density of macroplastics (637.12 items per hectare) on agricultural soil surfaces. Microplastic concentration, measured down to 90 cm depth, ranged from 0.00 to 56.18 particles per kg of dry soil weight. Maximum microplastic concentrations were found in regularly ploughed topsoils. After 34 years without sewage sludge application, macro- and microplastic loads were signifcantly higher on former application areas, compared to surrounding areas without history of direct sewage application. We found that anthropogenic ploughing was mainly responsible for plastic spread, as opposed to natural transport processes like erosion. Furthermore, small-scale lateral to vertical heterogeneous distribution of macro- and microplastics highlights the need to determine appropriate sampling strategies and the modelling of macro- and microplastic transport in soils

Die flächenhafte Ausweisung von Bodenfeuchteregimes: Eine Methode im Rahmen des pre-processing für die Modellierung des Landschaftswasserhaushaltes

In dem Einzugsgebiet „Obere Brachtpe“ (2,6 km², Sauerland, Deutschland) wurden mit einer konventionellen Bohrstock-Kartierung (319 Bodenprofile) die verschiedenen Böden mit ihren spezifischen pedohydrologischen Eigenschaften erfasst und darauf aufbauend das Klassifikationskonzept zur Ableitung von Bodenfeuchteregime-Typen nach Zepp (1995) angewendet. Anhand dieser Informationen konnte die vorherrschende Wasserbewegungsrichtung und folglich verschiedene Abflussprozessflächen ausgewiesen werden. Um dieses Raumgliederungskonzept hinsichtlich der Frage, ob es zum pre-processing für die Modellierung des Landschaftswasserhaushalts beitragen kann, zu verfizieren, wurden zwei Niederschlags-Abfluss-Ereignisse tracerhydrologisch untersucht. Die Resultate zeigen, dass sich das Klassifikationskonzept nach Zepp (1995) zur hydrologischen Raumgliederung eines Einzugsgebietes gut eignet, es wird aber auch deutlich, dass neben den Bodeneigenschaften insbesondere die Lage der schnell reagierenden Flächen (Sättigungsflächen) innerhalb des Einzugsgebietes und deren Konnektivität mit dem Vorfluter entscheidend für den Beitrag schneller Abflusskomponenten sind. Besteht eine direkte Verbindung zwischen Sättigungsflächen und Vorfluter, so ist eine schnelle Abflussreaktion und ein hoher Anteil an Ereigniswasser zu erwarten.In the catchment “Obere Brachtpe” (2,6 km², Sauerland, Germany) the soils and their specific pedohydrological characteristics were investigated using a conventional drilling rig (319 drills). Based on the results the classification concept for deducing the soil moisture types by Zepp (1995) was applied so that the dominant flow directions and dominant runoff process areas could be detected. The question if this classification concept could be used as tool for the pre-processing within the runoff modelling was verified by the tracer hydrological investigation of two rainfall runoff-events. The results confirm that the classification concept by Zepp (1995) is an applicable tool for the spatial delineation of a catchment in hydrological response units. On the other hand it became apparent that in addition to the soil properties, the location of saturated areas within the catchment and their connectivity to the channel are especially important for the contribution of accelerated runoff components. If there exists a direct connection between the saturated areas and the channel a fast runoff reaction and a high portion of event water can be estimated

Investigating microplastic dynamics in soils: Orientation for sampling strategies and sample pre‐procession

Studies on microplastics in soils is currently being established as a new research field. So far, mainly 'explorative studies' have been carried out to detect microplastics in different soil environments. To generate a deeper understanding of microplastics dynamics, 'systematic studies' are required. Such research must built on a targeted sampling strategy and considerate fieldwork and sample handling. From literature enquiry, a five-stage methodological workflow was deduced for studies on microplastics in soils. In the present review, the spatial representation of soils/soilscapes with microplasticsin soils research is conceptually and practically assessed. We discuss judgmental, randomized, and metric soil sampling strategies. Then, we explain sample pre-processing and give a brief overview of methods for microplastics identification and quantification. We conclude that the establishment of the novel field of research 'microplasticdynamics in soils' requires more intensive consideration of soil sampling strategies. Assoil is a complex medium and the soilscape is spatially heterogeneous, we highlight systematic sampling strategies as the best possible options for sophisticated research. However, no overall optimum methodology can be defined because the specific strategy must be in line with the particular research question. For all studies on microplastics in soils, practical improvement is needed to prevent contamination of soilsamples with plastics during sampling and sample pre-processing

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come

Different forest cover and its impact on eco-hydrological traits, invertebrate fauna and biodiversity of spring habitats

Headwater springs in the German Low Mountain Ranges are local ecotone habitats and biogeographical islands embedded in and interlinked with their adjacent landscape. The structure of forests reflects the eco-hydrological conditions in substrate type occurrence, microhabitat richness and biodiversity in forest springs. This study considers effects from different forest land cover by comparing spring habitats in deciduous beech forests and coniferous spruce forests on eco-hydrological structures and biodiversity. Study areas include six different forest landscapes in the Low Mountain Ranges in Central Germany in Hesse and Thuringia. Hydro-morphological structure mapping and invertebrate sampling was executed within a multi-habitat sampling regime, which involves sampling plots being allocated according to the cover ratio of the occurring substrata. Aquatic and terrestrial spring zones are considered with respect to an ecotone approach. Some in situ measurements were implemented, such as pH values, to assess the acidity of the spring water. Results show obvious differences in acidity, substrate type cover ratios and biodiversity in deciduous and coniferous forest springs. Conifer forest springs were found tending to acidification while deciduous forest springs were slightly alkaline. Deciduous forest springs had higher cover ratios of organic microhabitats as well as a higher biodiversity in species richness and total number of individuals. Although it was not possible to clearly distinguish one direct key factor of fauna assemblages, negative effects from forest management practices (e.g. monoculture plantations of conifer forest) on spring habitats can be concluded