Ute Wollschläger

Catchments as reactors: a comprehensive approach for water fluxes and solute turnover

Sustainable water quality management requires a profound understanding of water fluxes (precipitation, run-off, recharge, etc.) and solute turnover such as retention, reaction, transformation, etc. at the catchment or landscape scale. The Water and Earth System Science competence cluster (WESS, http://www.wess.info/) aims at a holistic analysis of the water cycle coupled to reactive solute transport, including soil–plant–atmosphere and groundwater–surface water interactions. To facilitate exploring the impact of land-use and climate changes on water cycling and water quality, special emphasis is placed on feedbacks between the atmosphere, the land surface, and the subsurface. A major challenge lies in bridging the scales in monitoring and modeling of surface/subsurface versus atmospheric processes. The field work follows the approach of contrasting catchments, i.e. neighboring watersheds with different land use or similar watersheds with different climate. This paper introduces the featured catchments and explains methodologies of WESS by selected examples.

Continuous and simultaneous measurement of reflector depth and average soil-water content with multichannel ground-penetrating radar

Ground-penetrating radar is a fast noninvasive technique that can monitor subsurface structure and water-content distribution. To interpret traveltime information from single common-offset measurements, additional assumptions, such as constant permittivity, usually are required. We present a fast ground-penetrating-radar measurement technique using a multiple transmitter-and-receiver setup to measure simultaneously the reflector depth and average soil-water content. It can be considered a moving minicommon-midpoint measurement. For a simple analysis, we use a straightforward evaluation procedure that includes two traveltimes to the same reflector, obtained from different antenna separations. For a more accurate approach, an inverse evaluation procedure is added, using traveltimes obtained from all antenna separations at one position and its neighboring measurement locations. The evaluation of a synthetic data set with a lateral variability in reflector depth and an experimental example with a large variability in soil-water content are introduced to demonstrate the applicability for field-scale measurements. The crucial point for this application is the access to absolute traveltimes, which are difficult to determine accurately from common-offset measurements.

Investigation of parameters controlling the soil sink of atmospheric molecular hydrogen

Enclosure measurements have been performed on a bare mineral soil at an experimental field site near Heidelberg, Germany. From observed molecular hydrogen (H2) mixing ratio changes in the enclosure, deposition velocities were calculated ranging from 8.4 × 10−3 to 8.2 × 10−2 cm s−1 and with an annual mean value of 3.1 × 10−2 cm s−1. In the studied range of 2– 27 ◦C, the uptake showed a significant temperature dependence. However, this turned out not to be the primary driving mechanism of the uptake flux. Soil moisture content, co-varying with temperature, was identified as the major parameter being responsible for the diffusive permeability of H2 in the soil and the final rate of H2 uptake. A simple Millington–Quirk diffusion model approach could largely explain this behaviour and yielded a diffusion path length of H2 in the studied soil of only 0.2–1.8 cm, suggesting that total H2 consumption occurs within the first few centimetres of the soil. The diffusion model, when applied to continuous measurements of soil moisture content, atmospheric pressure, temperature and the mixing ratio of H2 in the atmosphere, could largely reproduce the measured deposition flux densities, assuming a mean thickness of the diffusion path length of 0.7 cm.

The Bode hydrological observatory: a platform for integrated, interdisciplinary hydro-ecological research within the TERENO Harz/Central German Lowland Observatory

This article provides an overview about the Bode River catchment that was selected as the hydrological observatory and main region for hydro-ecological research within the TERrestrial ENvironmental Observatories Harz/Central German Lowland Observatory. It first provides information about the general characteristics of the catchment including climate, geology, soils, land use, water quality and aquatic ecology, followed by the description of the interdisciplinary research framework and the monitoring concept with the main components of the multi-scale and multi-temporal monitoring infrastructure. It also shows examples of interdisciplinary research projects aiming to advance the understanding of complex hydrological processes under natural and anthropogenic forcings and their interactions in a catchment context. The overview is complemented with research work conducted at a number of intensive research sites, each focusing on a particular functional zone or specific components and processes of the hydro-ecological system.

TERENO-SOILCan: a lysimeter-network in Germany observing soil processes and plant diversity influenced by climate change

The aim of TERENO (TERrestrial ENvironmental Observatories) is to collect long-term observation data on the hydrosphere, biosphere, pedosphere, lower atmosphere and anthroposphere along multiple spatial and temporal gradients in climate sensitive regions across Germany. The lysimeter-network SOILCan was installed as a part of TERENO between March and December 2010 within the four observatories. It represents a long-term large-scale experiment to study the effects of climate and management changes in terrestrial ecosystems, with particular focus on the impact of these changes on water, energy and matter fluxes into groundwater and atmosphere. SOILCan primarily focuses on soil hydrology, the carbon and nutrient cycle and plant species diversity. Time series measurements of states and fluxes at high spatial and temporal resolution in the soil and biosphere are combined with remote sensing information for the development and calibration of process-based models simulating impacts of climate change in soil processes at field to regional scale. Within the framework of SOILCan, 132 fully automated lysimeter systems were installed at 14 highly equipped experimental field sites across the four TERENO observatories. Relevant state variables of grassland and arable ecosystems are monitored characterizing climate, hydrology and matter fluxes into the atmosphere and within the hydrosphere as well as plant species diversity. Lysimeters are either being operated at or near their original sampling location or were transferred within or between the four TERENO observatories thereby using temperature and rainfall gradients to mimic future climatic conditions (space for time), which allow measuring impacts of climate change on terrestrial ecosystems. The lysimeters are cultivated as grassland (intensive, extensive and non-used) or arable land, the latter with a standardized crop rotation of winter wheat—winter barley—winter rye—oat. This publication describes the general design of the SOILCan experiment including a comprehensive description of the pedological characteristics of the different sites and presents a few exemplary results from the first years of operation.

Coupling of groundwater and surface water at Lake Willersinnweiher: Groundwater modeling and tracer studies

Abstract.Coupling between groundwater and surface water at Lake Willersinnweiher, a gravel pit lake in the Upper Rhine Graben without any surface in- or outflow, was investigated using both a groundwater model and the tracers 18O and SF6. Based on groundwater modeling, recharge and discharge areas around the lake as well as the residence time of the lake water were determined in a regional context. The uncertainty of the simulated flow field was assessed by sensitivity analysis. The tracers 18O and SF6 were measured in several observation wells and piezometers around the lake as well as in the lake’s water column. They were used to verify groundwater flow directions found in the modeling. We found that the groundwater-lake interaction model had a large uncertainty even though relatively detailed information on input data was available. Independent information obtained from the environmental tracers allowed us to improve and verify the model.

Bayesian inversion of Mualem-van Genuchten parameters in a multilayer soil profile: A data-driven, assumption-free likelihood function

This paper introduces a hierarchical simulation and modeling framework that allows for inference and validation of the likelihood function in Bayesian inversion of vadose zone hydraulic properties. The likelihood function or its analogs (objective functions and likelihood measures) are commonly assumed to be multivariate Gaussian in form; however, this assumption is not possible to verify without a hierarchical simulation and modeling framework. In this paper, we present the necessary statistical mechanisms for utilizing the hierarchical framework. We apply the hierarchical framework to the inversion of the vadose zone hydraulic properties within a multilayer soil profile conditioned on moisture content observations collected in the uppermost four layers. The key result of our work is that the goodness-of-fit validated likelihood function form provides empirical justification for the assumption of multivariate Gaussian likelihood functions in past and future inversions at similar sites. As an alternative, the likelihood function need not be assumed to follow a parametric statistical distribution and can be computed directly using nonparametric methods. The nonparametric methods are considerably more computationally demanding, and to demonstrate this approach, we present a smaller dimension synthetic case study of evaporation from a soil column. The main drawback of our work is the increased computational expense of the inversion.

On the reliability of current GPR ground wave methods for determining near-surface water contents

We explore the stability of the Ground-Penetrating Radar (GPR) ground wave signal and its applicability for measuring near-surface water contents along a 0.6 km long measurement line, crossing several vegetated sand dunes at a semi-desert site in Northwestern China. We find that the direct ground wave signal is a stable proxy for measuring near-surface soil moisture. However, the absolute water content may be difficult to establish without additional auxiliary information (e.g. through TDR point measurements). This is mainly due to limitations of the current feature-to-feature evaluation of the air- and ground wave wavelets.

Observation of volumetric water content and reflector depth with multichannel ground-penetrating radar in an artificial sand volume

The performance of the multichannel ground-penetrating radar (GPR) method introduced by [1] to simultaneously estimate reflector depth and average volumetric soil water content is tested on an artificial soil volume. The test site consists of several layers of sand which are partly water saturated. Volumetric water content and reflector depth deduced from the GPR measurements are compared with in situ measurements. It is shown that both reflector depth and volumetric water content can be reconstructed with an accuracy of about 0.1 m and 0.03 … 0.04, respectively. Possibilities for improving the multichannel GPR method are identified in terms of the employed ray path model and the measurement setup.

Cosmic‐ray Neutron Rover Surveys of Field Soil Moisture and the Influence of Roads

Measurements of root-zone soil moisture across spatial scales of tens to thousands of meters have been a challenge for many decades. The mobile application of Cosmic-Ray Neutron Sensing (CRNS) is a promising approach to measure field soil moisture non-invasively by surveying large regions with a ground-based vehicle. Recently, concerns have been raised about a potentially biasing influence of local structures and roads. We employed neutron transport simulations and dedicated experiments to quantify the influence of different road types on the CRNS measurement. We found that the presence of roads introduces a bias in the CRNS estimation of field soil moisture compared to non-road scenarios. However, this effect becomes insignificant at distances beyond a few meters from the road. Measurements from the road could overestimate the field value by up to 40 % depending on road material, width, and the surrounding field water content. The bias could be successfully removed with an analytical correction function that accounts for these parameters. Additionally, an empirical approach is proposed that can be used on-the-fly without prior knowledge of field soil moisture. Tests at different study sites demonstrated good agreement between road-effect corrected measurements and field soil moisture observations. However, if knowledge about the road characteristics is missing, any measurements on the road could substantially reduce the accuracy of this method. Our results constitute a practical advancement of the mobile CRNS methodology, which is important for providing unbiased estimates of field-scale soil moisture to support applications in hydrology, remote sensing, and agriculture.