Groundwater Recharge at Forested Hard Rock Aquifers
Country / Region: Germany
Begin of project: November 1, 2018
End of project: July 31, 2021
Status of project: July 31, 2021
Background:
Estimating groundwater recharge in forested hard rock aquifers is generally difficult because, on the one hand, the quantities of percolation water, which result mainly from the near-surface processes of interception, evaporation, transpiration and other storage components, and on the other hand, the hydraulic properties of the hard rock can often only be recorded or determined unsatisfactorily. In particular, the recording of the water path through the root system of the plants and the exemplary description or parameterisation of this process is mostly unsatisfactory so far. In addition, water storage in unsaturated hard rock has so far received little attention, although it may play a major role in the dynamics of groundwater recharge. For the estimation or prognosis of groundwater recharge by means of models, a better knowledge of the processes and reservoirs, or their model-based description and parameterisation, is therefore necessary.
Groundwater from hard rock with forested catchment areas plays an important role in Germany in the drinking water supply, as the water is comparatively low in anthropogenic pollution and, for example in the low mountain ranges, covers the local water supply in many places via spring catchments.
The focus is on the following questions:
- How does the water uptake of roots behave depending on natural boundary conditions, e.g. weather conditions, soil water content, influence of transpiration, depth of water uptake?
- How large is the real water uptake of the plants depending on these boundary conditions?
- What is the extent of groundwater recharge and what is its dynamic?
The investigations are complementary to the TOPSOIL project, which investigates agricultural land. As a further delimitation to the TOPSOIL project, the focus here is on the unsaturated hard rock, which has so far received little attention as a water reservoir. The investigations proposed here are carried out in cooperation with the Northwest German Forest Research Institute (NW-FVA) and the Institute of Geoecology at the Technical University of Braunschweig.
Geophysical measuring equipment
Source: BGR
Methods:
The water uptake of the plants under real boundary conditions and the soil-groundwater path are quantitatively recorded by artificial irrigation with water with a defined deuterium signature. The investigations are carried out on research sites from the NW-FVA's long-term soil monitoring in Solling with beech and spruce stands.
In soil, sampling is carried out by suction probes and supplemented by soil sampling and subsequent soil water extraction. The sampling is combined with the measurement of the water content using a time domain reflectometry (TDR) probe and the measurement of the suction tension using a tensiometer. Long-term soil-physical measurements and model parameters for both sites are available, as well as the measuring technique mentioned above. The soil-plant path is recorded by sampling the xylem water over the stomata and measuring the xylem flow. Supporting this is the in-situ measurement of the isotope signature in the vapour phase of the soil and in the plants.
The exploration of the soil-groundwater path is supported by geophysical measurements. For this purpose, the infiltration processes at the sites are observed by high-resolution electrical resistance tomography (ERT - Electrical Resistivity Tomography), both during artificial irrigation and under natural conditions. The aim of the investigations is to determine the speed at which the water flows through the unsaturated zone and to determine the heterogeneity of the flow paths. The combination of ERT with TDR and tensiometers, as planned in the project, is crucial to be able to deduce the water content changes and thus the dynamics from the electrical resistance. In addition, the petrophysical parameters will be determined from sample material to allow a quantitative statement.
Project contributions:
Partner:
- Northwest German Forest Research Institute (NW-FVA)
- Technical University Braunschweig, Institute of Geoecology, Junior Research Group Isotopes in Water in the Ground-Plant-Atmospheric System