On trends of drought in groundwater dominated hydrosystems in Central Europe

Media type: E-Book

Title: On trends of drought in groundwater dominated hydrosystems in Central Europe

Contributor: Hellwig, Jost [Author]; Stahl, Kerstin [Degree supervisor]

Corporation: Albert-Ludwigs-Universität Freiburg, Fakultät für Umwelt und Natürliche Ressourcen

Published: Freiburg: Universität, 2020

Extent: Online-Ressource

Language: English

DOI: 10.6094/UNIFR/154830

Identifier:

Keywords: Grundwasser ; Dürre ; Klimaänderung ; Modellierung ; Statistik ; (local)doctoralThesis

Origination:

University thesis: Dissertation, Universität Freiburg, 2019

Footnote:

Description: Abstract: Groundwater is the largest storage of freshwater worldwide and in Central Europe. It is the main source of public water supply and of crucial for agricultural irrigation, river discharge and surface water quality. As detailed observations of groundwater are scarce, analyses and predictions related to this topic are usually subject to large uncertainties. Particularly in the context of altered hydrological fluxes due to climate change it is important to broaden the scientific knowledge of the groundwater system and its sensitivity to climate. In consideration of numerous drought events in Central Europe during the last years, the groundwater’s reaction to drought and its role for the propagation of drought is of particular interest. This thesis comprises five studies which extend the understanding of Central European groundwater systems under drought and assess potential future changes based on the groundwater’s sensitivity to climate. Droughts propagate through the hydrological cycle. Therefore, analyses on (groundwater) drought usually assess precipitation deficits as a first step to identify the relevant events. Due to the large number of available meteorological data sets, this step is not unambiguous. Study 1 of this thesis compares different data sets demonstrating the differences and commonalities resulting from different data set resolutions and interpolation algorithms. Altogether it can be concluded, that the absolute precipitation of low-resolution data sets is not an accurate measure for drought studies on a small scale whereas relative measures such as standardized indices are more reliable. These results serve as a basis for the methodology of the following studies. The water quality of surface water depends on a variety of factors. Study 2 investigates the relationship between the deterioration of water quality during dry periods and catchment characteristics. It is shown that different properties are responsible for the responses of different water quality parameters. For example, there is a strong linkage between the geology in the catchment – which determines the chemical load of the groundwater – and the changes of sulfate concentration in the surface water during low flow. On the opposite, changes in the concentrations of chloride and nitrate during dry periods are mainly driven by the amount of sewage discharge within the catchment. Every groundwater system reacts on different time scales to precipitation deficits corresponding to its local hydrogeological characteristics. Study 3 analyses the characteristic groundwater-baseflow response times to precipitation for a set of natural catchments across Germany. Based on that, potential future changes in the annual minimum flow are estimated using common features of climate projections for Central Europe. In most parts of Germany – especially in the Central German Uplands with its fractured rocks aquifers – response times are rather short, indicating a high sensitivity to decreasing summer precipitation. Hence, decreases in annual minimum flow are likely for the future in these catchments. On the contrary, porous aquifers often have much longer response times, therefore increases in winter precipitation could offset the decreased summer precipitation and no substantial changes in annual low flow are expected for these aquifers. Groundwater systems are complex and highly heterogeneous over short distances. As long-term and near-natural observations are scarce, groundwater models are often inevitably the tools for research on groundwater-related processes. In study 4 a transient MODFLOW groundwater model is set up for Germany on a resolution of 1 km² and extensively evaluated. The model is able to depict the heterogeneous behaviour of groundwater head fluctuations due to different hydrogeological conditions. The aquifers differ in their sensitivity to short and long-term meteorological droughts and heterogeneous patterns of groundwater drought severity emerge for different drought events. The characteristic groundwater response time to precipitation is a good proxy to characterize different groundwater reaction types. It ranges in Germany from few months to several years and is strongly linked to hydrogeological and topographic parameters. Changes in climate are expected to alter the hydrological cycle including drought. In study 5 the groundwaters’ and baseflows’ sensitivity to changes in recharge is assessed using model scenarios. Drier antecedent conditions due to longer drought events could lead to more severe groundwater drought events. On the opposite, a seasonal shift of recharge towards more winter and less summer recharge is not likely to exacerbate groundwater droughts much due to the delayed groundwater response. As baseflow reacts on shorter time scales it is more prone to severer short-term droughts. The groundwater and baseflow sensitivity to future climate changes is in general strongly linked to the local hydrogeological conditions. In conclusion, this thesis demonstrates that the groundwaters’ reaction to drought is in Germany very heterogeneous. Characteristic responses to precipitation as well as trends of changes are hardly scalable due to the locally varying hydrogeological conditions. For detailed projections on water availability and water quality during drought – both under current and future climate conditions – multiple factors must be considered. Therefore, groundwater models are inevitable for extensive analyses and recent progress of groundwater representation in large-scale models offers new opportunities for enhanced predictions. Additionally, the groundwater response time to precipitation allows for an assessment of the groundwaters’ sensitivity to drought and potential future impacts. This thesis contributes to a profound understanding of the delayed and smoothed groundwater response to drought in Central Europe facilitating a more effective future water management

Access State: Open Access

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