We aim to understand and assess the complex interactions and cumulative effects of multiple stressors on river systems over time scales of several decades. Hydrological processes and extremes greatly impact the dynamics of other stressors, but the interactions remain poorly understood, particularly at larger scales. Our research will investigate long-term multi-stressor trajectories in Central Europe, based on large-scale model simulations. We will (1) couple a large-scale hydrological model with a groundwater model to simulate the dynamics of flows between surface water and groundwater, for the first time representing low flow conditions and desiccation events in Central European catchments over 70 years; (2) simulate water temperature, biological oxygen demand, salinity and their interactions in relation to hydrology; (3) analyse the spatio-temporal patterns of long-term degradation and recovery trajectories focusing on low flows and desiccation events that trigger stressor cascades; (4) assess the impact of climate change and human activities on river discharge; and (5) cooperate with other RESIST projects to upscale biotic consequences of hydrological changes to the Central European scale. This project tests Main Hypothesis MH 1.1 of RESIST, focusing on the assessment of low flow and desiccation events in river ecosystems that trigger multiple stressor effects at the catchment scale. In contrast to RESIST’s other hydrological project (A14), we will simulate stressor interactions at the large scale in Central European catchments, yet will collaborate with A14 on process understanding gained in the case study Kinzig and on complementary data analyses. In Phase II, A24 will assess, understand, and simulate the baseline conditions, long-term trajectories, and stressor interactions in Central European catchments at a spatio-temporal resolution that is relevant to the biota and required by the partner projects. This will be achieved through data collection, data analysis and improvement of simulations of the ReWaterGAP/WaterGAP3 modelling framework. We will analyse the dynamics of multiple stressors, i.e. stressor cascades caused by short and prolonged drought events, under changing climate conditions and the value of using counterfactual climate data to explore the contribution of climate change impacts (i.e. climate-related systems). The project is fundamental to upscaling efforts and will provide a deeper understanding of the contribution of climate change to the timing of degradation, which is essential for the assessment of primary and secondary stressors.

Kolja Maaß (Ruhr University Bochum)

Long-term multi-stressor trajectories in Central European rivers

Rivers and their ecosystems are shaped by a complex interplay of natural processes and human activities. Many ecosystem stressors are affected by hydrological processes and extremes, but their interactions are still poorly understood, particularly at the large scale. In A24, we investigate how multiple environmental stressors - such as drought, water temperature, salinity, and biological oxygen demand (BOD) - interact and impact river systems in Central Europe over several decades. Our goal is to better understand the long-term dynamics of river degradation and recovery in the face of climate change and increasing human pressures.

A key focus of our research is the close connection between surface water and groundwater. Groundwater provides essential baseflow to rivers, especially during dry periods, thereby maintaining ecological flow. The interaction between surface water and groundwater is crucial in times of droughts, but it depends on local conditions and varies across space and time. To capture these dynamics and improve the simulation of low flows and desiccation events, we integrate the gradient-based groundwater model G³M with the WaterGAP3 framework. The modelling framework also accounts for human activities such as water use, reservoir management, and pollution input that affect water resources in terms of both quantity and quality. Additionally, we use the WorldQual model to simulate water quality parameters (water temperature, salinity, BOD). All models operate at a spatial resolution of 5x5 arcminutes (approximately 9x9 km at the equator).

This model integration allows us to simulate low-flow conditions, desiccation events, and their effects on water quality in Central Europe over the past 70 years. The coupled model will be tested and validated against freely available streamflow and groundwater level observations. We will compare our large-scale results on groundwater recharge and baseflow with those from the physically based SWAT+ model for the Kinzig catchment and assess the potential for further model improvement. The outcomes of A24 are relevant for the upscaling of ecological findings and improve understanding of timing and duration of river degradation and recovery phases.

Contact: Kolja.Maass@hydrology.ruhr-uni-bochum.de

First Supervisor: Prof. Dr. Martina Flörke (Ruhr-University Bochum)
Second Supervisor: Prof. Dr. Robert Reinecke (JGU Mainz)