title: A Contribution to Improving Baseflow Modeling in Low Mountain Ranges
Creators: name: Kissel, Michael Bryan
Publisher: name: UNSPECIFIED
Year: 2025

description: Baseflow is a vital component of the water balance of a catchment. Its consideration in integrated water resources management is important as it impacts the water quality and water temperature, as well as sustains the flow in rivers and streams during dry periods. Therefore, it is an essential ecological factor. Climate change will negatively affect baseflow, e.g., the amount of baseflow, in many catchments in the future. Low mountain ranges are especially at risk of increased water stress and consequent social-economic conflicts due to climate change. Unfortunately, baseflow is difficult to measure. Therefore, a plethora of methods exists to estimate baseflow from observations of total flow. 
However, these methods cannot make predictions for future scenarios, taking climate change and anthropogenic impacts into consideration. For this reason, hydrological models, which simulate the rainfall runoff process in a catchment, are important tools for the investigation of future scenarios for operational planning. The aim of this thesis is to improve the modeling of baseflow in low mountain ranges with hydrological models. To this end, a study is conducted in a field laboratory in the Fischbach catchment in Hesse, Germany, which is a typical catchment in the German low mountain range. A conductivity mass balance method, recursive digital filters, as well as non-continuous separation 
methods are evaluated in the Fischbach catchment. The hydrological model BlueM.Sim is used in this study as it is representative of typical hydrological models, e.g., SWAT and HEC-HMS. The main findings 
of this study are: 
1. The conductivity mass balance baseflow separation method is applicable in low mountain ranges. The months June to September and November to May are best suited for the estimation of the electric conductivity of the baseflow and quick flow components, 
respectively. Both weekly and continuous monitoring are suited for the estimation of the electric conductivity of these components. 
2. The Eckhardt filter, with a calibrated BFI max parameter, is recommended for continuous baseflow separation in low mountain ranges. The Kille method is recommended for the calibration of the BFI max parameter. 
3. The physically based soil moisture approach in the current state of BlueM.Sim was able to reproduce the hydrological characteristics of the Fischbach catchment. At a daily timescale, periods are evident in which the modeled recessions do not match the observed recessions due to the single linear reservoir not being the optimal baseflow model structure in low mountain ranges. A direct calibration with baseflow time series is not possible in the current state of BlueM.Sim. 
4. Simulated total flow duration curves of the conceptual monthly factor-based approach for baseflow modeling can agree very well with those of observed total flow. However, for successful validation, a solution had to be chosen that less consistently reproduced the flow 
duration curve in the calibration period. The current factor-based approach is only applicable for the representation of long-term average conditions. 
5. The greater data requirements, longer computation time and complex calibration of the soil moisture approach make it unfeasible for widespread use in operational planning. The factor-based approach may be a suitable compromise regarding data requirements, computation 
time and calibration requirements. 
6. The implementation of a stepwise calibration significantly improved the physical plausibility of model results, especially regarding baseflow. 
7. The implementation of a parallel linear reservoir baseflow model in the soil moisture approach significantly improved the model results regarding baseflow. Parallel linear reservoirs are the superior model structure for baseflow modeling in low mountain ranges, regardless of 
calibration strategy. 
8. The novel modified factor approach significantly improved baseflow modeling with this conceptual approach. Unlike the original factor approach, this approach can represent different hydrological conditions. Therefore, this approach is of great interest for operational 
planning. 
9. The methodology of the hydrological model evaluation mostly fulfills the requirements of a comprehensive evaluation test as suggested in the literature. Therefore, this strongly suggests the suitability of BlueM.Sim for impact studies due to climate change.