title: On synergies between spatially-distributed, physically-based simulations and field observations in catchment hydrology
Creators: name: Glaser, Barbara
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: surface saturation
subject: thermal infrared imagery
subject: preferential flow
subject: catchment hydrology
subject: process understanding
subject: spatio-temporal variability
subject: field observations
subject: hydrological modelling
subject: HydroGeoSphere
subject: 550

description: A fundamental research objective in catchment hydrology is to investigate and understand the hydromechanical processes occurring within natural watersheds. A wide variety of different observation and field methods exists for providing insights into the hydrological functioning of hillslopes and catchments. However, all existing methods are limited in their spatial or temporal application. Consequently, there are limitations in the understanding on if and how the occurrence and relevance of different hydrological processes vary in space and time. A possibility to overcome the methodological limitations of field measurements is the complementary use of hydrological models. Yet hydrological models are only rarely used in real synergy with field observations and the existing studies mostly apply conceptual, bucket-type models, which are not particularly suitable for investigating the spatio-temporal variability and relevance of different processes. This dissertation presents two examples of applying a spatially-distributed, physically-based hydrological model in synergy with various conventional and innovative field data. The first part of the thesis (Study 1) focuses on the relevance of preferential flow as commonly observed in plot-scale field experiments for explaining the long-term response of catchment discharge and soil moisture. The study assesses the importance of the plot-scale field observations by applying a spatially-distributed, physically-based model and comparing the results of simulations with and without preferential flow parametrisations at plot and catchment scale against appropriate field data. The second part of the thesis (Study 2 - 4) addresses the intra-catchment variability of the dynamic development of surface saturation. It consists of three consecutive observation and simulation studies, following the idea to first employ a comprehensive data set of field observations to verify the consistency of the internal structures and processes of a model with reality before using the model to support the interpretation and overcome the limitations of field investigations. The first of the three studies (Study 2) assesses the practicability of applying thermal infrared imagery during different seasons and hydrological conditions and at various locations across a catchment to map and quantify surface saturation. The following study (Study 3) applies a spatially-distributed, physically-based catchment model in combination with the field data set collected in Study 2. On the one hand, it evaluates the capability of the model to reproduce the intra-catchment variability of the spatial patterns and temporal dynamics of surface saturation. On the other hand, the identified matches and mismatches between observation data and simulation results are used to infer which key factors control the spatio-temporal occurrence of surface saturation. The last study (Study 4) uses the comprehensively validated model of Study 3 to analyse the generation of surface saturation in space and time with regards to the immediate mechanism of water delivery and to different geographical source areas. All studies were carried out for the 42 ha forested Weierbach catchment, a well-established research catchment in western Luxembourg with complex hydrological behaviour. The simulations were performed with the integrated surface-subsurface model HydroGeoSphere. Study 1 revealed that small-scale observations of preferential flow are not informative to infer the influence of preferential flow paths at catchment scale. Although vertical preferential flow paths needed to be considered to simulate the plot-scale field observations, it was not suitable to apply the appropriate parametrisations at catchment scale. Instead, fast lateral flow paths showed to outweigh the role of vertical preferential flow at the catchment scale. Study 2 showed that thermal infrared imagery is a reliable method for collecting a comprehensive data set of surface saturation patterns and dynamics when considering some precautions for the image acquisition and post-processing. Study 3 demonstrated that the model can mostly reproduce the observed surface saturation patterns and dynamics, including locally varying patterns of surface saturation occurrence and varying relations of the local saturation extent to catchment discharge. The model performance suggested that the occurrence of surface saturation was mainly conditioned by exfiltration of groundwater into micro-topographic depressions, but that there must be additional influencing factors and processes, such as local heterogeneities in the subsurface, varying influences of precipitation or upstream water, and hysteretic formation processes. Study 4 proved that the surface saturation in the Weierbach catchment is largely composed of water that is delivered to the riparian zone and streambed by return flow from the subsurface. The spatio-temporal variability of the mixing of water from different subsurface stores and contributions from other water sources such as overland flow or precipitation showed to be smaller than expected. Overall, the results of the different studies demonstrated the great potential of synergies between spatially-distributed, physically-based modelling and field observations to substantially advance current process understanding in catchment hydrology, both for specific study sites and for general questions and conceptual perceptions.