Towards repeatable catchment experiments

Wider research context: The rainfall-runoff process is of high interest in catchment hydrology as it directly impacts the quantity and quality of available freshwater. It is influenced by a complex interplay of hydrometeorological variables and catchment properties that complicates the isolation of the effect of individual variables. This calls for new conceptual frameworks that advance our understanding of hydrological processes at the catchment scale. Objectives: The main aim is to better characterize the influence of hydrometeorological variables on runoff generation and catchment-wide water transport by utilizing naturally reoccurring patterns in hydrological flux and state variables as repeated experiments. Approach: First we will define hydrologically similar rainfall and catchment wetness (soil water content and groundwater) patterns by a data-driven and a modeling approach. Rainfall and catchment wetness will be defined as hydrologically similar if their respective runoff reactions are similar as measured by objective functions. Second, water transit time measures (transit time distribution (TTD), fraction of young water (Fyw)) will be defined as similar if a) for TTD their respective simulated isotope tracer in runoff or b) for Fyw the sine waves fitted to the isotope tracer in runoff are similar. Once identified, the hydrologically similar patterns will be searched in real-world data of three study catchments (forest, grassland, agriculture) and the respective runoff reactions will be analyzed. A similarity in the runoff reaction indicates repeatability of rainfall-runoff processes under similar conditions (repeatable experiment) while different runoff responses for similar patterns will be explained by hydrometeorological variables to characterize their influence on the rainfall-runoff process. Additional hydrological modeling will give further insights into catchment-internal reasons for similar or different runoff reactions and enables generalization of results for other catchments Innovation: Repeated catchment experiments in the field are currently impossible due to financial, administrative, and technological constraints. This study circumvents the problem by utilizing naturally reoccurring patterns in hydrologic time series and uses them as repeated experiments to advance our understanding of the rainfall-runoff process. The main outcome of this project will be an advanced understanding of the influence of hydrometeorological variables on the runoff process which can be further used to investigate the rainfall-runoff processes of other catchments. The proposed method can be transferred to other catchments in different climatic regions, has the potential to estimate transit times without long tracer time series and can be used to design measurement networks. Primary researchers involved: Dr. Michael Stockinger, Univ.-Prof. Dr. Christine Stumpp

How quickly rainfall becomes runoff impacts river water quantity, river water quality, and potential water management practices and their consequences. Rainwater uses different quick and slow flow paths on top of or through the soil to reach a river. The hydrological and meteorological factors that impact the transit time of rain to the river were investigated by this project. Among these factors are for example land use (e.g., forest, agriculture), rainfall intensity, or the wetness condition of soil during rainfall. To achieve this, on the one hand factors were investigated during similar runoff events and soil wetness conditions, and on the other hand the transit times of rain and influencing processes were analyzed using a hydrological model. Main results of this project include the development of a novel method to analyze these factors, namely comparing similar runoff events of an area. Using this, we were able to show that rain in areas influenced by maritime climate led to similar runoff events during similar soil wetness conditions, and rain in areas influenced by continental climates led to similar runoff events during similar rainfall characteristics. Furthermore, during similar soil wetness conditions the soil wetness several days before the event was important for runoff, and in some cases additionally rainfall intensities influenced the runoff. This was not only shown with similar runoff events, but also rainfall transit times were successfully modelled. Additionally, the project was able to improve modelling of rainfall transit times by investigating how to best model soil water and groundwater for plausible results.