Freiburger Schriften zur Hydrologie

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Band/volume 25: Wenninger J. (2007):

Prozesshydrologische Untersuchung im System Boden-Vegetation-Atmosphäre

The presented thesis „Studies of hydrological processes in the soil-vegetation-atmosphere system“ was carried out as part of the research projects “water balance of a dry forest stand” (Wasserhaushalt eines Waldes auf einem Trockenstandort) and “Use of geoelectrical resistiv­ity methods combined with tracer techniques to explore hydrological processes” (Einsatz geophysikalischer Methoden in Verbindung mit Tracermethoden in der Abfluss­bildungsfor­schung). This study contributes to process knowledge of the water fluxes in the soil‑vegetation‑atmosphere system at varying scales and study sites. Regarding the emerging changes of the global climate it is important to understand the hydrological processes in detail. This understanding is a basic requirement for the development of conceptual models and an appropriate hydrological modelling. Tracer methods are known as appropriate tools to investigate water fluxes, source areas and runoff components of a hydrological system. However it is not possible to answer all questions with this approach. In this thesis, classical hydrometric measurements – e.g. precipitation, runoff, soil moisture and groundwater level – tracer hydrological investigations – stable isotopes and geogene tracer – as well as geophysi­cal methods – 2D direct current resistivity – were applied. Combining the results of the experimental methods, insights on conceptual process knowledge of the hydrological soil‑vegetation‑atmosphere system could be gained. The soil represents an important balance- and turnover volume for various questions and diverse scientific fields. From a hydrological point of view major questions are processes of soil water movement and related to that observation of the temporal dynamics of groundwater recharge, separation of different runoff components and runoff generation processes and water balance calculations and dewatering by evapotranspiration. The process knowledge of this important hydrological area allows further predictions on impacts of future changes in climate and land use. The experiments were carried out at the Forest Meteorological test site Hartheim in Germany (plot scale) and within Weatherley catchment in South‑Africa (hillslope‑ and catchment scale).
The main part of the presented investigations was focused on research at the plot scale at the Forest Meteorological test site Hartheim (FMIF). The test site is an ecosystem characterised by restricted water conditions especially during the growing season. To investigate the temporal dynamics of the water balance, all important components of the hydrological water cycle were quantified. Dense temporal measurements of the meteorological conditions and the soil water balance allowed a detailed water balance calculation. The water balance at the FMIF confirms a deep infiltration depending on soil water conditions and precipitation characteristics. Within the investigation period of the hydrological year 2004 deep infiltration (and groundwater recharge - GWNB) could be estimated. The calculated GWNB could be verified, and the temporal dynamic could be shown with a one dimensional soil water model. Core of the tracer hydrological investigations is the use of the stable water isotopes. Stable isotopes were used as natural and applied tracers. Isotope signatures of precipitation were used as system input function to investigate unsaturated zone processes. Isotope compositions in soil water allowed statements on infiltration processes, mean residence times and evapora­tion. In the soil‑vegetation‑atmosphere system natural isotope signals were used to estimate source areas of water uptake by vegetation. Stable isotopes were used as applied tracers to gain further insights into the temporal dynamic of the transpiration process and infiltration within the soil system. Combining stable water isotopes with age dating methods led to a better knowledge of source areas of the groundwater at the FMIF. It could be shown that the groundwater originates mainly from local precipitation and lateral inflow. The application of geophysical methods allowed a non-invasive acquisition of subsurface conditions. With the aid of electrical resistivity surveys a two dimensional picture of structural characteristics could be drawn. By time‑lapse measurements at the same profile information about the variability of the resistivity values could be gained and with transfer functions it was possible to extrapolate point values of soil moisture to a larger scale.
Investigations at the hillslope and catchment scale in the Weatherley catchment allowed conclusions on dominating runoff generation processes through the use of various experimental methods. The application and combination of different field methods at the same investigation area led to better process understanding. This was particularly useful as each method has shortcomings and limitations regarding special and temporal resolution. The Analysis of hydrometric time series showed the formation of an episodic perched water table within the soil profile. The importance of this fast component during runoff events could be verified and quantified through tracer data and runoff component separations. Geoelectrical measurements provide spatial information about this hydrological important area. A combination with point measurements – like soil moisture, groundwater and depth of bedrock – allowed classifying the heterogeneous subsurface conditions. The results led to the confirmation and refinement of a conceptual model of runoff generating processes within the catchment.