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Abstract "Heft 25"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 25

Gerhard Brahmer

Wasser- und Stoffbilanzen bewaldeter Einzugsgebiete im Schwarzwald
unter besonderer Berücksichtigung naturräumlicher Ausstattungen und atmogener Einträge

Freiburg im Breisgau 1990

ISSN 0344-2691


Water and element fluxes were measured in three adjacent watersheds of the two ARINUS* experimental areas (SCHLUCHSEE and VILLINGEN) in the Black Forest between June 1987 and May 1989. The objective was to determine the portion of watershed internal element turnover, particularily in the pedosphere, in comparison to atmospheric depositions and their importance for nutrient loss and chemical dynamics in groundwater and surface water.

Plots for the measurement of open land precipitation (amount and chemistry) as well as other meterological parameters were established. In all watersheds, canopy throughfall and soil moisture were measured and soil seepage water was collected. For recording of runoff Thomson weirs were installed. In VILLINGEN, permanent wells allowed a direct sampling of groundwater. Soil chemical and physical parameters were determined at representative soil profiles based on a detailed mapping of the soil forms.

Water samples were taken weekly (in streams during heavy changes in water flow, more frequently) at different levels in the watersheds. The concentration of NH4+, Na+, K+, Ca2 + , Mg2 + , Al, Fe, Mn, Si, SO42-, NO3- and Cl- was analyzed. Furthermore, temperature, pH-value, alkalinity, UV-absorbance, color and specific electrical conductivity were determined.
To set up annual water budgets, precipitation, runoff and potential evapotran-spiration, calculated by means of different methods, were balanced. Differences in the water balance were checked with changes in soil moisture. The deviation from the difference between the climatic water balance and runoff at VILLINGEN compared less than 4 % off the amount of measured precipitation. At SCHLUCHSEE in contrast, negative differences, approximately 10 % of the measured precipitation, indicate an underestimation of the actual precipitation, especially during the snow abundant winter of the first study year.

To divide the runoff in direct interflow and indirect baseflow, a hydrochemical hydrograph separation was performed. Sodium concentrations in the streams were used as an indicator for runoff originating from groundwater since this element is released by silicate weathering, thus showing distinct differences between precipitation and seepage.
"(Effects of Restabilization Measures and Atmospheric Depositions  on the  Cycling of Nitrogen and Sulfur in the Eco- and Hydrosphere of Black Forest Sites)

Based on this model, the indirect runoff amount from SCHLUCHSEE accounts for 50-60 % of the total runoff, whereas in VILLINGEN only about 30 %. This is due to distinct differences in soil physical properties of the two study areas. In particular at VILLINGEN, poorly permeable subsoil horizons lead to a predominant lateral transport of water in the topsoil. Accordingly this results in a high percentage of direct runoff beeing unusual for forested watersheds.

Element input by open land precipitation is minor as compared to other forested areas in Central Europe. Deposition originates from long distance transport, since local emissions appear to be not important. From the statistical analysis of the element concentrations in open land precipitation, "mineral dust", "sea salt", and "acidic emissions" were found to be the main factors affecting precipitation chemistry. From the comparison between open land precipitation and canopy throughfall, the controlling factors responsible for the differences, "acidic interception", "foliar leaching", "sea salt" and "dust" were derived and quantified. Using these results, a model to seperate dry deposition from foliar leaching was modified and applied in the study area.

According to this model, the atmospheric inputs were deposited mostly by rainfall, whereas dry depositon, in accordance to the low concentration of pollutants in the ambient air, was not of major importance. The total atmospheric deposition In each of the adjacent watersheds correlated closely and showed only slight differences between the two study areas.

Water chemistry of the two study areas differs distinctly. The chemical composition of streamwater is mainly controlled by the respective hydrological pathways. Depending on the time to reach equilibrium as well as the chemical properties of soil and bedrock materials, different patterns of interaction between aqueous and solid phase emerge. At VILLINGEN, a considerable amount of direct runoff moves laterally in the acidic upper soil horizons or humus layers whereas at SCHLUCHSEE vertical water transport dominates according to the high water conductivity of the soil. In the SCHLUCHSEE area, groundwater runoff has a large influence on the chemical compositon of the streams owing the long time of contact between water and bedrock. Direct effects of the current atmospheric depositions on the streamwater are not seen since site specific characteristics and processes are more prevelant.

Based on estimations of annual elemental fixation by the stand, element fluxes in open land and canopy throughfall, total deposition, fixation of the stand biomass, watershed output, net export from the watershed and net mobilization of elements in the watershed were balanced. Despite a similar atmospheric deposition of S and N, the turnover of both elements in the two study areas differs   considerably.   Given   the   same   low   sulfur   input,   twice   this      amount   is released from the SCHLUCHSEE watersheds, whereas at VILLINGEN, sulfur output equals input. Nitrogen deposition at VILLINGEN is almost completely retained in the watershed, whereas at SCHLUCHSEE, N export is in the order of the input. The different pattern of element output in both areas results from different retention and mobilization processes along respective hydrological pathways.

A comparison of the input-output budgets with those from other areas illustrates the wide range of natural variation of elemental turnover in watersheds. To evaluate the effects of changed element inputs on the hydro- and pedosphere, extensive deterministic models should be used.

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