Proceedings zur 6. Fachtagung BIOMET - Deutsche ...

To study climate and land-use effects separately the watershed model BROOK90 (FEDERER,
1995) was used. It was parameterised by objectively derived areal parameters; no specific
calibration was done to fit observed and modelled runoff or ET (SEEGERT, 1998). In the period
1968 to 1995 a very good resemblance between measured and modelled ET was found
(Fig.3). Since about 1996 a clear change in this relationship is observed: The measured watershed
ET is typically larger than the modelled ET by up to 30%. Several hypotheses have been
tested for this surprisingly large difference including measurement insufficiencies (i) of the
runoff measurements or (ii) the meteorological input to BROOK90. A proof of such a large
change was not detected, neither in the stream gauge records nor in the meteorological records,
despite several inhomogenieties in those data (Wollner, pers. communication). Alternatively
land-use change was tested. Here, the most pronounced change is at one hand the reduction
of clear cuts (until 1990 about 1-2% of the area of the catchment annually) and on the
other hand the planting of beech in the under storey of the spruce canopy (about 10% of the
area of the catchment since mid of the 1990ies). Other possible influences are a change in the
draining of the catchment, especially the change in the maintenance of artificial drainage
which would lead to a change in the runoff coefficient of the catchment.
4 Discussions and Conclusions
Actual evapotranspiration (ETa) showed a very distinct behaviour after 1995. While potential
ET increased only marginally, runoff statistics show a significant change which lead to a
much higher actual ET of the watershed. However, changes in land-use point toward a possible
influence of recent changes in forest management. It is not clear yet whether those
changes can be attributed to changes in land-use alone or a combination of measurement insufficiencies,
changes in run-off characteristics due to less artificial drainage or changes in
land-use. Additional modelling with adapted parameters needs to be done to isolate these effects.
If correct, the consequences of those changes might be severe and should be part of the
risk analysis both in forestry and water issues.
Acknowledgement
The authors are grateful to the German tax payers, the Saxonian Ministry of Science and Art
(as the major supporters of TU Dresden) and many projects that helped to continue those
measurements over so many years, as well as to many students from undergradutae to graduate
and PhD level for their enthusiasm and help.
References
ATV-DVWK-Merkblatt M 504 (BERNHOFER, C., GUGLA, G., GOLF, W., GÜNTHER, R.,
JANKIEWICS, P., KLÄMT, A., MENZEL, L., MIEGEL, K., OLBRICH, H.-D. & U. WENDLING),
2002: Verdunstung in Bezug zu Landnutzung, Bewuchs und Boden. - ATV-DVWK-
Regelwerk. 144pp.
FEDERER, C. A., 1995: BROOK90: A Simulation Model for Evaporation, Soil Water and
Straemflow, Version 3.1ff bzw. 4.0, Computer Freeware and Documentation – USDA
Forest Service, Durham, USA.
FRANKE, J., GOLDBERG, V., EICHELMANN, U., FREYDANK, E. & C. BERNHOFER, 2004: Statistical
analysis of regional climate trends in Saxony. - Climate Research. 27:145-150.
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