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| Titel |
The estimation of evapotranspiration from wetland sites - the impact of soil physical properties near saturation |
| VerfasserIn |
Enrico Frahm, Bärbel Tiemeyer, Thomas Salzmann, Konrad Miegel |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040601
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| Zusammenfassung |
| The evapotranspiration (ET) is a significant process within the soil-plant-atmosphere
continuum (SPAC). Especially on wetland sites ET is an important component of the water
balance due to the high biomass and ET of wetland vegetation. Thus, comprehensive
knowledge of all hydrological processes is the basis for a sustainable management of
wetlands. Determining wetland ET still suffers from large uncertainties as it is
notoriously difficult to measure directly due to its inherent complexities and small scale
spatial and temporal variations. Consequently, there is a wide range of approaches
to derive evapotranspiration losses indirectly from other parameters, but all have
their significant assets and drawbacks. One of the commonly used methods is the
interpretation of diurnal ground water fluctuations (DGF), which has been successfully
applied to estimate the ET of phreatophytic vegetation (ETGW). The basic idea
behind this method is the assumption of a directly coupled system of incoming solar
radiation, vegetation ET, water transport within the plants and water uptake by the
root system of the phreatophytic vegetation from both the vadose zone and the
groundwater. Such a system is characterised by a strong diurnal cycle and significant
DGFs.
In the presented study, DGFs measured in a rewetted riverine fen in North-Eastern
Germany were analysed to estimate ETGW. With maximum daily values of 5.9 mm for
reed (Phragmites australis) and 7.9 mm for willow (Salix spp.), the method yields
generally plausible results. However, a comparison of the time series of ETGW
and ET according to the Penman-Monteith method (ETPM) shows considerable
discrepancies. Despite continuous sufficient water supply the ETGW results fall up to
90 % below the results of ETPM. The aim of the presented study was to identify
processes explaining these differences. As a first step, we could identify a clear
connection between these errors and the hydrological conditions: The difference
between ETGW and ETPMis increasing with a decreasing depth to groundwater.
Three working hypotheses might help to explain the observed differences between
ETGW and ETPM: a) a depth-dependent specific yield, b) diurnal changes in the
hysteresis of the water retention function and c) the presence of hydraulic lift as
an active system of water uptake and release by the phreatophytic roots. These
hypotheses are evaluated by field measurements of meteorological parameters,
groundwater level, tension and soil water content. Additionally, results of laboratory
tests – evaporation and multi-step outflow experiments – will be used to asses the
hypotheses.
Here, we provide evidence for the influence of all these phenomenona on the results of
ETGW for the first time. The presentation will focus on their relevance for both the
applicability of the DGF method to estimate ET and for the wetland SPAC in general. |
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