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| Titel |
Extending periodic eddy covariance latent heat fluxes through tree sap-flow measurements to estimate long-term total evaporation in a peat swamp forest |
| VerfasserIn |
A. D. Clulow, C. S. Everson, M. G. Mengistu, J. S. Price, A. Nickless, G. P. W. Jewitt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 19, no. 5 ; Nr. 19, no. 5 (2015-05-28), S.2513-2534 |
| Datensatznummer |
250120724
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| Publikation (Nr.) |
 copernicus.org/hess-19-2513-2015.pdf |
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| Zusammenfassung |
A combination of measurement and modelling was used to find a pragmatic
solution to estimate the annual total evaporation from the rare and
indigenous Nkazana Peat Swamp Forest (PSF) on the east coast of Southern
Africa to improve the water balance estimates within the area. Actual total
evaporation (ETa) was measured during three window periods (between
7 and 9 days each) using an eddy covariance (EC) system on a
telescopic mast above the forest canopy. Sap flows of an understory tree and
an emergent tree were measured using a low-maintenance heat pulse velocity
system for an entire hydrological year (October 2009 to September 2010). An empirical
model was derived, describing the relationship between ETa from the
Nkazana PSF and sap-flow measurements. These overlapped during two of the
window periods (R2 = 0.92 and 0.90), providing hourly estimates of
ETa from the Nkazana PSF for a year, totalling 1125 mm (while rainfall
was 650 mm). In building the empirical model, it was found that to include
the understory tree sap flow provided no benefit to the model performance. In
addition, the relationship between the emergent tree sap flow with ETa
between the two field campaigns was consistent and could be represented by a
single empirical model (R2 = 0.90; RMSE = 0.08 mm h−1).
During the window periods of EC measurement, no single meteorological
variable was found to describe the Nkazana PSF ETa satisfactorily.
However, in terms of evaporation models, the hourly FAO Penman–Monteith
reference evaporation (ETo) best described ETa during
the August 2009 (R2 = 0.75), November 2009 (R2 = 0.85) and
March 2010 (R2 = 0.76) field campaigns, compared to the
Priestley–Taylor potential evaporation (ETp) model
(R2 = 0.54, 0.74 and 0.62 during the respective field campaigns).
From the extended record of ETa (derived in this study from
sap flow) and ETo, a monthly crop factor (Kc) was
derived for the Nkazana PSF, providing a method of estimating long-term swamp
forest water-use from meteorological data. The monthly Kc
indicated two distinct periods. From February to May, it was between 1.2 and
1.4 compared with June to January, when the crop factor was 0.8 to 1.0. The
derived monthly Kc values were verified as accurate (to one
significant digit) using historical data measured at the same site, also
using EC, from a previous study.
The measurements provided insights into the microclimate within a subtropical
peat swamp forest and the contrasting sap flow of emergent and understory
trees. They showed that expensive, high-maintenance equipment can be used
during manageable window periods in conjunction with low-maintenance systems,
dedicated to individual trees, to derive a model to estimate long-term
ETa over remote heterogeneous forests. In addition, the contrast
in annual ETa and rainfall emphasised the reliance of the Nkazana
PSF on groundwater. |
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