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Eddy covarianceAdvances in both instrumentation and theory have made it possible to measure, non-invasively and over long periods, the net ecosystem flux of carbon from vegetated areas using the eddy covariance technique. This technique has recently been applied to measurements of forested areas, and in the past to crops for short term micrometeorology studies . However, very few measurements have been made of wetland ecosystems. The work reported here is, as far as we are aware, a first attempt to characterize the carbon fluxes of tropical wetland vegetation. Our objective was to characterise the papyrus ecosystem carbon budget at scales from the leaf to the whole canopy using measurements made with conventional small scale techniques, such as leaf photosynthesis and respiration of plant components and detritus, alongside eddy covariance measurements. The flux measurements obtained in this way were then compared with predictions from WIMOVAC (Windows Intuitive Model of Vegetation response to Atmosphere and Climate Change) which is a process-based model of leaf physiology and canopy structure (Humphries and Long, 1995) to validate leaf to ecosystem scaling. The ultimate aim of this work is to determine the contribution that papyrus swamps make to carbon cycling in the African tropical wetlands and to examine the hypothesis that papyrus swamps are potentially a major sink for carbon in the tropics. Ecosystem fluxes of CO2, H2O, momentum, sensible and latent heat were measured using the Eddy covariance technique . The system used consisted of commercially available instrumentation: a 3 axis symmetrical sonic anemometer (Solent A1012r, Gill Instruments, Lymington, UK), an infra-red gas analyser (IRGA Li 6262, Li-COR, Lincoln, Nebraska) in closed path mode and custom written software (EddySol, University of Edinburgh) which calculated the fluxes in real time, performing complex co-ordinate rotation and flux averaging as required. A full description of the equipment and EddySol software used in this work is given by Moncrieff et al. (1994), Grace (1995) and Moncrieff et al. (1997). Eddy covariance measurements were made at a height of 7 m above ground level with air drawn at 6l (litres) min-1 down to the IRGA. This was 2-3 m above the tallest papyrus culms and gave a flux footprint in which 95% of the footprint area was contained within the region of homogenous papyrus canopy (Schuepp, 1990). The mast was located on a concrete base (approx. 3 m x 3 m) at a distance of 150 m from the landward edge of the papyrus swamp. Averaged fluxes were recorded at 15 minute intervals for an initial 4 week period from 2 August, 1995 followed by a second period of one week starting on 11 March, 1996. These periods corresponded with, respectively, the coolest (monthly mean @ 15.0° C ) and warmest (monthly mean @ 17.9° C ) times of the year at the experimental site (Muthuri, 1985 and Muthuri et al., 1989). Span calibration of the CO2 and H2O channels of the IRGA was performed at 3 day intervals using chemical absorption columns for zero values and a compressed gas source of CO2 at 600 m mol mol-1 + 10 m mol mol-1 (Linde Gas, England) and a portable dew point generator (Li-610, Li-COR) for span values. Outline Measurement Procedure
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