Studying the spatial variability of methane flux with five eddy covariance towers of varying height

Studying the spatial variability of methane flux with five eddy covariance towers of varying height

van den Bulk, W.C.M.
Hensen, A.
Bosveld, F.C.
van Huissteden, J.
Peltola, O.
Helfter, C.
Belelli Marchesini, L.
Haapanala, S.
Laurila, T.
Lindroth, A.
Nemitz, E.
Rockmann, T.
Mammarella, I.
Vermeulen, A.T.

2015

In this study, the spatial representativeness of eddy covariance (EC) methane (CH4) measurements wasexamined by comparing parallel CH4fluxes from three short (6 m) towers separated by a few kilometresand from two higher levels (20 m and 60 m) at one location. The measurement campaign was held on anintensively managed grassland on peat soil in the Netherlands. The land use and land cover types are toa large degree homogeneous in the area.The CH4fluxes exhibited significant variability between the sites on 30-min scale. The spatial coef-ficient of variation (CVspa) between the three short towers was 56% and it was of similar magnitude asthe temporal variability, unlike for the other fluxes (friction velocity, sensible heat flux) for which thetemporal variability was considerably larger than the spatial variability. The CVspadecreased with tem-poral averaging, although less than what could be expected for a purely random process (1/vN), andit was 14% for 26-day means of CH4flux. This reflects the underlying heterogeneity of CH4flux in thestudied landscape at spatial scales ranging from 1 ha (flux footprint) to 10 km2(area bounded by the shorttowers). This heterogeneity should be taken into account when interpreting and comparing EC measure-ments. On an annual scale, the flux spatial variability contributed up to 50% of the uncertainty in CH4emissions. It was further tested whether EC flux measurements at higher levels could be used to acquirea more accurate estimate of the spatially integrated CH4emissions. Contrarily to what was expected, fluxintensity was found to both increase and decrease depending on measurement height. Using footprintmodelling, 56% of the variation between 6 m and 60 m CH4fluxes was attributed to emissions from localanthropogenic hotspots (farms). Furthermore, morning hours proved to be demanding for the tall towerEC where fluxes at 60 m were up to four-fold those at lower heights. These differences were connectedwith the onset of convective mixing during the morning period.

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http://resolver.tudelft.nl/uuid:6f988d97-701c-4a4c-899b-60a1b35baf6c

849455

ECN-W--15-031

ECN, Petten

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