THE PROPAGATION OF ERRORS IN LONG-TERM MEASUREMENTS OF LAND-ATMOSPHERE FLUXES OF CARBON AND WATER

For surface fluxes of carbon dioxide, the net daily flux is the sum of daytime and nighttime fluxes of approximately the same magnitude and opposite direction. The net flux is therefore significantly smaller th an the individual flux measurements and error assessment is critical i n determining whether a surface is a net source or sink of carbon diox ide. For carbon dioxide flux measurements, it is an occasional misconc eption that the net flux is measured as the difference between the net upward and downward fluxes (i.e. a small difference between large ter ms), This is not the case. The net flux is the sum of individual (half -hourly or hourly) flux measurements, each with an associated error te rm. The question of errors and uncertainties in long-term flux measure ments of carbon and water is addressed by first considering the potent ial for errors in flux measuring systems in general and thus errors wh ich are relevant to a wide range of timescales of measurement, We also focus exclusively on flux measurements made by the micrometeorologica l method of eddy covariance. Errors can loosely be divided into random errors and systematic errors, although in reality any particular erro r may be a combination of both types. Systematic errors can be fully s ystematic errors (errors that apply on all of the daily cycle) or sele ctively systematic errors (errors that apply to only part of the daily cycle), which have very different effects. Random errors may also be full or selective, but these do not differ substantially in their prop erties. We describe an error analysis in which these three different t ypes of error are applied to a long-term dataset to discover how error s may propagate through long-term data and which can be used to estima te the range of uncertainty in the reported sink strength of the parti cular ecosystem studied.