Analytical Techniques for Measuring Fluxes of CO₂ and CH₄ from Hydroelectric Reservoirs and Natural Water Bodies

Abstract

Hydro-Québec and its partners have been measuring greenhouse gases (GHG) gross fluxes from hydroelectric reservoirs and natural water bodies since 1993. Over the years the methods have changed with a constant aim for improvement. The methods used are: the thin boundary layer, the use of floating chambers with in situ or exsitu laboratory analysis and the use of floating chambers coupled to an automated instrument (NDIR or FTIR). All these methods have their pros and their cons. Over the years many tests were done to compare the methods.

There is no significant difference in the results obtained with the in situ or exsitu laboratory analysis. For CO₂ fluxes, the number of results rejected is similar for the NDIR and the laboratory analysis methods. For CH₄ fluxes, the number of results rejected is three times lower with the floating chamber with in situ laboratory analysis than with the other methods. The precision for duplicate measurements of fluxes is similar for all methods with floating chambers. In general, the thin boundary layer method tends to measure lower fluxes than the floating chamber method with laboratory analysis and there is no good correlation between the two methods. Fluxes measured with automated instruments (specially for fluxes > 5000 mg·m⁻²·d⁻¹) tend to be higher compared with the laboratory analysis method but the correlation between the two methods is very good (R²=0.92 for CO₂). The method with the less logistical constraints is the floating chamber coupled to an automated instrument. This method enables the sampling of about 5 times more sites in the same amount of time as the method with laboratory analysis. The floating chamber coupled to an automated instrument has therefore been retained as the method of choice by Hydro-Québec for GHG gross flux measurements over water bodies.