Energy balance closure and footprint analysis using Eddy Covariance measurements in Eastern Burkina Faso, West Africa

Abstract. The quality and the representativeness of the first long-term Eddy Covariance measurements in the savanna zone of West Africa were investigated using the energy balance closure and the footprint analysis. The quality and representativeness of the first long-term Eddy Covariance measurements over the West African savanna were investigated using the energy balance closure and the footprint analysis. The analysis covered four contrasting periods such as the complete dry season (January to March 2004), the dry to wet transition period (April to May 2004), the rainy season (June to September 2004) and the wet to dry transition period (October to November 2004).

The results show that the overall energy balance closure can be considered as satisfactory over the whole dataset. The regression fit between (Rn−G) and (H+λE) was significant (P<0.05) with a coefficient of determination (r²) of 0.80 and a slope of 0.88, while the intercept was 25W/m². The energy balance closure was affected by rain during the rainy season (r²=0.69), and by sampling problems during the transition periods (R² were 0.80 and 0.86, respectively).

The footprint analysis shows that the fetch ranged between 20 m (daytime) and 800 m (nighttime). This range showed that the fetch was adequate and fluxes sampled were representative, especially during the rainy season when the vegetal cover was dominated by crops and grasses with scale length of a few meters. During the dry season when the surface is free from crops and grasses, the measurements were also representative as about 60% of the trees around the station were contributing to the measured fluxes. However, during the transition periods some sampling problems appeared, less than 30% of the trees were contributing to the measured fluxes. The relevance of the dominant wind direction in the representativeness of the measurements was also discussed.