Energy partitioning over the West African savanna: Multi-year evaporation and surface conductance measurements in Eastern Burkina Faso

Seasonal variability of the energy partitioning was analyzed with a combination of eddy fluxes of sensible and latent heat and weather data on intensely farmed land in the savanna area of Eastern Burkina Faso, West Africa. The analysis covers two rainy seasons (MayOctober 2003 and 2004), one dry season (December-April 2004), one dry to wet transition period (May-June 2004) and two wet to dry transition periods (October-November 2003 and 2004). This is the first long-term flux measurement reported upon for this part of the world. The inter-annual partitioning of the available energy between the latent heat (actual evaporation) and sensible heat flux shows a conservative relationship between a decrease in annual rainfall (33%) and the sum of the increase in sensible heat flux (20%) and decrease in latent heat flux (10%). The latent heat flux was the main consumer of the available energy during the rainy season (71%), while sensible heat flux was dominant during the dry season (77%). In terms of the rainy season water balance, 1229 mm of rainfall was observed in 2003, of which 351 mm evaporated, while 825 mm of rainfall was observed in 2004, of which 268 mm evaporated. During the dry season, the latent heat flux was strongly coupled to the atmosphere, with the decoupling coefficient (omega) ranging from 0.18 to 0.4. These low values were due to low soil moisture availability and high vapor pressure deficit (VPD > 4 kPa). During the rainy season, the latent heat flux was decoupled from the atmosphere (0.6 =< (omega) < 0.9). Following this pattern of coupling and decoupling, a new formulation of actual evaporation and surface conductance was proposed. The results are thought to be relevant inputs for eco-hydrological models in the semi-arid region of West Africa.