A SIMPLE SYSTEM FOR AUTOMATED LONG-TERM BOWEN-RATIO MEASUREMENT

The Bowen ratio method is widely used to estimate fluxes of sensible heat and evaporation between a crop and the atmosphere. The most critical points of this method are the measurements of vertical humidity gradients, and long-term operation. We propose a simple system, using a capacitive hygrometer and alternate sampling of air at two levels with pumps to measure humidity gradients. It is well adapted to long-term measurements: low power consumption, low charge of maintenance, and low cost. It has been tested by comparison of hourly sensible and latent heat flux measurements performed by both eddy correlation and Bowen ratio, over a bare soil field and a fully evaporating canopy. Results are satisfactory over a large range of fluxes (0-400 W m-2) under daytime conditions. The main sources and magnitude of errors were investigated under the contrasted situations of bare soil and fully evaporating canopy. The system was shown to give good flux estimates even with very low humidity gradients.