EVALUATION OF ERRORS ASSOCIATED WITH BOWEN RATIO ENERGY BALANCE METHOD OVER GRASS IN AKURE, NIGERIA.

Abstract

One of the standard techniques used to measure evapotranspiration indirectly is Bowen ratioenergy balance (BREB) method. It is a micrometeorological technique, and it is widely used because of its simplicity, robustness, and affordability. This method has been applied in this study to partition available energy (Rn – G) into turbulent fluxes at the study site in Akure, Nigeria during wet and dry periods (July to December, 2014). BREB method estimates sensible and latent heat fluxes from measures of vertical gradients of air temperature and vapour pressure. Both net radiation and soil heat flux are measured directly. These are then combined in surface energy balance equation. Results show that apart from errors of measurement of net radiation and soil heat flux introduced into the estimates of turbulent fluxes, Bowen ratio-energy balance method often produces totally unacceptable results. Wrong signs, and extremely inaccurate magnitude of latent and sensible heat fluxes which occurs when Bowen ratio approaches -1 are common with this method. These problems occur as a result of resolution limits of instruments used. Also, under moist conditions, the BREB method can give good results for evapotranspiration estimates, but the method may not be so accurate under very dry conditions. Under moist conditions, error analysis showed that relative error in evapotranspiration is only small if the relative error in Bowen ratio is also small. In dry conditions, however, the absolute error in evapotranspiration is always fairly small because of the small value of evapotranspiration itself. In this study, errors associated with Bowen ratio-energy balance method are evaluated. A method to find the range of Bowen ratio around -1 which produces inaccurate flux estimates of latent and sensible heat fluxes is presented. The excluded region of Bowen ratio is not constant but depends on the vapour pressure differences measured in each averaging period and the resolution limits of the device used. If temperature and vapour pressure gradients and the resolution limits of the sensors are known, spurious data obtained from the BREB method would easily be recognized. Objective criteria to eliminate spurious data are also presented.