A review of the Bowen ratio surface energy balance method for quantifying evapotranspiration and other energy fluxes

The Bowen ratio energy balance (BREB) method of indirect measurement of latent and sensible heat fluxes and other major components of the surface energy balance is accepted as one of the robust approaches in the fields of agricultural engineering, micrometeorology, water resources research, hydrology, and related disciplines. The adoption of this technology in quantification of surface energy fluxes in practical applications is mainly attributed to the method's performance and robustness in measurement of such fluxes for various agro-ecosystem surfaces in different climates. The method determines the Bowen ratio (β) by means of measured gradients of atmospheric temperature and air moisture content (actual water vapor pressure) and applies β in the energy balance equation to solve for the latent and sensible heat flux. The main assumption in the Bowen ratio theory is that the energy transfer coefficients for latent heat (K_V ) and sensible heat fluxes (K_H ) are equal. This assumption is made because all energy scalars are carried by the same eddies, therefore, these scalars are associated at the same boundary layer of the evaporating surface. A basic criterion of this method is that the air temperature and water vapor pressure are measured (above an evaporating surface) at such heights that the horizontal gradient of air temperature and water vapor pressure can be neglected. The equality (or similarity) assumption (K_V = K_H) has been proven to be valid for a range of field and vegetation surfaces in various climates. However, the assumption has also been proven invalid for some heterogeneous vegetation surfaces. Nevertheless, successful application of the BREB method to measure surface energy fluxes has been reported for many different types of terrestrial surfaces, including agricultural fields, grasslands, forestry, lakes, wetlands, ocean, etc. This article reviews the history; the main scientific, theoretical, and technical principles; the operational characteristics; and some of the advances in instrumentation of the BREB method. Some studies utilizing the BREB method are also presented.