On the equality assumption of latent and sensible heat energy transfer coefficients of the bowen ratio theory for evapotranspiration estimations: Another look at the potential causes of inequalities

Evapotranspiration (ET) and sensible heat (H) flux play a critical role in climate change; micrometeorology; atmospheric investigations; and related studies. They are two of the driving variables in climate impact(s) and hydrologic balance dynamics. Therefore, their accurate estimate is important for more robust modeling of the aforementioned relationships. The Bowen ratio energy balance method of estimating ET and H diffusions depends on the assumption that the diffusivities of latent heat (K_V) and sensible heat (K_H) are always equal. This assumption is re-visited and analyzed for a subsurface drip-irrigated field in south central Nebraska. The inequality dynamics for subsurface drip-irrigated conditions have not been studied. Potential causes that lead K_V to differ from K_H and a rectification procedure for the errors introduced by the inequalities were investigated. Actual ET; H; and other surface energy flux parameters using an eddy covariance system and a Bowen Ratio Energy Balance System (located side by side) on an hourly basis were measured continuously for two consecutive years for a non-stressed and subsurface drip-irrigated maize canopy. Most of the differences between K_V and K_H appeared towards the higher values of K_V and K_H. Although it was observed that K_V was predominantly higher than K_H; there were considerable data points showing the opposite. In general; daily K_V ranges from about 0.1 m²·s^-1 to 1.6 m²·s^-1; and K_H ranges from about 0.05 m²·s^-1 to 1.1 m²·s^-1. The higher values for K_V and K_H appear around March and April; and around Septemberand October. The lower values appear around mid to late December and around late June to early July. Hourly estimates of K_V range between approximately 0 m²·s^-1 to 1.8 m²·s^-1 and that of K_H ranges approximately between 0 m²·s^-1 to 1.7 m²·s^-1. The inequalities between K_V and K_H varied diurnally as well as seasonally. The inequalities were greater during the non-growing (dormant) seasons than those during the growing seasons. During the study period, K_V was, in general, lesser than K_H during morning hours and was greater during afternoon hours. The differences between K_V and K_H mainly occurred in the afternoon due to the greater values of sensible heat acting as a secondary source of energy to vaporize water. As a result; during the afternoon; the latent heat diffusion rate (K_V) becomes greater than the sensible heat diffusion rate (K_H). The adjustments (rectification) for the inequalities between eddy diffusivities is quite essential at least for sensible heat estimation, and can have important implications for application of the Bowen ratio method for estimation of diffusion fluxes of other gasses.