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
Document Type Article
Irmak, Kilic, & Chatterjee in MDPI Climate (2014) 2. Copyright © 2014, the authors. Licensee MDPI, Basel, Switzerland. Open access, Creative Commons Attribution license 4.0.
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 (KV) and sensible heat (KH) 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 KV to differ from KH 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 KV and KH appeared towards the higher values of KV and KH. Although it was observed that KV was predominantly higher than KH; there were considerable data points showing the opposite. In general; daily KV ranges from about 0.1 m2·s−1 to 1.6 m2·s−1; and KH ranges from about 0.05 m2·s−1 to 1.1 m2·s−1. The higher values for KV and KH appear around March and April; and around September and October. The lower values appear around mid to late December and around late June to early July. Hourly estimates of KV range between approximately 0 m2·s−1 to 1.8 m2·s−1 and that of KH ranges approximately between 0 m2·s−1 to 1.7 m2·s−1. The inequalities between KV and KH 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, KV was, in general, lesser than KH during morning hours and was greater during afternoon hours. The differences between KV and KH 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 (KV) becomes greater than the sensible heat diffusion rate (KH). 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.