Handily measuring sensible and latent heat exchanges at a bargain: A test of the variance-Bowen ratio approach

Eddy covariance has been the gold standard used to quantify on-farm crop water use and validate evapotranspiration and remote sensing models, but it comes with a high cost. Development of surface renewal allowed cheaper implementations, but it still requires complex data processing. Additionally, the thermocouple used in this method is vulnerable to breakage. With the depleting water resources in the western U.S., there needs to be a method that is cost-effective and simple to quantify water use across wide range of crops. Here, we employed the variance-Bowen ratio approach at an alfalfa site to measure sensible and latent heat flux (λE) using 1) a sonic anemometer and 2) an air temperature and relative humidity sensor (T-RH). The variance-based sensible heat flux, using a sonic anemometer and a T-RH sensor, agreed strongly (slope = 0.98, R2 = 0.96, n = 3726) with eddy covariance. The modified-Bowen-based latent heat flux (λEβ) also showed very good results (slope = 0.89, R2 = 0.91, n = 3773) with some uncertainty. We then assessed cases where λEβ performed lesser based on heat advection measurements and remote sensing observations from ECOSTRESS. The problematic λEβ was attributed as a product of the advection of heat and moisture from upwind fields because the assumed equal diffusion of heat and water no longer stands under advection. Overall, our results suggest that the variance-Bowen ratio is a robust, inexpensive, yet user-friendly approach to measure sensible and latent heat flux. Given its reduced cost, this approach also has the potential for measuring horizontal heat advection and validating remote sensing models across many different crop types and ecosystems. Furthermore, the inclusion of advection measurement could be crucial in addressing certain remote sensing models (i.e. DisALEXI and PT-JPL) as they systematically omit any advection components.