Evaluating eddy covariance cotton ET measurements in an advective environment with large weighing lysimeters

Abstract

Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface sensible and latent heat fluxes and evapotranspiration (ET) from local to regional scales, and in crop coefficient development. Therefore, the objective was to evaluate the accuracy of EC systems in measuring sensible heat (H) and latent heat (LE) fluxes. For this purpose, two EC systems were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas High Plains, during the months of June and July 2008. Sensible and latent heat fluxes were underestimated with an average error of about 30%. Most of the errors were from nocturnal measurements. Energy balance (EB) closure was 73.2–78.0% for daytime fluxes. Thus, daylight fluxes were adjusted for lack of EB closure using the Bowen ratio/preservation of energy principle, which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint expanded beyond the crop field boundary. As a result, ET values remarkably matched lysimetric ET values, with a ‘mean bias error ± root mean square error’ of −0.03 ± 0.5 mm day⁻¹ (or −0.6 ± 10.2%).

Appendix

Conversion of daytime average LE fluxes into ET rate

$$ {\text{ET}} = \left( {{\frac{{3,600 \times N \times {\text{LE}}}}{{\lambda_{\text{LE}} \times \rho_{\text{w}} }}}} \right) $$

(16)

where ET is evapotranspiration (mm day⁻¹) converted from daytime average EC-measured LE (W m⁻²). λ_LE is the latent heat of vaporization (MJ kg⁻¹), equal to (2.501–0.00236 T_a), being T_a in °C units, and ρ_w is water density (~1 Mg m⁻³). The 3,600 is a time conversion of s h⁻¹; while the N is the number of bright sunshine hours per day. N is computed as follows:

$$ N = \left( {{\frac{24}{\pi }} \times \omega_{\text{s}} } \right) $$

(17)

where ω_s is the sunset hour angle (radians), computed as:

$$ \omega_{\text{s}} = {\text{arc}}\;\cos \left[ { - \tan \left( \Upgamma \right)\tan \left( \delta \right)} \right] $$

(18)

where Γ is the location latitude (radians) and δ is the solar declination angle (radians).

$$ \delta = 0.409 \times \sin \left( {{\frac{{2\pi \times {\text{DOY}}}}{366}} - 1.39} \right) $$

(19)

where DOY is the day of the year and 366 is the number of days in a leap year. In our case, 2008 was a leap year; otherwise the number should be 365 for a regular year.