Comparison of Surface Renewal and Eddy Covariance Estimates of Sensible Heat and Evapotranspiration over a Small Vineyard in a Semi-Arid Heterogeneous Landscape

Abstract

Evapotranspiration (ET), evaporation from water surfaces and soil and transpiration from plants, is a key factor in irrigation scheduling and agricultural water management. Sensible and latent heat fluxes are the main factors in studying water-energy interactions between land, canopy and atmosphere. Latent heat flux will be directly converted to obtain evapotranspiration measurements. In this experiment, sensible heat flux obtained from the Surface Renewal (SR) system was measured using a fine wire thermocouple. This sensible heat flux was then combined with soil heat flux and net radiation measurements to estimate the latent heat flux from as a residual from the energy balance equation. Sensible heat fluxes acquired from the Eddy Covariance (EC) system was measured using the IRGASON. Similarly, the latent heat flux was also obtained from the IRGASON and not as a residual from the energy balance equation. The experiment was done over a vineyard in the Bekaa Valley, Lebanon between October 2021 and December 2021. Half-hourly results showed a good agreement between both sensible heat fluxes with an R2 of 0.76, R of 0.87 and an RMSE of 35 W/m2. The highest mean difference and mean percentage difference between both sensible heat fluxes came from the wind range of angles between 14.3º to 74.9º. Wind coming from this direction passed over urban areas and a fallow field. This might have increased the sensible heat flux of the EC as the system requires a higher fetch. There was a poor correlation between the latent heat fluxes in both systems where R2 was 0.3, R was 0.6 and RMSE was 45 W/m2. This difference can be attributed to errors from the instruments measuring the net radiation and soil heat flux as well as the correction method used. ET derived from the SR was 34% higher than that of the EC. This difference can be attributed to the mode of work of each system. Major rainfall event of 6.8 mm caused an increase in evapotranspiration values of 2.5 mm and 1.4 mm in the SR and EC respectively. Comparison between the available energy and the turbulent fluxes for the energy balance closure showed a very good agreement with an R2 of 0.9, R of 0.95 and an RMSE of 39 W/m2. Further adjustments can be applied to improve performance of the SR such as producing robust fine wires and creating a stand-alone SR system independent of an EC system.