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Open AccessArticle

On the Use of the Eddy Covariance Latent Heat Flux and Sap Flow Transpiration for the Validation of a Surface Energy Balance Model

1
Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali—DICAM, Università degli Studi di Palermo, Viale delle Scienze, Bld. 8, 90128 Palermo, Italy
2
Dipartimento Scienze Agrarie Alimentari ed Agro-ambientali, Università di Pisa, Via del Borghetto, 80, 56124 Pisa, Italy
*
Author to whom correspondence should be addressed.
Remote Sens. 2018, 10(2), 195; https://doi.org/10.3390/rs10020195
Received: 11 October 2017 / Revised: 24 January 2018 / Accepted: 26 January 2018 / Published: 29 January 2018
(This article belongs to the Section Biogeosciences Remote Sensing)
Actual evapotranspiration is assessed via surface energy balance at an hourly rate. However, a robust estimation of daily evapotranspiration from hourly values is required. Outcomes of surface energy balance are frequently determined via measures of eddy covariance latent heat flux. Surface energy balance can be applied on images acquired at different times and spatial resolutions. In addition, hourly actual evapotranspiration needs to be integrated at a daily rate for operational uses. Questions arise whether the validation of surface energy balance models can benefit from complementary in situ measures of latent heat flux and sap flow transpiration. Here, validation was driven by image acquisition time, spatial resolution, and temporal integration. Thermal and optical images were collected with a proximity-sensing platform on an olive orchard at different acquisition times. Actual latent heat fluxes from canopy and sap flux at tree trunks were measured with a flux tower and heat dissipation probes. The latent heat fluxes were then further analyzed. A surface energy balance was applied over proximity sensing images re-sampled at different spatial resolutions with resulting latent heat fluxes compared to in situ ones. A time lag was observed and quantified between actual latent heat fluxes from canopy and sap flux at the tree trunk. Results also indicate that a pixel resolution comparable to the average canopy size was suitable for estimating the actual evapotranspiration via a single source surface energy balance model. Images should not be acquired at the beginning or the end of the diurnal period. Findings imply that sap flow transpiration can be used to measure surface energy balance at a daily rate or when images are found at an hourly rate near noon, and the existing time lag between the latent heat flux at the canopy and the sap flow at the trunk does not need to be taken into account. View Full-Text
Keywords: flux tower; heat dissipation technique; time lag; spatial resolution; acquisition time flux tower; heat dissipation technique; time lag; spatial resolution; acquisition time
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MDPI and ACS Style

Maltese, A.; Awada, H.; Capodici, F.; Ciraolo, G.; La Loggia, G.; Rallo, G. On the Use of the Eddy Covariance Latent Heat Flux and Sap Flow Transpiration for the Validation of a Surface Energy Balance Model. Remote Sens. 2018, 10, 195.

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