Modelling surface energy fluxes over a dehesa (oak savanna) ecosystem using a thermal based two source energy balance model (TSEB) I

Authors: ANDREU, A. - Ifapa Centro Alameda Del Obispo; Kustas, William - Bill; POLO, M.J. - Universidad De Cordoba; CARRARA, A. - Universidad De Valencia; GONZALEZ-DUGO, M.P. - Universidad De Cordoba

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2018
Publication Date: 6/15/2018
Citation: Andreu, A., Kustas, W.P., Polo, M., Carrara, A., Gonzalez-Dugo, M. 2018. Modelling surface energy fluxes over a dehesa (oak savanna) ecosystem using a thermal based two source energy balance model (TSEB) I. Remote Sensing. https://doi.org/10.3390/rs10040567.
DOI: https://doi.org/10.3390/rs10040567

Interpretive Summary: The savanna is among the most variable, complex and extensive biomes on Earth, supporting agriculture and rural economies. These water-limited ecosystems are highly sensitive to changes in both climate, and in land-use/management practices. The application of remote sensing observations with process-based land models estimating water and energy exchange enables monitoring ecosystem status, and thus provides useful information for water resource and land management decisions. The utility of the Two-Source Energy Balance (TSEB) model for estimating water and energy fluxes is evaluated over a Mediterranean oak savanna region (called the dehesa) in southern Spain covering nearly 2.5 million hectares. A detailed analysis of TSEB formulations that account for vegetation architecture (tree overstory and grass understory) as well as remote sensing retrieval of vegetation structure and cover, important inputs to TSEB, was conducted. With just a few modifications to the TSEB formulations and remotely sensed retrievals accounting for the unique dehesa vegetation architecture and their conservative water-use tendencies, the remote sensing-based TSEB is considered adequate for monitoring dehesa ecosystem water use and stress. This information can be used by environmental and land management agencies for assessing dehesa ecosystem health.

Technical Abstract: Savanna is among the most variable, complex and extensive biome on Earth, supporting livestock and rural livelihoods. These water-limited ecosystems are highly sensitive to changes in both climate conditions, and in land-use/management practices. The integration of earth observation (EO) data into process-based land models enables to monitor the ecosystem status, improving its management and conservation. In this paper the utility of the Two-Source Energy Balance (TSEB) model for estimating surface energy fluxes is evaluated over a Mediterranean oak savanna (dehesa). A detail analysis of TSEB formulation was conducted, evaluating how the vegetation architecture (multiple layers) affects the roughness parameters and wind profile, as well as the reliability of EO data to estimate the ecosystem parameters. The results suggest that the assumption of a constant oak leaf area index is acceptable for the purposes of the study and the use of spectral information to derive vegetation indices is sufficiently accurate, although green fraction index may not reflect phenological conditions during the dry period. Although the hypothesis that a separately wind-speed extinction coefficient for each layer was partially addressed, the results showed that taken a single oak coefficient is more precise than a bulk system coefficient. The accuracy of the energy fluxes estimations, with an adjusted Priestly-Taylor coefficient (0.9) reflecting the conservative water-use tendencies of this semi-arid vegetation and a roughness length formulation which integrates the tree structure and the low fractional cover, is considered adequate for monitoring the ecosystem water use (RMSD ~40 W/m^2).