EGU2020-21742, updated on 20 Jan 2021
https://doi.org/10.5194/egusphere-egu2020-21742
EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Monitoring evapotranspiration and water stress of Mediterranean oak savannas using optical and thermal remote sensing-based approaches

Elisabet Carpintero, Ana Andreu, Pedro J. Gómez-Giráldez, and María P. González-Dugo
Elisabet Carpintero et al.
  • IFAPA. Institute of Agricultural and Fisheries Research and Training of Andalusia. Avd. Menéndez Pidal s/n, 14071 Cordoba, Spain (elisabet.carpintero@juntadeandalucia.es)

In water-controlled systems, the evapotranspiration (ET) is a key indicator of the ecosystem health and the water status of the vegetation. Continuous monitoring of this variable over Mediterranean savannas (landscape consisting of widely-spaced oak trees combined with pasture, crops and shrubs) provides the baseline required to evaluate actual threats (e.g. vulnerable areas, land-use changes, invasive species, over-grazing, bush encroachment, etc.) and design management actions leading to reduce the economic and environmental vulnerability. However, the patched nature of these agropastoral ecosystems, with different uses (agricultural, farming, hunting), and their complex canopy structure, with various layers of vegetation and bare soil, pose additional difficulties. The combination of satellite mission with high/medium spatial/temporal resolutions provides appropriate information to characterize the variability of the Mediterranean savanna, assessing resource availability at local scales.

The aim of this work is to quantify ET and water stress at field-scale over a dehesa ecosystem located in Southern Spain, coupling remote sensing-based water and energy balance models. A soil water balance has been applied for five consecutive hydrological years (between 2012 and 2017) using the vegetation index (VI) based approach (VI-ETo model), on a daily scale and 30 m of spatial resolution. It combines FAO56 guidelines with the spectral response in the visible and near-infrared regions to compute more accurately the canopy transpiration. Landsat-8 and Sentinel-2 images, meteorological, and soil data have been used. This approach has been adapted to dehesa ecosystem, taking into account the double strata of annual grasses and tree canopies. However, the lack of available information about the spatial distribution of soil properties and the presence of multiple vegetation layers with very different root depths increase the uncertainty of water balance calculations. The combination with energy balance-based models may overcome these issues. In this case, the two-source energy balance model (TSEB) has been applied to explore the possibilities of integrating both approaches.  ET was estimated using TSEB in the days with available thermal data, more accurately assessing the reduction on ET due to soil water deficit, and allowing the adjustment of water stress coefficient in the VI-ETo model.

The modeled ET results have been validated with field observations (Santa Clotilde; 38º12’N, 4º17’ W; 736 m a.s.l.), measuring the energy balance components with an eddy covariance system and complementary instruments. The VI-ETo model has proven to be robust to monitor the vegetation water use of this complex ecosystem. However, the integration of the energy balance modelling has improved the estimations during the dry periods, with highly stressed vegetation, enabling a continuous monitoring of ET and water stress over this landscape.

How to cite: Carpintero, E., Andreu, A., Gómez-Giráldez, P. J., and González-Dugo, M. P.: Monitoring evapotranspiration and water stress of Mediterranean oak savannas using optical and thermal remote sensing-based approaches , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21742, https://doi.org/10.5194/egusphere-egu2020-21742, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-21742, Hamideh Nouri, 06 May 2020

    Very interesting research. You suggested that TSEB works better for dry periods. What are the challenges for wet periods?

    • AC1: Reply to CC1, Elisabet Carpintero, 07 May 2020

      Finally, the methodology following the VI-ETo model (optical remote sensing-based approach) was applied over all the seasons, and the model was capable of accurately reproduce the low ET summer rates of this complex ecosystem. The combination of the TSEB and VI-ETo approach will be a future objective that we will address soon. Although in general, the application of models based on thermal or optical images during wet seasons is always challenging due to data gaps due to clouds.

      • CC3: Reply to AC1, Hamideh Nouri, 07 May 2020

        Thanks Elisabet. All the best on your research.

  • CC2: Comment on EGU2020-21742, Stenka Vulova, 07 May 2020

    Very interesting study. I am also working on quantifying ET, but for urban vegetation (using thermal and optical UAV data). Which literature describes how soil parameters and vegetation parameters are derived to compute Ks and Ke? (for the VI-ET0 approach)

    The presentation focuses on the VI approach - did you also try TSEB or use thermal data? How did other methods perform? And, fo you think this method could be applied to urban vegetation, which is more heterogeneous?

    • AC2: Reply to CC2, Elisabet Carpintero, 10 May 2020

      This work has been accepted to be published at Water Journal (MPDI): Carpintero, E., Andreu, A., Gómez-Giráldez, P.J., Blázquez, A., González-Dugo, M.P. Remote sensing based water balance to monitor evapotranspiration and water stress of a Mediterranean oak-grass savanna. After a short time, you will find in the paper the complete description of the methodology explaining how soil parameters and vegetation parameters are derived to compute Ks and Ke.

      In this work, we exclusively have applied the VI-ETo approach. The combination of the TSEB and VI-ETo approach will be a future objective that we will address soon. There are a couple of papers published with the analysis of the TSEB model over this area, and addressed by Andreu et al. (2018). References:

      Andreu, A., Kustas, W.P., Polo, M.J., Carrara, A., González-Dugo, M.P., 2018a. Modeling surface energy fluxes over a dehesa (oak savanna) ecosystem using a thermal based two-source energy balance model (TSEB) I. Remote Sens. 10, 1–27. https://doi.org/10.3390/rs10040567

      Andreu, A., Kustas, W.P., Polo, M.J., Carrara, A., González-Dugo, M.P., 2018b. Modeling surface energy fluxes over a dehesa (oak savanna) ecosystem using a thermal based two source energy balance model (TSEB) II-Integration of remote sensing medium and low spatial

      • CC4: Reply to AC2, Hamideh Nouri, 12 May 2020

        Thanks you very much for the explanation and sharing refs.