Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 14, 3909-3926, 2017
https://doi.org/10.5194/bg-14-3909-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
07 Sep 2017
A biophysical approach using water deficit factor for daily estimations of evapotranspiration and CO2 uptake in Mediterranean environments
David Helman1, Itamar M. Lensky1, Yagil Osem2, Shani Rohatyn3, Eyal Rotenberg3, and Dan Yakir3 1Department of Geography and Environment, Bar-Ilan University, Ramat Gan 52900, Israel
2Department of Natural Resources, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
3Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
Abstract. Estimations of ecosystem-level evapotranspiration (ET) and CO2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach using remote sensing (RS) and meteorological data (RS–Met) is adjusted to extreme high-energy water-limited Mediterranean ecosystems that suffer from continuous stress conditions to provide daily estimations of ET and CO2 uptake (measured as gross primary production, GPP) at a spatial resolution of 250 m. The RS–Met was adjusted using a seasonal water deficit factor (fWD) based on daily rainfall, temperature and radiation data. We validated our adjusted RS–Met with eddy covariance flux measurements using a newly developed mobile lab system and the single active FLUXNET station operating in this region (Yatir pine forest station) at a total of seven forest and non-forest sites across a climatic transect in Israel (280–770 mm yr−1). RS–Met was also compared to the satellite-borne MODIS-based ET and GPP products (MOD16 and MOD17, respectively) at these sites.

Results show that the inclusion of the fWD significantly improved the model, with R =  0.64–0.91 for the ET-adjusted model (compared to 0.05–0.80 for the unadjusted model) and R =  0.72–0.92 for the adjusted GPP model (compared to R =  0.56–0.90 of the non-adjusted model). The RS–Met (with the fWD) successfully tracked observed changes in ET and GPP between dry and wet seasons across the sites. ET and GPP estimates from the adjusted RS–Met also agreed well with eddy covariance estimates on an annual timescale at the FLUXNET station of Yatir (266 ± 61 vs. 257 ± 58 mm yr−1 and 765 ± 112 vs. 748 ± 124 gC m−2 yr−1 for ET and GPP, respectively). Comparison with MODIS products showed consistently lower estimates from the MODIS-based models, particularly at the forest sites. Using the adjusted RS–Met, we show that afforestation significantly increased the water use efficiency (the ratio of carbon uptake to ET) in this region, with the positive effect decreasing when moving from dry to more humid environments, strengthening the importance of drylands afforestation. This simple yet robust biophysical approach shows promise for reliable ecosystem-level estimations of ET and CO2 uptake in extreme high-energy water-limited environments.


Citation: Helman, D., Lensky, I. M., Osem, Y., Rohatyn, S., Rotenberg, E., and Yakir, D.: A biophysical approach using water deficit factor for daily estimations of evapotranspiration and CO2 uptake in Mediterranean environments, Biogeosciences, 14, 3909-3926, https://doi.org/10.5194/bg-14-3909-2017, 2017.
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Short summary
A remote-sensing biophysical approach for estimating daily evapotranspiration and carbon uptake was tested at Mediterranean forest and non-forest sites in Israel after accounting for root zone water deficit. A newly developed mobile lab system and an active FLUXNET station were used for validation. The model successfully tracked observed changes in fluxes, showing promise for a reliable ecosystem-level assessment. Changes in water use efficiency due to afforestation were assessed in Israel.
A remote-sensing biophysical approach for estimating daily evapotranspiration and carbon uptake...
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