<p>Olive trees are one of the most important crops in the Mediterranean basin (10.5 Mha), accounting for 97.5% of the world&#8217;s olive cultivation area with relevant social and economic benefits and ecological consequences. Concretely, it takes up 2.7 Mha in Spain, of which more than 1.6 are in Andalusia. Olive cultivation demands climate-smart management to facilitate crop adaptation to climate scenario and predictable development. A more efficient water use and management optimization is an especially important issue and, therefore, quantifying and modeling evapotranspiration (ET) is essential.<br>However, given the lack of a satellite thermal mission with both high spatial resolution and frequent revisit time, we have evaluated in this work a data fusion methodology (Gao et al., 2012) that combines Sentinel-2 and Sentinel-3 images with the two-source energy balance model (Norman et al.,1995) proposed by Guzinski & Nieto et al. (2019). Estimates of actual ET were produced at 20 m resolution from January 2016 to December 2019 in an irrigated olive grove in Southern Iberian Peninsula. Preliminary results have been validated (every 5-10 days depending on Sentinel images availability and cloud cover) by ground-based in situ data using Eddy Covariance (EC) technique, showing mean absolute errors between estimated values and those obtained by EC: 156 Wm<sup>-2</sup> (net radiation), 76 Wm<sup>-2</sup> (soil heat flux), 36 Wm<sup>-2</sup> (sensible heat flux), 210 Wm<sup>-2</sup> (latent heat flux).</p><p>Gao, F., Kustas, W. P., & Anderson, M. C. (2012). A data mining approach for sharpening thermal satellite imagery over land. Remote Sensing, 4(11), 3287&#8211;3319. https://doi.org/10.3390/rs4113287</p><p>Guzinski, R., & Nieto, H. (2019). Evaluating the feasibility of using Sentinel-2 and Sentinel-3 satellites for high resolution evapotranspiration estimations. Remote Sensing of Environment, 221, 157&#8211;172. https://doi.org/10.1016/j.rse.2018.11.019</p><p>Norman, J. M., Kustas, W. P., & Humes, K. S. (1995). Source approach for estimating soil and vegetation energy fluxes in observations of directional radiometric surface temperature. Agricultural and Forest Meteorology, 77(3&#8211;4), 263&#8211;293. https://doi.org/10.1016/01681923(95)02265Y</p>
<p>The Mediterranean region has a great surface of olive crops where about 98% of the world&#8217;s olive agricultural area is represented. Of these lands, 2.7 Mha are in Spain with more than half concentrated in the Southeastern Iberian Peninsula. In this type of crop, the maintenance of natural herbaceous-cover in the alleys from autumn to spring is a common practice to protect the soil against erosion, but little is known yet about its contribution to CO<sub>2</sub> and H<sub>2</sub>O fluxes and their seasonal variability. The eddy covariance technique is used worldwide to measure GHG fluxes at the ecosystem level. Additionally, this technique has been used successfully to measure fluxes below the canopy in closed forests and pastures. In this regard, continuous monitoring of eddy covariance CO<sub>2</sub>/H<sub>2</sub>O fluxes above and below the trees canopy was carried out in an irrigated olive grove (<em>Olea europaea L.</em>) in the Southeastern Iberian Peninsula in the hydrological year 2020-2021. The olive trees are distributed in a plantation frame of 12&#215;12 m and the area is divided into two plots: 1) with natural herbaceous-cover from autumn to spring, then cut and left on the surface (hereafter HC); and 2) kept herbaceous-free by glyphosate-based herbicide application (hereafter HF). Each plot has two eddy covariance towers, one above the canopy (ecosystem tower) and the other below the canopy (subcanopy tower).</p><p>A comparison between fluxes measured with the subcanopy towers and those measured with ecosystem towers showed the need for wind-direction filtering of the fluxes measured at the subcanopy level. Results show the relevance of selecting those fluxes coming from wind directions where the alleys are located in order to get accurate subcanopy CO<sub>2</sub>/H<sub>2</sub>O fluxes, avoiding those eddies coming from the olives. Regarding seasonal variability of the CO<sub>2</sub>/H<sub>2</sub>O fluxes measured at the subcanopy level, preliminary results showed that the HC plot behaved as a C light sink in winter (Dec., Jan., Feb.), being February the month with the most absorption averaging around 1.5 g C m<sup>-2</sup> day<sup>-1</sup> while HF behaved as C neutral. In the month before mowing (March), HC behaved as a sink, absorbing, on average, around 2.5 g C m<sup>-2</sup> day<sup>-1</sup>, while HF acted as a light source emitting around 0.2 g C m<sup>-2</sup> day<sup>-1</sup>. After mowing (from April to June) both HC and HF acted as sources, with HC yielding the largest values in April (around 2.1 g C m<sup>-2</sup> day<sup>-1</sup>). Finally, in summer and autumn (from July to Nov.) both HC and HF appear to behave as weak C sources at the subcanopy level.</p>
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