**1. Introduction**

Currently, the most prevailing group of EO methodologies for the estimation of ETc is the Energy Balance (EB) algorithms, and more specifically, residual methods [1]. Remotely sensed EB algorithms convert satellite-sensed radiances into land surface characteristics, such as albedo, leaf area index, vegetation indices, surface roughness, surface emissivity and surface temperature, to estimate ET as a "residual" of the land surface Energy Balance equation [2,3]. The most recent EB models differ mainly in how Sensible Heat (H) is estimated [1]. These models include the Two-Source Model (TSM), where the Energy Balances of soil and vegetation are modeled separately, and then combined to estimate total LE, the Surface Energy Balance Algorithm for Land (SEBAL) and the Mapping Evapotranspiration with Internalized Calibration (METRIC), which use hot and cold pixels within the satellite images to develop an empirical temperature difference equation, and the Surface Energy Balance Index (SEBI) based on the contrast between wet and dry areas [4–6]. Other variations in SEBI include the Simplified Surface Energy Balance Index (S-SEBI) and the Surface Energy Balance System (SEBS) [7].

In this paper, an actual daily evapotranspiration (ETa) computation methodology is presented as a contribution to a European-funded research project named "HubIS". Obtaining useful spatial information and describing difficult physical processes through remote sensing is important for developing better agricultural practices. The applied

**Citation:** Spiliotopoulos, M.; Alpanakis, N.; Tziatzios, G.A.; Faraslis, I.; Sidiropoulos, P.; Sakellariou, S.; Karoutsos, G.; Dalezios, N.R.; Dercas, N. Estimation of Remotely Sensed Actual Evapotranspiration in Water-Limited Mediterranean Agroecosystems for Monitoring Crop (cotton) Water Requirements. *Environ. Sci. Proc.* **2023**, *25*, 9. https://doi.org/ 10.3390/ECWS-7-14200

Academic Editor: Athanasios Loukas

Published: 14 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

approach was first operated by the European Space Agency (ESA) using the Sen-ET plugin, and the proposed methodology is an improvement of ESA's method. The proposed methodology framework consists of seventeen separate steps, with the outcome being the actual daily evapotranspiration flows estimation at a 20 × 20 m spatial resolution. The proposed methodology is applied to cotton in Thessaly, Greece, for the 2021 growing season. The results are very satisfactory and indicate the suitability of Sen-ET SNAP software to estimate daily actual evapotranspiration, as well as spatial variability throughout the crop. The methodology can be applied for effective irrigation managemen<sup>t</sup> in data-scarce rural regions.

### **2. Materials and Methods—Study Area**

In this study, a combination of Sentinel-2 and Sentinel-3 images for daily crop evapotranspiration estimation is presented and applied to cotton fields in Thessaly, Greece, which is considered to be a water-limited, Mediterranean, agricultural area. The simulation program used is the Sen-ET SNAP software. The plug-in uses satellite images from Sentinel-2 and Sentinel-3 and meteorological data from the Weather Research and Forecast (WRF) model. The purpose of the Sen-ET SNAP plug-in is to enable the estimation of daily actual evapotranspiration rates (and other land–surface energy fluxes) at the field scale.

The initial version of Sen-ET used in SNAP, developed by ESA, uses the two-source Energy Balance (TSEB) model [8] with Sentinel-2, Sentinel-3 and meteorological data from ECMWF (ESA, 2020) [9,10]. The adopted methodology in the present approach follows the initial 17 steps based on the TSEB model (Table 1). However, a modification of the initial Sen-ET SNAP used in the present study takes advantage of meteorological data taken from the WRF model, instead of ECMWF. This approach stands as a modification and improvement from the original plug-in. Real-time data produced by the WRF are inserted into the proposed methodology, finally giving ETa values in mm/day with a spatial resolution of 20 m × 20 m.

**Table 1.** The 17 steps created for the operation of Sen-ET.


The study area is the region of Thessaly, which is the most cultivated region in Greece, (Figure 1). The annual quantity of water consumption is about 1.422 hm3, with 92%, i.e., approximately 1305.5 hm3, being used to meet the irrigation needs. Almost 500,000 ha are cultivated in Thessaly, while almost 250,000 are irrigated. Seventy-six (76) % of irri-

gation amounts comes from groundwater systems through legal or illegal drilling, while only twenty-four (24) % comes from surface water systems [9]. The physical condition of groundwater systems in Thessaly is not satisfactory both in quantitative and quality terms [9]. In the present study, seven experimental plots were used in the research program "HubIS" and were analyzed and processed.

**Figure 1.** Study area.
