**5. Conclusions**

Exploiting the capability of the IASI instrument to perform simultaneous retrievals of surface and thermodynamical parameters, we have developed an index called the water deficit index, or *wdi*. The index is intended to be used in the case of evident droughts, as it can assess the severe water deficit of vegetation, and in particular, forests.

The tool has been exemplified in a target area in the south of Italy, which suffered from an intense drought and heatwave in 2017. When the heatwave is developing, we have shown, with the help of correlative observations of surface soil moisture and the leaf area index, that *wdi* can assess the severity of the water deficit. Of particular interest is the anti-correlation with the surface soil moisture. The soil water content and the ability of the soil to transport water to the roots govern the transpiration rate of vegetation. In cases where the *wdi* becomes large, we have found that *ssm* gets smaller, which shows how *wdi* is capable of capturing processes occurring at the surface–atmosphere interface.

The possible usage of *wdi* includes monitoring large forested areas for the increased risks of wildfire and assessing mitigation measures for regions whose green ecosystems are more fragile and in danger because of climate change.

**Author Contributions:** G.M. contributed to the overall conceptualization, organized the field campaigns and revised the paper; F.R. revised the paper, contributed to funding acquisition, and organized the field campaigns; I.D.F. developed the 2D-OI scheme; A.R. was involved in the formal analysis of the field campaign data and contributed to the final editing; L.S. contributed to the editing of the paper and was involved in the field campaigns; P.P. contributed to the analysis of IASI data; A.C. contributed to the implementation of the 2D-OI scheme; S.V. contributed to the software development and data acquisition; C.S. wrote the paper, and contributed to the funding acquisition and conceptualization of the *wdi* index. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Italian Ministry of University has supported this research in the framework of the project ARS01\_00405, "OT4CLIMA" (D.D. 2261 del 6.9.2018, PONR&I 2014-2020 and FSC).

**Data Availability Statement:** The IASI L1C data used in this study are directly available from EU-METSAT. They are received through the EUMETCast near real-time data distribution service. Surface soil moisture data were downloaded from the site https://land.copernicus.eu/global/products/ssm (accessed on 15 August 2022). Leaf area index data were downloaded from the site https://land. copernicus.eu/global/products/lai (accessed on 15 August 2022). All L2 IASI data computed in the paper are available on request from the authors. Data used to build up Figure 10 are available on request from the authors.

**Acknowledgments:** IASI has been developed and built under the responsibility of the Centre National d'Etudes Spatiales (CNES, France). Instrumentation flies onboard the Metop satellites as part of the EUMETSAT Polar System.

**Conflicts of Interest:** The authors declare no conflict of interest.
