## 4 Discussion  
A major characteristic of drought is the presence of extremely low soil moisture, either due to reduced precipitation and/or increased evapotranspiration [64]. This suggests that soil moisture can provide vital signals about drought conditions and its severity. When studying soil moisture deficit, it is usual to pay attention to the correlation between soil moisture anomaly and the following two frequently used drought indices: the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI) [65]. The World Meteorological Organization [65] suggested that the SPI estimated for time intervals between one and six months can be used for agricultural drought monitoring. Previous studies [66; 67] have used the SPI as a benchmark to compare soil moisture anomalies. In the current study, spatiotemporal variability of drought is analyzed using as reference parameter the soil moisture availability to vegetation, SMAI [42]. This biogeophysical index can account for onset/termination of different levels of drought severity being functionally related to soil moisture deficit, while SPI is a precondition for soil moisture deficit on a correlation principle. The obtained  
Figure 9: Land surface state and probable fire occurrence: Monthly mean LSASAF LST 0900 UTC for (**a**) August 2016 and (**b**) August 2017. Thermal anomaly detections by LSASAF FRP-Pixel product for (**c**) August 2016 and (**d**) August 2017. Monthly mean (2007–2018) LSASAF LST 1200 UTC values for (**e**) August. results are indicative of a strong functional negative linear relationship between SMAI and LST during periods of dry land surface anomalies. Using a long-term dataset of 12 years of records from the contemporary climate (2007-2018) over Bulgaria, Eastern Mediterranean region, LST is applied in the context of an Essential Climate Variable (ECV). The correlation varies with the changes of the ratio between evapotranspiration and soil moisture during 'wet/dry' periods, and it is also related to the type and state of vegetation cover and local climate. Thus, the current study provides an advanced framework for using LST retrievals based on IR satellite observations from the geostationary MSG satellite as a source of information for studying climatic aspects of dry land surface state anomalies in the context of coupled energy and water cycles. The reported behavior of the biophysical parameter LST underlines the role of land-atmosphere interactions for the establishment of dry land surface anomalies in the Eastern Mediterranean. The physical basis of this framework is the relationship between surface soil temperature and soil moisture. The variations of surface soil temperature are related to soil thermal properties and meteorological factors such as solar radiation, air temperature, relative humidity, wind, etc. When the soil surface is wet, evaporation is a major factor controlling surface heat loss. As the surface layer dries and the soil water supply cannot meet the evaporative demand, soil temperature is largely influenced by thermal inertia. Thus, the change of surface soil temperature can be an indication of soil water content. [PERSON] et al. (2016) found a significant relationship between the diurnal range of surface soil temperature and surface soil water content, and reported that the relationship was a function of soil type. The physical mechanism of this relation includes the following influences: changes in soil moisture affect the albedo and thermal diffusivity of the soil and the Bowen ratio (the ratio of the sensible to latent heat fluxes) in the atmospheric surface boundary layer. As the moist soil surface dries out, more of the incoming solar energy is reflected, and a larger fraction of the absorbed energy is used to heat the air and soil ([PERSON] _et al._, 2016). The heat flow into the soil increases at first, then decreases as the soil becomes very dry ([PERSON] _et al._, 2016).