## 3 Results  
### Temporal Variability of Biogeophysical Indices  
As a first step in the evaluation of the relationships between SMAI and LST, their temporal variability during 'dry' and 'wet' conditions was compared. The mean seasonal courses (March-October) for two selected 'dry'/'wet' years (2007/2010) within the test period 2007-2018 exhibit similar trends (Figure 2), to summer, LST is increasing. The drought occurrence in July 2007 \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Data** & **Temporal Resolution** & **Spatial Resolution** & **Type/Source** \\ \hline Soil Moisture Availability Index (SMAI) & Twice daily, 0600 and 1800 UTC & NIMH synoptic station network & SVAT model post-processing/ [Dd] \\ Land Surface Temperature (LST) & 15 min & SEVIRI, about 5 km over Bulgaria & LSASAF product/ MSG \\ Fire Radiative Power-Pixel (FRP-Pixel) & 15 min & SEVIRI, about 5 km over Bulgaria & FRP-Pixel product/MSG \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of data used and their characteristics. corresponds to highest mean LST values up to 40 \({}^{\circ}\)C while high SMA in July 2010 corresponds to lower LST (around 28 \({}^{\circ}\)C). The decreasing SMA is related to increasing of LST for both 'dry' and 'wet' years/periods, i.e., they have opposite mean annual trends (Figure 2a). Boxplots of the two indices illustrate their mean values and seasonal variability (Figure 2b,c). The box with notches provides an opportunity for a visual statistical test of a hypothesis for the difference in the medians of the presented data and, thus, to prove the significance of the conclusions drawn below. The attached plots in Figure 2b,c show different aggregation of data by months and years and, thus, by visual comparison of the corresponding notches, it can be found that there is sufficient reason to accept the statistical hypothesis that the medians of all are significantly different. There is a definite variability of both indices from month to month. During dry periods with depleted SMA, as in July 2007, there is a low variability in SMAI because of the high solar irradiance (leading to high energetic loading) and lack of precipitation. At optimum values of SMAI (March 2007) a low variability is present as well because of small energetic loading, while during wetter periods (July 2010, Aug 2007), the SMAI variation is more significant due to high energetic loading and existing precipitation. LST boxplots from March to October indicate a broader variation during dry periods, in contrast with those during wet periods where the higher SMAI mitigates the land surface heating and, in turn, decreases the amplitude of LST inter-monthly dynamics. The seasonal course of SMAI and LST for the region of Bulgaria is characterized by their monthly mean values on the basis of the 12-year test period (2007-2018) (Figure 3). The red lines show the median and first and fourth quartile averages of the same statistics for all individual boxplots shown on this plot. They are used to visually assess the degree of difference depending on the month. Based on the boxplot comparative analyses, the following main findings can be drawn. Mean SMAI and LST have opposed seasonal trends: an initial higher SMAI (optimum moistening) in spring (March) is steadily depleted in parallel to increasing insolation and vegetation development; it becomes significantly depleted in July (between dry and risk of drought stages according to the SMAI scale), reaches a minimum in August/September (risk of drought and drought, correspondingly), and starts to increase in autumn (October) (Figure 3a). Accordingly, LST starts from lowest mean annual values in March and steadily increases with a maximum in August (mean value of about 35 \({}^{\circ}\)C) (Figure 3b).