## 2 Materials and Methods
### Approach for Drought Assessment Using Satellite and Meteorological Data
#### 2.5.2 Design of the Drought Index Based on the TCI under Different Climatic Areas
The number of lags of TCI used in the DISS formula determines the degree of intensity of the drought conditions. This information could be dedicated to a specific type of vegetation cover, describing their temporal response lag to drought [41]. For mitigating drought effects on arable area, shorter periods are preferable, as 3-4 eight-day time steps of TCI. The 5 steps of the TCI model would be useful for detecting drought of the more drought-resistant plants. The statistical analysis revealed that the period of accumulation for the meteorological coefficient is longer than the time horizon of the satellite response. The 30 day HTC is significantly related to three steps of the eight-day TCI (24 days), as well as the 40 day HTC, to 4*8 days and 50 to 5*8 days, respectively. This is in accordance with the nature of the vegetation's condition, which is controlled by soil moisture, as well as with the delayed reaction to water deficit. Testing of the dependences between in situ and satellite indices in the moisture context was presented in [42]. The equation of the DISS index based on the 3 time-step moving average of TCI for agriculture area is as follows:  
\[DISS3_{t}\ =\ MedHTC30*\exp(-1.6+1.4*TCI_{t}+1.*TCI_{t-1}+.8*TCI_{t-2}) \tag{8}\]  
Figure 11: Comparison of Drought Information Satellite System (DISS) values for different MedHTC conditions. The distribution of HTC values across the territory of Poland presented in Figure 6 shows that the dominating values are around 1.0. Thus, HTC \(<\) 1, with some approximation, should be treated as indicating atmospheric conditions, which predestine drought conditions. The selection of thresholds for the classification of drought levels was based on the statistical analysis presented in Table 3, related to the probability of compliance of the DISS index in its particular ranges with the water deficit characterized by HTC. Statistical agreement with a water deficit above 80% was accepted as satisfactory for determining serious drought conditions; a value of 0.5 was chosen as the threshold of serious drought, because it is half of the 0-1 interval; and a value of 1 is related to average conditions in the area when the median HTC is equal to 1.0 (Figure 11). When the DISS values increase, the probability of water deficit decreases, until the level of DISS \(>\) 1.5; then, there is a high certainty that the area is not affected by drought. The probability grows with an increase in the time horizon when aggregating the satellite indices (three, four, and five time steps). Testing took place on a sample of approximately 50,000 observations using all available data. The percentage characteristics of the moisture conditions described by DISS within the agricultural area of Poland are presented in Table 3.  
Classification of the DISS index was done using the thresholds specified in Table 3. Five levels of dry/moisture conditions were distinguished for the agricultural areas:  
1. Drought--when DISS \(<\) 0.5
2. Drying--when DISS \(\geq\) 0.5 and DISS \(<\) 0.8
3. Average conditions--when DISS \(\geq\) 0.8 and DISS \(<\) 1.5
4. Good conditions--when DISS \(\geq\) 1.5 and DISS \(<\) 3.0
5. Wet/cold conditions--when DISS \(\geq\) 3.0