*3.1. The Characteristics of Droughts in the Period of 1981–2016*

SPI values were calculated separately for all eight weather stations for the time scales of 1, 3, 6, 9 and 12 months. Depending on the selected accumulation period, the range of SPI values was different for individual meteorological stations, as follows: SPI−1 from −3.54 to 3.59, SPI−3 from −3.01 to 2.94, SPI−6 from −3.32 to 2.65, SPI−9 from −3.47 to 2.93 and SPI−12 from −3.38 to 2.77 (Table 4). The longer the indicator's accumulation period, the lower the drought intensity. The most intense meteorological droughts were recorded in the following periods: 1982–1985, 1989–1996, 2002–2006, 2008–2009. The number of months with drought occurrences varied, depending on the period of accumulation and distribution of individual meteorological stations. In the 1-month accumulation period, the number of months with drought occurrences (values ≤ −1.0) ranged from 62 to 73 months i.e., from 14.3% to 16.9% of all the months in the analysed multi-year period of 1981–2016. On the other hand, wet months with SPI values ≥ 1.0 lasted from 52 to 71 months i.e., from 12.0% to 16.4% of the analysed time period. In the case of SPI−1, the droughts lasted the longest in Kłodawa, and were the shortest in Kołuda and Strzelno. The highest number of months with drought occurrences (SPI ≤ −1.0) in the 3-month accumulation period was recorded at the Kłodawa station (81 months), and the lowest in Kołuda Wielka (66 months). In the longer accumulation periods i.e., 6, 9 and 12 months, the number of months was similar and amounted to 60 to 75 months for SPI−6, 59 to 74 months for SPI−9 and 59 to 79 months for SPI−12.

Greater differentiation was recorded in the cases of hydrological drought occurrences (SRI) at the station in Łysek, No´c Kalina and Pako´s´c (Table 5). The SRI values in Łysek were characterized by the largest range i.e., from −5.22 to 2.00. The most intense droughts were recorded from April to December of 2016, which was related to relatively minor discharges of the Note´c River during this period. At the No´c Kalina station, the SRI values recorded were from −3.16 to 2.49. The most intense hydrological droughts were recorded from May to September of 1990, depending on the period of accumulation. The SRI values in Pako´s´c were characterized by a lower amplitude and ranged from −2.33 to 3.05. The number of months with SRI values below −1.0 ranged from 75 to 81 months, and in No´c Kalina—from 77 to 101 months.


**Table 5.** The Parameters of a hydrological drought (SRI) in different time scales in the period of 1981–2016.

The hydrological droughts (SRI ≥ −1.0) at the station in Łysek were characterized by a short duration, from 6.5% to 8.3% of the analysed period, and high intensity. The number of wet months (SRI ≥ 1.0) ranged from 6.0% to 9.3% of all the months of the analysed multiannual period. The largest number of months with SRI values ≤ −1.0 was recorded at the No´c Kalina station i.e., from 17.9% to 23.4% of the analysed multi-year period, while the wet months constituted between 13.4% and 17.6% of this period. The hydrological station in Pako´s´c closes the catchment area of the Upper Note´c River. The lower intensity of hydrological droughts at the Pako´s´c station and their much shorter duration compared to the No´c Kalina and Łysek stations may have resulted from anthropogenic activities carried out in the area. These activities were mainly related to the lignite open pits, which ran south of the Łysek and No´c Kalina stations (Lubstów), as well as the operation of a new open pit mine located north of the Łysek station (Tomisławice).

The spatial distribution of the number of months with meteorological drought occurrences in the catchment area of the Upper Note´c is shown in Figure 3. The number of months with droughts in the accumulation period of 1 month does not present large spatial differentiation. Larger differences are noticeable in the period of 3- and 12-month accumulation. In the case of SPI−3, the months with droughts in the southern part of the catchment lasted the longest, while in the SPI−12 period they were the longest in the western part of the catchment.

Figure 4 shows the average values of SPI from 8 stations as well as the development of hydrological drought occurrences (SRI) recorded at the station in Pako´s´c. The number of months with averaged meteorological droughts is between 13.4 and 15% of all the analysed months. As presented in Figure 4, the development of hydrological droughts is related to meteorological droughts.

**Figure 4.** *Cont.*

**Figure 4.** The development of meteorological and hydrological droughts in the period of 1981–2016 for different accumulation periods (*n* = 1, 3, 6, 9 and 12), SPI—average values from 8 stations, SRI for Pako´s´c.
