*2.2. Standardized Precipitation Index (SPI) and Standardized Runoff Index (SRI)*

Two indicators were used to determine droughts: the Standardized Precipitation Index (SPI), which defines meteorological droughts, and the Standardized Runoff Index (SRI), which defines hydrological droughts. Data from 8 meteorological stations: 5 located within the catchment area of the Note´c River (Pako´s´c, Strzelno, Sompolno, Izbica Kujawska, Kołuda Wielka) and 3 in its close vicinity (Kłodawa, Gniezno and Janowiec Wielkopolski) were used to determine meteorological droughts.

The Standardized Precipitation Index (SPI) is one of the most frequently used indicators of a meteorological drought, and was developed on the basis of the normalization of precipitation probabilities [60]. This indicator defines a precipitation deficit and allows the monitoring of droughts in different time frames. The SPI is recommended by the World Meteorological Organisation (WMO) for determining the phenomenon of drought [61]. For more information on the formulation of SPI, its advantages and limitations, see papers [62–64].

The SPI was calculated on the basis of monthly precipitation totals for 5 meteorological stations in the upper catchment area of the Note´c River (Izbica Kujawska, Strzelno, Kołuda Wielka, Sompolno and Pako´s´c) and 3 stations in its close vicinity (Kłodawa, Gniezno and Janowiec Wielkopolski). The daily precipitation data were aggregated into monthly time scales, and fitted to a two-parameter gamma distribution function. The SPI was calculated for each month at different timescales. Therefore, 5 different time series were analysed, i.e., 1-, 3-, 6-, 9- and 12-months. SPI values define the deviation from the median expressed in units of standard deviation, which was calculated according to the formula:

$$SPI\_{\prime} \; SRI = \frac{f(\mathbf{x}) - \mathbf{u}}{\sigma} \tag{1}$$

*SPI*, *SRI*—Standardized Precipitation Index, Standardized Runoff Index

*f*(*x*)—transformed sum of precipitation, discharges

*μ*—mean value of the normalized index *x*

*σ*—standard deviation of index *x*

In order to calculate the SPI, the compliance of the distribution of the transformed variable *f*(*P*) with the normality distribution was tested using the *x*2—Pearson normality test [65].

The Standardized Runoff Index (SRI) is calculated according to the same procedure as the SPI, however it is based on the discharge data [66–69]. A 2-parameter logarithmic function was used as a normalizing function when calculating the SRI [70]. The detailed calculation method is presented in the study [9,47]. The probabilities were transformed into standard normal distribution. The application of SPI allows for the differentiation of the intensity of a drought using a set of SPI thresholds: −1, −1.5, −2 and 1.0, 1.5, 2 for moderate, severe and extreme droughts and precipitations, respectively [28].

The proposed approach is based on the assessment of water resources under different hydroclimatic conditions and the determination of different intensity classes. The adoption of standardized indicators allowed for the classification of drought intensity, which is presented in Table 3. Extreme events were identified for the indicator values above 1.0, when rainy periods occur, and for the indicator values below −1.0, when there are droughts.

**Table 3.** The classification scale for SPI and SRI values.

