**1. Introduction**

Drought is a natural disaster characterized by long-term water scarcity [1–3]. Drought is one of the most serious natural threats that causes damage to various aspects of the environment, society, and economy [4,5]. No universal definition of drought has been established due to the wide variability in water supply and demand worldwide [6,7]. There are four categories of drought in the literature: meteorological, agricultural, hydrological, and socioeconomic [8–13]. Drought damage is a serious issue in numerous countries around the world. Due to its nature, drought is difficult to monitor, and its effects are often poorly documented. Among the various sectors of the economy, agriculture is one of the most vulnerable to drought, where its effects are also most noticeable [14–16]. Numerous reports and scientific articles indicate that forecasts of future climate conditions suggest an increase in the frequency and intensity of droughts in some regions of the world [17–21]. The increase in the frequency of drought occurrence in recent years has not been limited to arid and semi-arid regions [22–26], but has been gradually becoming more common in

**Citation:** Kubiak-Wójcicka, K.; Pilarska, A.; Kami ´nski, D. The Analysis of Long-Term Trends in the Meteorological and Hydrological Drought Occurrences Using Non-Parametric Methods—Case Study of the Catchment of the Upper Note´c River (Central Poland). *Atmosphere* **2021**, *12*, 1098. https://doi.org/10.3390/ atmos12091098

Academic Editors: Andrzej Walega and Agnieszka Ziernicka-Wojtaszek

Received: 16 July 2021 Accepted: 20 August 2021 Published: 25 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

regions with a temperate and humid climate [27–32]. Poland, which has one of the most limited water resources in Europe [33], is among those regions experiencing an increase in drought frequency [34]. The distribution of water resources in Poland has been diversified in terms of time and space. In the current climate, many regions of the country often suffer from water scarcity. In the future, this scarcity may become even more serious, and the availability of water resources might become limited. In recent years, only slight changes in annual precipitation totals have been recorded in Poland, however a noticeable shift has been observed in seasonal and monthly precipitation distribution [35,36]. Moreover, there have been quite significant changes in thermal conditions, characterized by a great increase in air temperature over a multi-year period [37]. As a result of these changes, temporary difficulties in water supply have been recorded in some areas of Poland [38]. This problem will be particularly harmful for the agricultural regions of the country. Polish agriculture is largely dependent on precipitation, which is highly variable both in terms of its temporal and spatial characteristics. Plant production is reliant mainly on water obtained from precipitation and available to plants by means of water-retentive soil [39]. In the event of a drought in agricultural areas, crop irrigation is required, especially during the growing season [40].

Drought studies in Poland have been conducted at a regional and local level. Previously published drought analyses mainly refer to the classification of drought types using various drought indicators [41–43], monitoring of drought conditions [44–47], as well as the characteristics of the drought, including its duration, intensity, size and frequency [48].

This study focuses on the research of drought trends in the period of 1981–2016, in a particularly drought-susceptible area of Poland. The analysis of drought trends in the long term might indicate the direction of possible near-future changes. The case study focuses on the catchment of the Upper Note´c River, which is a heavily exploited agricultural area with some of the lowest reserves of water.

The main objectives of this study included:


The obtained results will help the departments of state administration, responsible for water resources management, make informed decisions and establish a long-term local development strategy, regulating the sustainable management of water resources.

#### **2. Materials and Methods**

#### *2.1. Study Area and Dataset*

The research was carried out in Poland, a region located in a temperate climate zone with a predominance of polar-sea air masses. The amount of precipitation in Poland varies temporally and spatially. The average annual precipitation in Poland recorded in the period of 1981–2010 was 603 mm. The lowest annual precipitation total of 500 mm was observed in the central part of the country, and the highest—970 mm, in the south of Poland [49]. The area covered by the analysis includes the Upper Note´c catchment, closed by the water gauge in Pako´s´c (Figure 1). The catchment area up to the Pako´s´c station is 2301.98 km2. This catchment is located in the historical region of Kujawy, which is extremely important for agriculture. Arable land within the catchment area accounts for 76.07% of the catchment area, while forest areas account for 11.4%. It is also the region with the lowest annual precipitation in Poland and the area with the highest water shortages in agriculture [50].

**Figure 1.** Study area. Source: (**a**) own elaboration made with [51,52]. Map elaborated in the coordinate system: WGS 84/UTM zone 34N; (**b**) own elaboration made with [52–54], location of the stations based on [55,56] and Table 1, location of the lignite open pits based on [56,57]. Meteorological station Izbica Kujawska is out of catchment area according to coordinates from Table 1. Location of the hydrological station Pako´s´c is determined by cartographic issues. Map elaborated in the coordinate system: WGS 84/UTM zone 34N.


**Table 1.** Annual sum of atmospheric precipitation in the period of 1980–2016.

The data used in this work are derived from historical series of daily precipitation totals recorded at 8 meteorological stations (Izbica Kujawska, Kołuda Wielka, Pako´s´c, Sompolno, Strzelno, Gniezno, Janowiec Wielkopolski, Kłodawa). Air temperature measurements were obtained from the Kołuda Wielka station. Discharge data were obtained from 3 hydrological stations: Łysek, No´c Kalina and Pako´s´c. Meteorological and hydrological data for the period of 1980–2016 were obtained from the Institute of Meteorology and Water Management—National Research Institute. Daily values were converted into monthly values for the purpose of extended calculations.

In terms of annual precipitation, the Upper Note´c area is one of the regions with the lowest precipitation in Poland. Total annual precipitation in the period of 1980–2016 ranged from 500.1 mm (Kołuda Wielka) to 542.5 mm (Strzelno) (Table 1). The highest precipitation totals were recorded in 2010 at most meteorological stations. The exceptions were Kłodawa, with the maximum precipitation occurring in 2001, and Kołuda Wielka and Strzelno, with the maximum recorded precipitation in 1980. The lowest precipitation totals were recorded at most stations in 1989, with the exception of the stations in Gniezno and Janowiec, where the lowest annual precipitation were recorded in 1982, and Izbica Kujawska, where the

lowest totals occurred in 2011. Air temperature measurements were carried out only at the meteorological station in Kołuda Wielka. The average annual air temperature at the meteorological station in Kołuda Wielka in 1980–2016 was 8.5 ◦C, and the highest average annual air temperature was recorded in 1989 (9.8 ◦C) (Figure 2).

**Figure 2.** Average annual air temperature and average precipitation sums at the Kołuda Wielka meteorological station in the period of 1980–2016.

In terms of hydrology, the catchment area of the Upper Note´c is categorised as one of the areas with the most limited water resources [58]. According to Tomaszewski and Kubiak-Wójcicka [59], the average long-term unit runoff of the Note´c measured at the Pako´s´c water gauge in the period of 1951–2015 amounted to 2.41 dm3·s−1·km−2. This is the lowest unit runoff value recorded in Poland, with an average of 5.5 dm3·s−1·km−<sup>2</sup> [34]. Both precipitation and discharge in the catchment area of the Upper Note´c River are among the lowest in Poland. During the analysed period of 1980–2016, the unit runoff of the Note´c River at the Pako´s´c station was 2.21 dm3·s−1·km−2. The highest values of the maximum discharge were recorded in July 1980 at all hydrological stations. The maximum discharge at the Pako´s´c station was 69.3 m3·s−<sup>1</sup> (unit runoff 30.1 dm3·s−1·km−2), and the lowest discharge was recorded in October 2003 (0.23 dm3·s−1·km<sup>−</sup>2) (Table 2). At the No´c Kalina station, the lowest discharge was recorded in September 1989 and August 1992. However, the lowest discharge at the Łysek station was recorded from August to December 2015 and in January 2016, at which time virtually no discharge occurred in the watercourse bed.

**Table 2.** Hydrological characteristics of the Note´c River in the period of 1980–2016—annual discharge.

