**1. Introduction**

Drought, similar to floods, is a dangerous natural hazard that can affect almost every region of the world at any time. Its genesis and course depend on many factors, both natural and those resulting from human pressure. Unlike floods, drought develops gradually and exhibits a high temporal inertia, so its symptoms are often underestimated and mistakenly perceived as less of a threat to humans compared to other natural disasters. Long-term droughts affect all sectors of the economy and, as a result, society as a whole. Drought is mainly related to a rainfall deficit leading to a decrease in water supplies affecting the flora and fauna of a given region [1,2]. Meteorological drought is characterized by a deficit of precipitation, an elevated temperature, and low humidity. These anomalies then propagate to impact the surface water and groundwater sources, ecosystems, and human activities. The impact of drought on society, the environment, and the economy depends on its duration and spatial extent. Water stress or water deficit caused by drought has a substantial influence on low production in major agricultural crops [3,4].

The extent of the water deficit on the land surface can be quantified by various indices based on meteorological variables. These include the Palmer Drought Severity Index (PDSI), the Crop Moisture Index (CMI), the Surface Water Supply Index (SWSI), the Rainfall Anomaly Index (RAI), the Standardized Precipitation Index (SPI), and the

**Citation:** Achite, M.; Krakauer, N.Y.; Wał ˛ega, A.; Caloiero, T. Spatial and Temporal Analysis of Dry and Wet Spells in the Wadi Cheliff Basin, Algeria. *Atmosphere* **2021**, *12*, 798. https://doi.org/10.3390/ atmos12060798

Academic Editor: Alexander V. Chernokulsky

Received: 14 May 2021 Accepted: 17 June 2021 Published: 21 June 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/).

Standardized Precipitation Evapotranspiration Index (SPEI) [5,6]. The World Meteorological Organization has recommended that the standardized precipitation index (SPI) developed by McKee et al. [7] is used as a universal meteorological drought index because of its standardized form and the lower requirement of available data that is needed [8–11]. For example, a study described by Ekewzuo and Ezeh [12] and performed in West Africa used a 3-month SPI and showed that the most exposed area to extreme drought conditions occurs over the northern Sahel domain though the frequency of occurrence is very low.

In general, climate change has worsened the extremes of high temperature and both low and high precipitation, and thus has increased the risk of drought [13,14]. For example, Vilaj et al. [15] reported that the SPI reveals an increasing occurrence of droughts in Kerala, India, caused by a decreasing trend of extreme precipitation indexes and an increasing trend of extreme temperature indexes. Algeria is a good example of the worrying manifestations of climate change. As Hadour et al. [16] reported for the RCP8.5 scenario, a decrease in winter rains for the 2039, 2069, and 2099 horizons is projected, while the temperatures will increase. To help with water management under this increasing risk of drought, spatially detailed long-term meteorological data are needed. These data can inform an analysis of the tendency of meteorological drought indicators and better represent their spatiotemporal complexity. The western part of Algeria has experienced several droughts over the last century [16–18]. Drought effects within the country are modulated by the high heterogeneity of the spatial distribution of the rainfall [19].

The variability of precipitation and thus the variability of the drought intensity is linked with many physical mechanisms. For example, in the Iberian Peninsula, Vicente-Serrano et al. [20] linked the increasing drought tendency with greater atmospheric evaporative demand associated with temperature rises. Markonis et al. [21] showed that drier conditions over the Mediterranean are in accordance with a recent north–south polarization of drought patterns over Europe. Parry et al. [22] analyzed three major pan-European droughts in the second half of the twentieth century through synoptic conditions and large-scale circulation patterns, emphasizing that each major drought episode had its own unique spatiotemporal signature. Studies performed by Littman [23] evidenced the influence of the phase pace of the NAO (North Atlantic Oscillation) teleconnection pattern on the precipitation and temperature variability in Turkey. Kingston et al. [24] found that the combination of the NAO and the EA/WR circulation patterns was the most important driver of drought that the European land area was experiencing on a monthly time scale. López-Moreno and Vicente-Serrano [25] found opposing NAO–SPI relationships between northern and southern Europe. Precipitation and drought are also linked to sea surface temperature (SST) anomalies [26,27].

The Wadi Cheliff is the longest river in Algeria and plays a vital role in its socioeconomic development. The Wadi originates from the Saharan Atlas, near Aflou in the mountains of the Jebel Amour, and has a length of approximately 750 km, flowing into the Mediterranean Sea. Accordingly, the present paper shows the results from an analysis of meteorological drought performed on a large number of (150) rainfall stations with long-term precipitation records, which is a unique contribution for the Wadi Cheliff basin.

The aim of the paper is the spatiotemporal analysis of the Standardized Precipitation Index (SPI) variability on the Wadi Cheliff basin in the period 1970–2018. Run theory has been applied on the 12-month SPI series and some characteristics of the drought and wet spells have been identified. Additionally, trends of annual 12-month SPI are shown.
