*2.1. Study Area and Data*

The Wadi Cheliff Basin (WCB) covers an area of 43,750 km2 and lies between 00◦07 44 E and 03◦31 07 E and between 33◦53 13 N and 36◦26 34 N (Figure 1). The topography of the basin is complex and rugged. The altitude varies from −4 m to 1969 m.

**Figure 1.** Location of the study area and of the selected 150 rain gauges on an elevation map, along with the distribution of the average annual precipitation evaluated from the 150 rain gauges with a spline interpolation.

Climatically, the basin is arid and semi-arid. The mean annual temperature decreases gradually from the north to south as the elevation increases going upstream [28]. The mean annual precipitation recorded at different stations (1970–2018) ranged from 161 mm to 662 mm, 80% of which fell between November and March. For this study, datasets of several rainfall stations (Figure 1) with long-term annual precipitation records from 1970 to 2018 across the WCB were taken from the National Agency of the Water Resources (ANRH). However, the period of the records for these stations varies and some have missing records. To improve the data quality, only the observing stations with data series accounting for 70% or more of the overall period were chosen for our study. After excluding the stations with too many missing values, the double mass curve technique was used to analyze the remaining missing data. The data was subjected to quality control and data gap filling using the linear regression method. The period of study was chosen as 1970–2018, which is as long as possible based on the availability of recorded data for the majority of the stations in the region [29].
