## 3 Results and Discussion
### Spatio-Temporal Evolution of SPEI  
The results demonstrate the consistent categorization of the SPEI index. However, the limited number and distribution of measurement stations and the region's topographical complexity introduce uncertainties in the interpolated observed SPEI maps (Figures 5, S2 and S3). Figure 5 presents the spatial distribution of short-term drought events (SPEI-3) in March, highlighting significant climatic conditions during specific years. Notable events include extreme droughts in 2000 and 2012, moderate-to-extreme humidity in 1988, 1991, and 1996, and mild-to-severe humidity in 2009 and 2010, indicating substantial fluctuations. A comparison with the CHIRPS data revealed consistency in identifying extreme droughts in 2000 and 2012, but differences emerged in characterizing the humidity conditions. The CHIRPS tends to overestimate the intensity of short-term wet and dry conditions (Figure 5). Figure S2 presents the spatial distribution of medium-term drought events (SPEI-6) in June, revealing exciting patterns. According to the CHIRPS, the dry years range from having moderate to extreme conditions, including 1981, 1983, 1984, 1987, 1990, 1992, 1994, 1995, 2000, 2001, 2003, 2005, 2008, and 2012, and from 2015 to 2017. In contrast, the observed  
data identify dry years in 1981, 2000, 2001, 2005, 2008, and 2017. The wet years also show variation; the CHIRPS indicates high-level humidity in 1988, 1996, 2006, 2009, 2010, and 2011, while the observed data identify wet years in 1982, 1991, 1996, 2009, 2011, and 2018 (Figure S2). Figure 4: Variability in SPI-12 derived from observed data and CHIRPS satellite rainfall estimates for four stations (Adamna, El Massira Dam, Iloudjane, and Nkouris). Figure S3 presents the spatial distribution of December's long-term drought events (SPEI-12). Both the datasets consistently identify droughs in 1981, 1992, 2000, 2001, and 2017. However, the discrepancies in drought intensity between the observed data and the CHIRPS appear in specific years (Figure S3). These differences may be attributed to spatial and temporal resolution variations, potential data gaps, biases in CHIRPS data processing and calibration, the local climatic conditions, and the interpolation techniques. In the MS region, the limited number of precipitation measurement stations underscores the value of satellite data, which offer more comprehensive coverage. Other  
Figure 5: The spatial distribution of the monthly drought index SPEI-3 in March for each year from 1981 to 2018 in the MS region. studies [37; 61; 62] have emphasized the usefulness of satellite data in addressing data gaps in areas with sparse station coverage. The SPEI maps based on the rain gauge data reveal that the categorization accuracy between the observed and CHIRPS data is highest near the stations, but decreases with distance, particularly in the mountainous and foothill areas. This observation aligns with the findings from [60; 61], which also noted increased uncertainty with distance and low station density. Our results demonstrate that the CHIRPS-based SPEI effectively captures the occurrence and characteristics of some drought events at different timescales. However, significant spatial discrepancies between the CHIRPS-SPEI and the observed-interpolated SPEI for SPEI-3, SPEI-6, and SPEI-12 are evident. These discrepancies likely arise from the complex terrain, inadequate spatial scale, and low station density. Figure 5 illustrates the CHIRPS' tendency to overestimate the intensity of short-term wet and dry conditions compared to that of the observed data. Figures S2 and S3 reveal variations in medium- and long-term drought events, with the CHIRPS identifying more dry years than the observed data, particularly in complex terrains. These findings align with studies by the authors of [34] in Brazil, [49; 68] in China, and [71] in Ethiopia, who noted similar challenges in accurately capturing drought events using the CHIRPS-based SPI in areas with complex terrain and low station density.