Table 1 presents a bibliometric summary of drought indexes. This systematic inventory lists the following for each indicator: (i) the acronym and full name; (ii) the open-source packages available for calculation (R, Python); (iii) the seminal reference introducing the method; and (iv) the number of publications that have used it. This structure makes it possible to objectively assess the methodological practices of the international scientific community and identify de facto standards as well as persistent gaps.

Table 1. Inventory and bibliometric occurrence of drought indices.

Cross-reference with the dataset of Kchouk et al., (2021); Updated Database (MENA and Tunisia) February 2026.

This inventory reveals marked heterogeneity in the use of indexes, with a clear predominance of meteorological indices designed to quantify climate drivers, to the detriment of indices measuring hydrological or agronomic impacts. The Standardized Precipitation Index (SPI), as proposed by [14], is widely recognized as the absolute methodological standard, with 1843 publications representing nearly one-third of the analyzed corpus. This dominance can be explained by its simplicity: The model’s simplicity in terms of its input data, i.e., rainfall, as well as the simplicity of its calculation and the availability of open-source packages (SPEI in R, climate-indices in Python), have also contributed to its widespread use.

The PDSI [15] and SPEI [16] indices continue to be significant, with 875 and 780 occurrences, respectively. While the PDSI is outdated, it remains a historical reference that incorporates a simplified water balance (rainfall, temperature, and soil retention capacity). The most recent SPEI represents a significant advancement by incorporating evaporative demand (ETP), making it more sensitive to global warming. These two indices reflect the growing importance attached to the thermal dimension in the characterization of contemporary droughts.

With regard to agricultural drought, remote sensing has transformed monitoring practices. The NDVI (Normalized Difference Vegetation Index, [17]) has been cited in a total of 1308 publications, making it the preferred proxy for assessing large-scale vegetation water stress. Its success is based on free and continuous access to satellite data (NOAA-AVHRR, MODIS, Landsat) since the 1980s. Derived indices (EVI, VCI, VHI) complement this arsenal with 204, 185, and 104 occurrences, respectively, offering improved atmospheric corrections and temporal normalizations.

However, a methodological imbalance emerges when examining hydrological indexes. The Streamflow Drought Index (SDI, [18]), a critical tool for surface water resource management, has only been documented in 114 publications, which is significantly lower than the number of publications for the SPI. Despite the strategic importance of aquifers as a last-resort resource during crises, there is limited research on groundwater-specific indices (SGI: 2 publications; GRI: 3 publications). Similarly, storage indexes (Reservoir Level Index: 31 publications) are underrepresented, despite their direct operational variable status for decision support.

This overview reveals a structural bias in scientific literature: research overwhelmingly favors easily accessible and standardized forcing indexes (e.g., rainfall, temperature, vegetation) at the expense of impact indexes (e.g., flow rates, groundwater levels, stocks), which are nevertheless more relevant for crisis management.

Figure 2 illustrates the chronology of drought episodes documented in Tunisia over a span of more than thirteen centuries.

An analysis of this historical series reveals a heterogeneous temporal distribution of events, with increasing density over the centuries. The following three distinct periods can be identified: The drought patterns exhibited three distinct periods. The first period, spanning from 700 to 1500, was marked by sporadic documentation of events. The second period, from 1500 to 1850, witnessed a noticeable decline in drought frequency, which could be attributed to a lack of documentation. The third period, from 1850 to 2023, exhibited a notable surge in drought episodes. This heightened awareness of droughts, in terms of both duration and frequency, can be partly attributed to the increased accessibility of relevant documentation. Nevertheless, the recent period (1850-2023), and more specifically the early 20th century, stands out for its exceptionally high density of events (n = 20). While this trend is consistent with the era of industrialization and the associated rise in greenhouse gas concentrations [42], we acknowledge that improved archival coverage may also contribute to this apparent acceleration. Thus, the link between post-1850 drought frequency and anthropogenic climate change is presented here as a plausible hypothesis, pending formal attribution analysis that disentangles documentation bias from climatic signals. This finding aligns with observations indicating an acceleration of climate hazards in the Mediterranean region ([6][7]), particularly in Tunisia.

Additionally, the intersection of climatic events and historical milestones, such as dynasties, occupations, and independence, offers novel perspectives for examining the interplay between natural hazards and socio-political dynamics. This approach aligns with the recommendations outlined in [5] and [43], which underscores the necessity to incorporate the human dimension into the study of

Figure 2. Historical drought in Tunisia.

droughts. The authors assert that “humans are not merely passive victims but actors capable of altering the trajectory of the phenomenon”.

The historical database contains numerous records that support this complex interaction:

It has been observed that periods of prolonged drought often coincide with episodes of famine and political instability. For instance, the significant increase in dry events in the 11th century (1022, 1033, 1040, 1041, 1042, 1080, 1097) occurred before or coincided with the invasion of the Banu Hilal (tributes recorded in 1076), a major tribal migration that had a considerable impact on the demographic and social structure of Ifriqiya². This correlation suggests that climatic stresses may have acted as triggers or amplifiers of migratory movements and conflicts over resources.Subsistence crises have also been linked to epidemics. The drought of 1457 coincided with the plague in Tunis, illustrating the phenomenon of syndemic, whereby climatic hazards weaken populations through famine and malnutrition, thereby exacerbating their vulnerability to infectious diseases.The period of French protectorate (from 1881) saw an increase in recorded droughts (1867, 1881, 1882, 1911, 1914, 1920, 1927, 1928, 1935, 1943). This period also saw profound changes in agricultural structures and local revolts (Ali Ben Ghedhahem’s revolt in 1864-1867), regional famines, and successive waves of rural exodus. The accumulation of climatic stresses in this context of colonial domination likely contributed to the exacerbation of social tensions and resistance movements.Finally, despite the development of water infrastructure, the contemporary period is marked by persistent droughts. Recent events (2015, 2021, 2022, 2023) have occurred against a backdrop of major political tensions (the 2011 uprising, the 2021-2023 institutional crisis), reminding us that vulnerability to drought remains a key socio-political issue for post-revolutionary Tunisia.

It is important to note that these associations represent historically plausible links based on chronological correlations and documentary evidence, rather than statistically demonstrated causal effects. These observations underscore the complexity of drought assessment and highlight the need for nuanced climate indexes that can accurately capture its multifaceted nature. It is part of a coupled human-environment system in which rainfall deficits interact with governance structures, demographic dynamics, and geopolitical contexts. As discussed in [1], “the socio-economic dimension of drought emerges from the interplay between physical anomalies and societies’ response capacities, making this hazard an intrinsically interdisciplinary subject of study”.

Figure 3 emphasizes the significant heterogeneity in the application of drought indexes across the MENA region, reflecting both methodological and structural disparities between the countries in question. A clear predominance of meteorological indices can be observed across the entire basin, confirming the global bibliometric trends identified by [10] and [11]. As [6] notes, this emphasis on meteorological drivers facilitates international standardization, but it can also obscure the intricate processes underlying drought propagation to agricultural and hydrological domains.

Regarding agricultural drought, the map demonstrates cases a notable yet inconsistent presence of indexes derived from remote sensing, particularly NDVI. This dynamic is particularly evident in countries with relatively developed research capabilities or a high dependence on rain-fed agriculture, such as Morocco, Tunisia, and Egypt. The increasing use of these spatial indices addresses the need for large-scale monitoring of vegetation water stress, as evidenced by the research conducted by [7] in Tunisia.

However, despite the strategic role of surface water and groundwater for water security in the region, the hydrological indexes of direct impact related to these appear marginal or even absent. This methodological imbalance corroborates the

Figure 3. Bibliometric mapping of indexes related to types of droughts: case of the Middle East and North Africa region (1980-2026).

observations of [44], which point to gaps in hydrological monitoring networks and a lack of reliable flow data in the MENA region. This deficiency results in a structural mismatch between the available diagnostic tools and the actual needs of managers, who require information on water stocks rather than just deficits in rainfall.

Figure 4 emphasizes the methodological heterogeneity in the application of drought indexes in Tunisia. In accordance with global bibliometric trends identified by [12] and Table 1, there is a clear predominance of meteorological indices, particularly the Standardized Precipitation Index (SPI), which is the benchmark tool in Tunisian literature. This preference can be explained by several factors: The simplicity of calculating the SPI is advantageous because it requires only rainfall series. Furthermore, the availability and accessibility of precipitation data is beneficial. Finally, the World Meteorological Organization’s recognition of this index as the standard for monitoring meteorological droughts [45] is a significant asset.

However, recent literature reveals the emergence of indices incorporating the thermal dimension (SPEI) and remote sensing (NDVI, VCI), reflecting a growing awareness of evaporative demand and plant stress in the Mediterranean context. Despite this diversity, current monitoring has limitations, notably a lag between meteorological indices (SPI, SPEI) and actual impacts on water security. The

Figure 4. Drought indexes used in Tunisia (1980-2026).

development of hybrid indexes, which combine climate forcings and management variables (such as reservoir levels and withdrawals), is still limited by the accessibility of hydrological data (flows and piezometry).

Although the SPI is the benchmark tool due to its simplicity and the availability of rainfall data, recent studies highlight that no single index can effectively characterize drought across the entire territory. This discrepancy can be attributed to the diverse bioclimatic zones of Tunisia, which result in varied responses to rainfall deficits. Reference [7] Demonstrates that the SPI performs better in the humid northern forest region, while the PDSI, which incorporates the thermal dimension, is more suitable for the semi-arid central steppe. In the southeast, [46] recommend a multi-scale approach (SPI-3 to SPI-72) to capture both seasonal agricultural droughts and multi-year structural sequences. More recently, [47] underscored the significance of remote sensing indices (NDWI, NDTI) for monitoring strategic reservoirs such as Sidi Salem, where the contraction of water surfaces (−47.6% between 1985 and 2025) cannot be fully explained by rainfall deficits alone. These results underscore the necessity for a diversified monitoring strategy, integrating meteorological, agricultural, and hydrological indexes according to bioclimatic zones and water utilization