Evaluation of the performance of meteorological drought indices in Morocco: A case study of different climatic zones

Understanding drought occurrence and evolution is important in minimizing the impacts associated with it. This work assesses the performance of 10 commonly used meteorological indices to measure drought in Morocco. The studied indices are Deciles Index (DI), Percent of Normal Index (PNI), Z-Score Index (ZSI), China-Z Index (CZI), Rainfall Anomaly Index (RAI), Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), Palmer Drought Severity Index (PDSI), its self-calibrated variant (scPDSI) and Palmer Z Index (PZI). Rainfall and temperature gridded data is sourced from PERSIANN-CDR and ERA5, respectively, for the period 1983-2021. The study area exhibits three main climatic regimes; subhumid, semi-arid and arid, with a drying and warming climate, as depicted by the rainfall and temperature trends analysis. Results show that most rainfall-based indices perform relatively poorly in drought monitoring in the study area. DI and PNI appear to be inconsistent and abnormally responsive to rainfall. RAI reports droughts 56.5% more frequently and slightly underestimate drought intensity compared to other indices. Similarly, ZSI and CZI largely underestimate drought intensity. PDSI and scPDSI are computationally demanding, often underestimate drought intensity and overestimate drought duration by at least 115% compared to SPI and SPEI. Conversely, PZI can be used for drought onset detection as it reported droughts early compared to the other indices. SPI and SPEI perform overall better regarding their consistent drought identification and severity assessment. However, SPEI is found to be more suitable than SPI in the arid and semi-arid regions and performed better considering the warming climate of the country. Three climatic regimes were identified in Morocco and an increasing trend was observed for temperature. Ten calculated drought indices were compared via different analysis and their weaknesses were revealed. PET is an important factor that should not be disregarded for drought monitoring especially in arid and semi-arid regions. The SPEI overall represented droughts well, and the same can be said about SPI in subhumid regions, while the other indices showed various weaknesses. image