Time scale selection and periodicity analysis of grassland drought monitoring index in Inner Mongolia

Drought monitoring index is the basis for drought monitoring and early warning. SPEI index is characterized by multiple time scales, which time scale SPEI is more applicable to grassland drought monitoring continues to be a hot topic of research. To find the optimal time scale of SPEI for drought monitoring in Inner Mongolia, we randomly selected 11 stations in the lively area of Inner Mongolia grasslands and calculated the correlation coefficients between EVI and SPEI at different time scales for these stations. We founded that the SPEI6 is more applicability and practicability for drought monitoring in Inner Mongolia. Moreover, we applied wavelets to discuss drought periodic fluctuations in Inner Mongolia during 1960–2018. The drought evolution of Inner Mongolia grasslands was characterized by multiple time scales with small cycles nested within larger cycles during the study period. There were 4 types of cycles for the drought evolution of Inner Mongolia grasslands: 3–8 years, 4–32 years, 24–32 years and 6–24 years. Besides, we also found that the 17-year time scale was the primary period for the drought evolution of Inner Mongolia grasslands. Overall, the drought in Inner Mongolia grasslands has eased since 2017. The results will provide some theoretical basis for grassland drought monitoring and early warning. •SPEI6 has better applicability and practicability in the monitoring and analysis of drought in Inner Mongolia grassland.•The drought evolution of Inner Mongolia grasslands was characterized by multiple time scales.•The 17-year time scale was the primary period for the drought evolution of Inner Mongolia grassland.•The drought in Inner Mongolia grassland has eased since 2017.