Drought Frequency and Spatio-Temporal Variability in the Great Lakes Depression Region of Western Mongolia (1990–2020)

Drought has become an increasingly frequent and severe environmental and climatic phenomenon in arid and semi-arid regions in recent years. The Great Lakes Depression in Mongolia is one of the regions most sensitive to drought and aridity. This study investigates the spatial and temporal distribution, as well as the intensity of drought events in the Great Lakes Depression from 1990 to 2020 using remote sensing methodologies. Key climatic variables such as precipitation, air temperature, wind speed, and soil moisture were employed to estimate drought frequency. Additionally, satellite data from MODIS and Landsat 5 were used to calculate the Vegetation Condition Index (VCI), Temperature Condition Index (TCI), and Vegetation Health Index (VHI), providing a more detailed assessment of drought intensity and recurrence. These indices were further analyzed in relation to the Standardized Precipitation Evapotranspiration Index (SPEI) through statistical comparison. The results indicate that the early 1990s were relatively wet, while the period between 2000 and 2011 saw a significant increase in drought frequency, with events occurring more frequently and over shorter intervals. Lakes in areas fed by precipitation and river inflow were particularly sensitive to changes in moisture availability. During the 2000–2011 drought period, TCI declined by 1%, precipitation by 0.9%, and VCI by 0.6%. Increasing aridity, reduced soil moisture, and rising air temperatures in the region have contributed to greater drought intensity and frequency. For example, the years 1994 (VCI: 54.1%), 1997 (78.5%), and 1998 (65.7%) were relatively moist, with high precipitation; however, the corresponding TCI values for those years—29.7%, 35.8%, and 9.9%, respectively—indicate that temperature-induced evapotranspiration exceeded moisture input, resulting in increased drought coverage and frequency despite favorable rainfall conditions.