Assessing the utility of meteorological drought indices in monitoring summer drought based on soil moisture in Chongqing, China

Numerous drought indices have been developed to analyze and monitor drought condition, but they are region specific and limited by various climatic conditions. In southwest China, summer drought mainly occurs from June to September, causing destructive and profound impact on agriculture, society, and ecosystems. The current study assesses the availability of meteorological drought indices in monitoring summer drought in this area at 5-day scale. The drought indices include the relative moisture index (M), the standardized precipitation index (SPI), the standardized precipitation evapotranspiration index (SPEI), the composite index of meteorological drought (CIspi), and the improved composite index of meteorological drought (CIwap). Long-term daily precipitation and temperature from 1970 to 2014 are used to calculate 30-day M (M ₃₀), SPI (SPI₃₀), SPEI (SPEI₃₀), 90-day SPEI (SPEI₉₀), CIspi, and CIwap. The 5-day soil moisture observations from 2010 to 2013 are applied to assess the performance of these drought indices. Correlation analysis, overall accuracy, and kappa coefficient are utilized to investigate the relationships between soil moisture and drought indices. Correlation analysis indicates that soil moisture is well correlated with CIwap, SPEI₃₀, M ₃₀, SPI₃₀, and CIspi except SPEI₉₀. Moreover, drought classifications identified by M ₃₀ are in agreement with that of the observed soil moisture. The results show that M ₃₀ based on precipitation and potential evapotranspiration is an appropriate indicator for monitoring drought condition at a finer scale in the study area. According to M ₃₀, summer drought during 1970–2014 happened in each year and showed a slightly upward tendency in recent years.