Monitoring the extreme drought in the middle and lower reaches of the Yangtze River in 2019 from GRACE-FO satellites

Abstract

In the summer and autumn of 2019, an extreme drought lasting 0.5 year in the middle and lower reaches of the Yangtze River had a major impact on local agricultural production and the ecological environment. Traditionally, drought severity assessment relies on drought-related parameters observed by in situ measurements, such as precipitation, evapotranspiration, runoff, and soil moisture. In the past two decades, GRACE/GRACE-FO gravity satellite data have been widely used to estimate global and regional terrestrial water storage anomalies (TWSA), but there are still limited studies on the quantitative assessment of the severity of drought using GRACE/GRACE-FO data. The objective of this study was to analyse the drought severity in the middle and lower reaches of the Yangtze River in the summer and autumn of 2019 using GRACE/GRACE-FO data and to compare our estimates with the traditional drought index. The GRACE-based drought severity index (GRACE-DSI) is a monthly global drought severity index based on the TWSA from the GRACE/GRACE-FO (GRACE-TWSA). In this study, we used spherical harmonic (SH) products from the GRACE/GRACE-FO to obtain the TWSA. GRACE data processing includes replacing the degree-1 and C 20 , removing the 2004–2009 mean SH, filtering to reduce correlated errors, Gaussian smoothing, calculating the equivalent water layer thickness on 1° ´ 1° grids, and correcting leakage errors with scale factors. We then calculated the GRACE-DSI based on the GRACE-TWSA with the trend and seasonal cycles removed. To ensure consistency, we processed the standardised precipitation evapotranspiration index (SPEI) similar to the GRACE data and standardised the new SPEI as the SPEI-Z. Our results indicated that the GRACE-DSI can successfully monitor the occurrence and development of drought in the middle and lower reaches of the Yangtze River in spatiotemporal domains. The GRACE/GRACE-FO successfully detected the extreme drought event that occurred in the east of Hubei, north of Jiangxi, and south of Anhui during the summer and autumn of 2019. In July, the arid region was located in the north of the Yangtze River basin, including Henan and Shandong provinces. In August, the drought intensified and moved south to the middle and lower reaches of the Yangtze River, reaching a maximum in September, especially in Hubei, Jiangxi, and Anhui provinces. The drought was weakened in October, but intensified again in November and December and expanded through the middle and lower reaches of the Yangtze River to the coastal regions, such as Fujian and Guangzhou provinces. The mean time series of the GRACE-DSI over Hubei, Jiangxi, Anhui, and Hunan provinces showed good agreement with the SPEI-Z in both amplitude and phase on the six month time scale (SPEI06-Z) with a correlation coefficient of 0.84. The time series of the GRACE-DSI and SPEI06-Z both clearly showed the extreme drought events that occurred in Hubei, Jiangxi, Anhui, and Hunan provinces during the spring and summer of 2011 and the summer and autumn of 2019. Our results also indicate that the GRACE-FO has the same drought monitoring capability as the GRACE.