Terrestrial water storage changes and extreme drought and flood events in ASEAN based on GRACE gravity satellite data

Terrestrial Water Storage (TWS) changes are important for human livelihoods, agricultural production, and economic development. The Association of Southeast Asian Nations (ASEAN) are friendly neighbors, and also China’s largest trading partner. This region has abundant water resource. However, the changes in TWS are difficult to quantify due to limited measured data. And frequent drought and flood events occur in this region. This research analyzed the spatio-temporal characteristics of TWS changes in the ASEAN region from January 2002 to December 2022 based on GRACE gravity satellite data, and constructed the GRACE-DSI to analyze extreme drought and flood events in the Mekong River Basin. The results showed that: 1) TWS in the Indo-China Peninsula showed decreasing trend of −0.020 cm/month (P<0.05), and human activities such as agricultural irrigation and groundwater extraction are important reasons for the reduction of TWS. While in Kalimantan Island and Sumatra Island, the TWS were significantly increasing (0.023 cm/month, P<0.01 and 0.040 cm/month, P<0.01); 2) The TWS in the northern region of 10°N showed significant seasonal variation, gradually decreasing from November to April of the following year and increasing from May to October, with a seasonal variation range of −25 to 25 cm. The seasonal variation amplitude in the southern region of 10°N was only about 10 cm. The distribution of precipitation showed obvious spatial differences in the study area, with more precipitation in the south and less in the north. Precipitation plays a major role in the change of TWS. The annual precipitation in the northern region of 10°N was 1000-2 000 mm with a rainy season from May to October and a dry season from November to April of the next year. In the southern region of 10°N, the annual precipitation (more 3000 mm in some areas) was much higher than that in the north, but there was no obvious seasonal variation, resulted in a smaller amplitude variation in the monthly TWS anomaly. 3) During the study period of 2002 to 2022, the Mekong River Basin experienced several drought and flood events, such as the flood from July to December in 2011 and the drought event from October 2019 to September 2020. The seasonal distribution of precipitation on the Indo-China Peninsula where the Mekong River located was uneven and the seasonal amplitude showed an increasing trend. Under the background of significant rise in air temperature, potential evapotranspiration showed an increasing trend, which was an important reason for the frequent water drought disasters in the Mekong River Basin. The extreme drought and flood events in the Mekong River can accurately monitored by GRACE-DSI index, and they were affected by ENSO (The GRACE-DSI drought index was negatively correlated with the ENSO-ONI index, r=−0.408, P<0.01).