Characteristics of drought events using three-dimensional graph connectedness recognition method in the Yangtze River Basin, China

Drought events occur more frequently against global warming in recent years, resulting in a great pressureto the regional water resource management, ecosystem, and sustainable socio-economic development in many regions. Identification of drought events is essential to further understand characteristics of drought onset, evolution, and recovery. In this study, taking the Yangtze River Basin (YRB) as the case study, a new recognition method was proposed to identify the regional drought events using three-dimensional graph connectednessrecognition method. This new identification was first to recognize the three-dimensional continuum with no limitation, and then to split the continuum using the threshold of occurring and overlapping areas, finally to extract the drought events with three-dimensional characteristics. The number of times to calculate the overlaid area in space was reduced significantly compared to the drought identification using clustering method, which identifies drought patches in each month using the threshold of occurring area firstly, and then merges patches in adjacent months using the threshold of overlapping area. The spatial-temporal patterns of droughts were obtained to verify the efficiency of this new method using the three-month Standardized Precipitation Evapotranspiration Index (SPEI3)in YRBfrom 1960-2015. The results showed that: 1) 1.5% of the studyarea was considered as the threshold of occurring and overlapping areas, compared with the previous official reports. The drought events identified by the new method matched very well with the historical occurred onesinthe onset, termination, and propagationin YRB. There was a significant positive correlation between the drought intensity of single drought event and the moisture deficit of surface soil in the affected region. It proved the efficiency of the new method in identifying and extracting three-dimensional drought events; 2) 281 drought events are identified in YRB from 1960-2015 and the drought duration of 64 events are larger than 4 months; 3) The duration of long duration drought events (which duration is larger than 4 months) decreased, while the drought area and drought frequency increased. Long duration droughts occurred 43 times during 1960-1999, the average drought duration and drought area were 8.1 months and 2.74×104 km2, respectively. And long duration droughts occurred 21 times during 2000-2015, the average drought duration and drought area were 7.6 months and 3.50×104 km2, respectively. The frequency of long duration drought events in the middle region of YRB is higher than that in the eastern and western regions. 4) The drought events occurred more frequently in Shaanxi, Hubei, and Sichuan provinces, while the north Yunnan and Guizhou provinces presented a relatively lower frequency. Accumulative drought intensity was large in north Guizhou, northwest and east Sichuan, and Hubeiprovinces and Chongqing City, whereas, relatively small in the north Yunnan, Jiangxi and south Jiangsu provinces. The frequency and severity of drought events presented an increasing trend in the middle region of YRB, while a decreasing trend in the western and the eastern. The center of about 50% drought events moved to the northwest, mainly due to the water vapor transference and the migration of precipitation belt. The drought events identification method proposed in this studycan effectively recognize and extract drought events withthree-dimension characteristics. The finding can provide greatly promising guidance for drought and regional water resource management. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.