Synchronous–Asynchronous Encounter Probability of Drought and Waterlogging in Different River Basin Areas in the Middle Route of South-to-North Water Transfer Project in China

In this study, daily precipitation data of 265 meteorological stations in the Middle Route of the South-to-North Water Transfer Project region during 1961-2015, as well as 500 years of drought and waterlogging data at 120 stations provided by Chinese Academy of Meteorological Sciences are used to analyze the drought and waterlogging characteristics in one water source area (the upper reaches of Hanjiang River) and three water receiving areas (Tangbai, Huaihe, and Haihe river basins) and to calculate the efficiency of water transfer in three water receiving areas in the Middle Route of South-to-North Water Transfer Project. This study also investigates the future drought and waterlogging characteristics under the RCP 4.5 scenario based on the simulation results of 21 CMIP5 global climate models. In order to provide basis for scientific and reasonable operation of water resources in the Middle Route of South-to-North Water Transfer Project。 The results show that: (1) In the past 500 years, the water source area shows an increasing probability of occurrence of waterlogging, and probability of occurrence of drought decreases first and then increases. In the 20^th century the probability of occurrence of drought in the water source area reaches highest in the history (31.7%). Meanwhile, in last 20 years, drought and waterlogging events occur frequently and the probability of occurrence of a sudden switch between drought and waterlogging increases. (2) In the past 500 years, the Huaihe river basin shows the highest efficiency of water transfer (87.3%) while the Tangbai river basin shows the lowest (78.4%). (3) In the past 500 years, the water transfer in the water receiving areas tend to be unfavorable. Since the start of the 20^th century the probability of drought occurring simultaneously at each river basin and the water source region reaches a maximum, especially for the Hanjiang-Tangbai (Han-Tang) basin, where the probability is nearly 30%. Hanjiang-Tangbai, Hanjing-Huaihe (Han-Huai), and Hangjiang-Haihe (Han-Hai) basins favorable probabilities are 70.6%, 76.2% and 76.2, respectively. (4) In the past 500 years, the multi-year drought and waterlogging events occur in the water source area, with the longest drought lasting seven years. The probability of multi-year draughts occurring simultaneously in the water source area and water receiving areas is greater than for waterlogging. There were 34 instances of droughts lasting longer than 2 years and 24 instances of waterlogging events. These multi-year evets occurred more frequently in the Han-Tang and Han-Hai than the Han-Huai. (5) The water transfer efficiency is high during the main flood season (June to August) than other months. The Hehai river basin has the highest efficiency of 90.9% among three receiving water areas. During the non-flood season, The Hehai river basin still has a higher efficiency (85.5%) than the other two river basins, while The Tangbai river basin has the lowest efficiency of 81.8%. (6) The simulations show that during 2010 to 2100 period, The Tangbai, Huaihe and Hehai river basins have water transfer efficiency of 87.7%, 87.7% and 89.9%, respectively. Meanwhile, with increasing of precipitation during 2050 to 2100, there are more waterlogging years than drought years, therefore, it will benefit the water transfer of the project. However, in late 21th century, the risk of waterlogging in both the water source and water receiving areas simultaneously will increase.