Water loss by evaporation from China’s South-North Water Transfer Project

China’s South-North Water Transfer Project (SNWTP) is the longest and largest water transfer project in world history. However, the evaporative loss from SNWTP is still unclear. Here we estimated the water loss by evaporation from the open canal and reservoir of the Middle Route of SNWTP (MR-SNWTP), based on field experiments and three mathematical models (Penman equation, Penman-Monteith equation, and Priestley-Taylor equation). Results showed that the Penman equation was the most reliable model, thus it was used to evaluate the evaporative loss from MR-SNWTP. Under the original planned scenario, average annual evaporative loss from the open canal and accompanying reservoir of MR-SNWTP would be approximately 9.00 × 108 m3, of which 35.28% results directly from the construction of MR-SNWTP (3.34% of the planned total aqueduct diversion). However, during the first implement year of MR-SNWTP in 2015, the actual total water loss by evaporation was 6.43 × 108 m3, and the increased evaporative loss was 2.27 × 108 m3 due to the construction of MR-SNWTP, accounting for 8.57% of the actual total aqueduct diversion. This implies that the efficiency of MR-SNWTP from the perspective of water loss would be improved in the future with more water being transferred. Our results demonstrate that there is a general balance between water evaporation and vertical precipitation supplement of MR-SNWTP.