Evaluation of Phreatic Evaporation in Manas River Basin Plain by Bromine Tracing Method

The phreatic evaporation is the main discharge way of groundwater in arid inland basins,but its quantitative evaluation has great uncertainty,which leads to the difficulty of water balance analysis and water resources evaluation. In the plain area of the Manas River Basin in Xinjiang,saturated sodium bromide was applied at different sites in different land use types as a tracer in May 2017, and soil samples were collected in August 2017 and August 2018 for measuring bromide concentration. The evaporation rate of the phreatic water is determined based on the vertical migration of the bromide peaks. The vertical profiles of bromide ion concentration profiles show mainly a single peak moved upward. According to the upward moving distance of the peak,the annual average phreatic evaporation in the non-desert area is calculated as 33.59 mm. The evaporations of different land use types from strong to weak were cotton field,uncultivated land,forest and desert,and the annual average evaporations were 41.71 mm,34.01 mm,11.28 mm,8.58 mm,respectively. The results of bromide ion tracing method for evaluating the evaporation of phreatic water are consistent with the relevant literature. The phreatic evaporation rates are related to the land use type,the lithology of the aeration zone,the volumetric water content of the soil and the phreatic depth. The higher the clay content of the rock formation is,the higher the water content is,the closer to the low-lying area of the fine soil plain,the stronger phreatic evaporation is.