Abstract: By long term monitoring,it is found that there exists a stable amount of water evaporation in Mogao Grottoes,Gansu Province,China,which is the key factor causing the deterioration of the mural wall painting. In order to clarify the source of this evaporation,the Cave 72 is chosen as the research object,which is closed after a condensation-dehumidification temperature-humidity control system is installed. The water vapor is collected with the artificial condensation method. At the same time,the samples of the condensing water,precipitation, phreatic water,Daquan river water and surrounding rock water are obtained and their δD and δ¹⁸O are measured to reveal the source of cave evaporation water by the method of isotopic tracing. The monitoring result shows that the average value of δD and δ¹⁸O of evaporation water,precipitation water,Daquian River water phreatic water and surrounding rock water is (-46.41‰,-5.36‰), (-59.83‰,-7.91‰), (-72.22‰,-9.75‰), (-71.30‰,-9.70‰), (-27.55‰,4.23‰),respectively,which rules out the possibility that the precipitation is the source of the phreatic water. By the online the isotopes calculated,the δD and δ¹⁸O of precipitation of Yema Mountain are (-86.00‰,-12.00‰),Mogao local precipitation are (-60.00‰,-8.50‰),and its weighted average value of precipitation are (-5.30‰,-0.75‰),respectively. These show that the Danghe River is a reasonable source of the underground phreatic water and the caves evaporation water. So there exists a clear supply channel: The precipitation of Yema Mountain → Danghe River → phreatic water of Danghe → phreatic water of Mogao Grottoes/Daquan River → surrounding rock water→ evaporation water of cave. Owing to the δ values of Daquan River are (71.30‰,9.70‰),which is very close to that of phreatic water (72.22‰,9.75‰),at present it is hard to determine if there is a water source from Daquan River to the caves. At the same time,the study finds that the evaporation of the dry surrounding rock water also obeys the isotope fractionation effect of temperature,i. e. the evaporation values of isotopes are negatively correlated with temperature. The surrounding rock selects the relatively positive water in the process of the phreatic moisture migration. The evaporation water,therefore,the δ value relative to the surrounding water itself is negatively,but significantly higher than that of phreatic water. In addition,isotopic of evaporating moisture is also sensitive to the humidity of surrounding rock caves. Under the same temperature,however,with different relative humidity,the δ values of surrounding rock hygroscopic moisture are different. More drier the surrounding environment is,more higher the δ values are. In a certain extent,the dry degree of surrounding rock determines the isotope values of the water in surrounding rocks. Under the periodic fluctuations of temperature,the absorption and desorption of surrounding rock are formed the power pump source of phreatic evaporation. The determining of the water source in the cave has laid foundations for protecting the cultural relic of cave. It not only has important significance for the preventive protection of Mogao Grottoes cultural relics but also provides an important reference to protect the other caves on the Silk Road.

Key words: Mogao Grottoes, water, isotope