Abstract: Water bodies with size smaller than 1 km₂ occupy 40.2% of the total area of inland water bodies. The evaporation of small water bodies is one of the important processes in inland water cycle. However, evaporation over small water bodies is either ignored in water cycle studies or estimated based on the equation of large water bodies. Actually, the driving process and controlling mechanism for evaporation over small water bodies is significantly different from that over large water bodies. Meanwhile, there are significant technical difficulties and knowledge gaps in the observation and estimation of evaporation over small water bodies, due to the lacking of direct observation. The eddy covariance and isotopic mass balance methods are applicative to observe and estimate evaporation over small water bodies. Here, we reviewed the applications of these two methods, and focused on the difficulties and challenges. Furthermore, we evaluated the mechanisms underlying evaporation at different time scales based on the principle of conservation of energy, with emphasis on the impact of advectionentrainment process on evaporation. The technical challenges of the eddy covariance method for evaporation observation of small water bodies mainly include source signal pollution in source regions and spectrum correction, while the key issue for isotopic method is the accurate quantification of kinetic fractionation factor under natural conditions. The size of signal source region of evaporation with eddy covariance observation over small water bodies is around hundreds of meters, while the isotopic mass balance methods could be used over the whole water bodies. We concluded that evaporation over small water bodies could be observed more accurately with the combination of the two methods, which would facilitate quantifying the contribution to the regional and global water cycle.

Key words: evaporation, eddy covariance, isotopic mass balance, advection-entrainment process