Calculation of Kinetic Fractionation of Open-Water Evaporation Over an Upland Reservoir

Based on the fact that many water reservoirs built in headwater catchments receive no water input during dry season, we modified the Craig-Gordon model and proposed an approach to calculate kinetic fractionation factors in open-water evaporation taking into account isotopic compositions of precipitation and of lake water, lake volume, and land-based meteorological data. The approach is centered around the determination of lake water isotope enrichment rate over time during dry periods. We then applied this approach for calculating kinetic fractionation factors, turbulence index, and open-water evaporation for a small reservoir located in a sub-tropical monsoonal climate. We found that the kinetic fractionation factors, epsilon K,H and epsilon K,O are, respectively 3.11 parts per thousand +/- 0.41 parts per thousand and 2.50 parts per thousand +/- 2.15 parts per thousand, corresponding to the turbulence index of 0.43 +/- 0.19. Evaporation rates in open-water estimated by our approach is 1.55 times of the land-based evaporation gauged in the nearby meteorological station. Wind speed, represented by turbulence index, is proved as an important climatic factor influencing kinetic fractionation. This study confirms that wind and surface area factors, which have been neglected in several isotope mass balance models, should be included into calculation process. Isotopes are forms of the same element that differ in atomic mass. Water molecules (H2O) have lighter isotopes of H (1H, 16O) and heavier isotopes (2H, 18O). The evaporation of water favors the evaporation of the lighter isotope molecules over heavier isotope, via a process called fractionation. Well defined fractionation factors help determine precisely the degree of water loss that can be expected in an open water system due to evaporation. Such an accurate determination of seasonal evaporation loss would in turn help water managers better understand water resource management, especially in a drought prone area. Today, climate change has increased the intensity and frequency of extreme weather events. Many people in the world now live under water stress conditions. Therefore, a precise determination of evaporation loss in open-water bodies is necessary to increase population resilience to climate change. This study proposes a straight forward approach to determine kinetic fractionation factors (the others are equilibrium fractionation factors) in a desiccating lake during the dry season and apply them to a sub-tropical reservoir in Viet Nam. The kinetic fractionation factors are found to be highly dependent on wind speed variability while evaporation loss is more dependent on temperature and humidity. A modified Craig-Gordon model was developed to calculate the kinetic fractionation factor of water isotopic compositions in upland reservoir Kinetic fractionation is significantly influenced by wind condition for an upland reservoir in sub-tropical monsoon climate Open-water evaporation rate is re-evaluated as 1.55 times of the land-based monitoring data