基于稳定氢氧同位素的盐水与纯水蒸发差异分析

水面蒸发是水循环的重要部分,目前大量的研究集中在淡水或低盐度咸水体蒸发,仅以淡水的蒸发特性或计算方法应用于全部水体并不准确。研究盐水蒸发与淡水蒸发的差异对蒸发过程机理研究、推动蒸发模型的创新改进至关重要。研究中使用蒸发皿对比盐水与纯水的蒸发过程,利用稳定氢氧同位素比较盐水与纯水蒸发的动力学分馏过程,结果表明：盐水蒸发量较纯水少,但仍存在蒸发日内变化规律,蒸发速率与气温变化规律同步,并在正午前后达日最大值;随着蒸发的进行,重同位素在液相富集,盐度对于H/D分馏有更显著的抑制作用;盐水与纯水的蒸发线拟合均有良好的线性关系,盐分使水体蒸发受到更强的非平衡分馏影响;热红外拍摄液面观测到蒸发过程中盐水液面温度始终高于纯水0.1~2.2℃,平均温差达1℃,这是因为盐水蒸发量较纯水小,更少的热量通过潜热释放;Craig & Gordon模型计算蒸发水汽同位素特征值,表明随着蒸发的进行,蒸发水汽组分重同位素也在不断富集,但程度不如剩余水体,检验盐水与纯水蒸发水汽氢氧同位素拟合方程却无明显差异,与前述结论相悖,说明该方程在小尺度上的应用还有待研究。     

Stable isotope fractionations in the evaporation of water: The wind effect

We performed pan evaporation experiments with the objective of exploring the behaviour of the long-standing Craig–Gordon (C–G) stable isotope model for evaporation under different conditions of air turbulence. The water lost through evaporation was automatically replenished so that a steady isotopic composition was reached, the value of which depended on the isotopic composition of the replenishment water and environmental parameters like temperature, relative humidity and isotopic composition of the atmospheric vapour, and the air turbulence index. The pans were exposed to artificial winds ranging from 0 to 2.5 m/s to change the air turbulence index, which governs the repartition between vapour transported by molecular diffusion and turbulent diffusion. Our data revealed that for wind speeds >0.5 m/s the isotopic composition of the evaporating water deviated from that predicted by the C–G model. This deviation was hypothetically attributed to microdroplets of liquid water removed by the wind without any isotopic fractionation. Isotope mass balance equations allowed us to quantify this water loss, which at wind speeds of ~2 m/s reached 10% of the total evaporation losses. An alternative kinetic evaporation model was proposed whereby the equilibrium layer and the atmospheric laminar layer above the evaporating water of the C–G model were destroyed by the wind and evaporated water molecules were directly injected into the atmosphere. In this model, the isotopic fractionations were due to the slower kinetics of hydrogen bond breakage between molecules in liquid water when heavy isotopes are involved. Accordingly, our data suggested that for isotope water balance studies where winds are frequently above 2 m/s, the C–G model may be inadequate without appropriate corrections for spray vaporization, or the introduction of appropriate kinetic isotope fractionation factors.     

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

The stable isotopes of hydrogen and oxygen (δ²H and δ¹⁸O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run‐off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.