Evaporation and transport of water isotopologues from Greenland lakes: The lake size effect
Evaporation and transport of water isotopologues from Greenland lakes: The lake size effect
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DOI：
10.1016/j.quascirev.2015.07.029
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发表时间：
2013
影响因子：
4
通讯作者：
Xiahong Feng;A. Lauder;E. Posmentier;B. Kopec;R. Virginia
中科院分区：
文献类型：
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作者：
Xiahong Feng;A. Lauder;E. Posmentier;B. Kopec;R. Virginia
湖泊蒸发通量的同位素组成用于许多水文和古气候研究，这些研究有助于限制湖泊的水收支和/或推断气候条件的变化。水面蒸发的同位素通量通常使用最初由Craig和Gordon（1965）构想的零维（0-D）模型来计算。这种模型通常有层流和湍流层，假设稳态条件，忽略水平变化。特别是没有考虑平流对同位素变化的影响。虽然这种经典的处理方法可以用于大型开放地表水体的某些部分，例如海洋或大湖，但它可能不适用于相对较小的水体，因为有限的取水量不能使陆地和水面的空气完全平衡。水蒸汽浓度和同位素比值的水平和垂直梯度可能在湖泊上空形成。这些梯度反过来又影响水蒸气的蒸发通量及其同位素比率，这是0-D模式无法充分预测的。在格陵兰岛Kangerlussuaq，当风速为1-5 m/s时，相对干燥和同位素耗尽的空气在5 km范围内平流到几个湖泊表面时，我们首次观测到蒸汽和同位素梯度的垂直和水平分量。我们使用稳态二维模型模拟了湖上空气中的蒸汽和同位素分布，其中垂直扩散输送与水平平流平衡。我们的观测结果验证了该模型，并将其用于计算水蒸气的蒸发通量及其同位素比。在零风速的特殊情况下，模型简化为一维。将一维模型的结果与二维模型的结果进行了比较，以评估平流和无平流条件下同位素通量的差异。由于湖泊上空的风平流改变了水同位素的浓度、梯度和蒸发通量，因此它改变了湖泊的水平衡和同位素比率以及它们之间的关系。这些影响对小湖泊影响最大。如果在从湖泊同位素推断水平衡时忽略了风平流，就会产生一个误差，其大小取决于湖泊的大小。我们称之为“湖泊大小效应”。对于风向长度小于500 m的湖泊，平均δ18O和δD比一维模型的通量值至少低2‰。δ18O比δD在水平衡计算中的相对误差要大得多；前者大约是后者的8倍。这一结果表明，用δD计算的水量平衡对湖泊大小的差异和/或其随时间的变化不太敏感。一维模型的结果也与可比较的0-D模型的结果进行了比较。由于垂直蒸汽和同位素梯度始终存在（即使在没有平流条件下），如果使用在任意高度测量的环境空气中的相对湿度和同位素比率进行0-D模式计算，则可能无法获得正确的通量值。通常，在2米或10米的标准气象测量会导致低估蒸气通量的δ18O和δD值。这项工作提供了平流对蒸发同位素通量影响的第一个量化。流动水汽分析与二维模拟相结合的方法可以应用于其他环境条件，其中平流对同位素通量的影响大小取决于当地气象和水文变量之间的关系。我们的研究结果还表明……
Isotopic compositions of evaporative flux from a lake are used in many hydrological and paleoclimate studies that help constrain the water budget of a lake and/or to infer changes in climate conditions. The isotopic fluxes of evaporation from a water surface are typically computed using a zero dimensional (0-D) model originally conceptualized by Craig and Gordon (1965). Such models generally have laminar and turbulent layers, assume a steady state condition, and neglect horizontal variations. In particular, the effect of advection on isotopic variations is not considered. While this classical treatment can be used for some sections of large open surface water bodies, such as an ocean or a large lake, it may not apply to relatively small water bodies where limited fetch does not allow full equilibration between air from land and the water surface. Both horizontal and vertical gradients in water vapor concentration and isotopic ratios may develop over a lake. These gradients, in turn, affect the evaporative fluxes of water vapor and its isotopic ratios, which is not adequately predicted by a 0-D model.We observed, for the first time, the vertical as well as horizontal components of vapor and isotopic gradients as relatively dry and isotopically depleted air advected over the surfaces of several lakes up to a 5 km fetch under winds of 1–5 m/s in Kangerlussuaq, Greenland. We modeled the vapor and isotopic distribution in air above the lake using a steady state 2-D model, in which vertical diffusive transport balances horizontal advection. The model was verified by our observations, and then used to calculate evaporative fluxes of vapor and its isotopic ratios. In the special case of zero wind speed, the model reduces to 1-D. Results from this 1-D model are compared with those from the 2-D model to assess the discrepancy in isotopic fluxes between advection and no advection conditions.Since wind advection above a lake alters the concentrations, gradients, and evaporative fluxes of water isotopes, it alters the water balance and isotope ratios of the lake and the relationship between them. These effects are greatest for small lakes. If wind advection is neglected in the inference of water balance from lake isotopes, an error is thus introduced, the magnitude of which depends on lake size. We refer to this as the “lake size effect”. For lakes less than 500 m in length along the wind direction, the average δ18O and δD of vapor flux are at least 2‰ lower than the corresponding flux values from the 1-D model. The magnitude of the resulting relative error in water balance calculations is much greater if using δ18O than δD in mass balance calculations; the former is about eight times the latter. This result argues that water balance calculated with δD is less sensitive to the difference in lake size and/or its change over time.The 1-D model result is also compared with that from a comparable 0-D model. Since vertical vapor and isotope gradients always exist (even under no advection conditions), one may not obtain correct flux values if the relative humidity and isotopic ratios in ambient air measured at an arbitrary height are used for the 0-D model calculation. Typically, the standard meteorological measurements at 2 or 10 m would result in an underestimate of the δ18O and δD values of the vapor flux.This work has provided the first quantification on the effect of advection on isotopic fluxes of evaporation. The method of mobile vapor analysis combined with 2-D modeling can be applied to other environmental settings, in which the size of advection effect on isotopic fluxes depends upon relationships among local meteorological and hydrological variables. Our results also suggest that …