海河流域降水稳定同位素的云底二次蒸发效应

云底二次蒸发导致的同位素动力分馏可显著影响观测的降水同位素组成和大气水线。本文利用海河流域7个监测站点的降水δ²H和δ¹⁸O数据,分析了云底二次蒸发对流域降水同位素的影响。结果表明：流域降雨水样的大气水线为δ²H=7.19δ¹⁸O-0.74,显著不同于降雪水样的大气水线(δ²H=8.42δ¹⁸O+15.88);流域降雨,特别是小降雨(<5 mm)事件,易受到云底二次蒸发的影响,导致其大气水线的斜率和截距均随着降雨量的减小而减小。流域降雨同位素的云底二次蒸发主要受气温和相对湿度控制,随着气温的升高和相对湿度的减小,云底二次蒸发加剧,导致观测的地面降雨富集重的同位素,同时伴随的同位素动力分馏导致流域降水过量氘(d)值以及大气水线的斜率和截距均减小。与平原地区相比较,流域山间盆地地区受“雨影效应”影响,气候相对干燥,其降雨同位素受更强的云底二次蒸发影响。观测期间,流域小的降雨事件占总降水事件的42%,故云底二次蒸发对流域降水同位素具有重要的影响。

Influence of below-cloud secondary evaporation on stable isotope composition in precipitation in the Haihe River Basin,China

Measurement of stable water isotopes (δ²H and δ¹⁸O) at the watershed scale can provide a diagnostic indication for hydrological processes and water cycling of the watershed. Kinetic isotope fractionation associated with below-cloud secondary evaporation can significantly affect the observed precipitation isotopic composition and local meteoric water line. Stable isotope composition of δ²H and δ¹⁸O in precipitation was investigated in the Haihe River Basin, northern China based on observations at seven stations from July 2012 to January 2013. Linear regression between δ²H and δ¹⁸O values of rain samples yielded a correlation equation of δ²H=7.19δ¹⁸O-0.74, which is significantly different from that based on snow samples (δ²H=8.42δ¹⁸O+15.88). Due to the influence of below-cloud secondary evaporation on rain isotopes, the slope and intercept of correlation between δ²H and δ¹⁸O for small amount rainfall samples (<5 mm) progressively decreased with decreasing precipitation, varying from 6.73 to 7.61 and 5.28‰ to -11.04‰, respectively. Correlation of isotope values with local temperature, relative humidity, and precipitation amount provides evidence that small amount rainfall samples underwent secondary evaporation accompanied by mass dependent isotope fractionation during their descent from the cloud base to the ground. Compared to sites in the plain areas, rain stable isotopes at sites in the mountainous areas tended to be influenced by below-cloud secondary evaporation due to the dry atmosphere caused by the rain shadow effect. Since about half of the precipitation events in the observation period were rain samples with amount less than 5 mm, below-cloud secondary evaporation had an important influence on isotope composition of precipitation in the Haihe River Basin. Our study suggests that analysis of isotope composition of individual rainfall events can provide some valuable insight into below-cloud secondary evaporation effect, which is masked to a large extent by analysis of monthly precipitation isotope data.