摘要
黑河流域属于干旱半干旱区内陆河流域,冰川、戈壁、沙漠以及绿洲等自然景观并存。流域降水水汽来源复杂,时空分布变化大,降水对流域生态环境有着重要影响。黑河流域大气降水特征以及水汽来源的研究,将对深入研究黑河流域乃至整个西北干旱区的水循环特征,指导干旱区生态环境的重建与恢复有重要的意义。环境同位素对大尺度流域的降水及水汽来源进行分析有其优越性。2002年10月至2004年9月期间对黑河流域上游和中游六个采样点进行了大气降水水样的采集。在样品的实验室分析及数据分析的基础上,对大气降水中的氢氧同位素的时空分布及其与各种影响因子的关系进行了深入的探讨,并结合后向轨迹法对水汽来源进行了深入研究;同时根据西水站2002至2003年的气象资料,对西水大气降水中的氢氧同位素组成进行了模拟。在综合研究分析的基础上获得了如下成果:
1.对历次大气降水事件中的δ18O及其月降水加权平均值的分析发现:黑河流域降水中的氢氧同位素表现出明显的“温度效应”,δ18O和温度T之间的线性关系与海拔有一定的关系,即海拔越高,直线的斜率越大,δ18O/△T越大;大气降水中δ18O的月加权平均值和降水量之间无明显的关系,但从逐次降水事件上分析发现δ18O与降水量之间存在着“降水量效应”。降水中δ18O的月加权平均值变化范围从下游戈壁沙漠带的-2.2‰到山区的-9.2‰,随着采样点海拔的增加,δ18O呈现出下降的趋势,特别是1690m的莺落峡至2569m的西水之间呈现出明显的“高度效应”。通过分析西水、莺落峡、甘州、平川和正义峡降水中δ18O的加权平均值,得到δ18O的高度梯度为-0.47‰/100m;模拟δ18O值随时间的变化后发现,每一个取样点降水中δ18O值的季节性变化呈现明显的正弦波趋势;
2.建立的黑河流域地方大气降水线(LMWL)方程为:δD=7.82δ18O+7.63,相关系数r=0.99。降水线斜率低于全球大气降水线斜率8,符合黑河流域干燥的环境特征和较强的蒸发条件;
3.研究了流域降水中氘盈余(d)的分布规律及其影响因素,并结合后向轨迹法(Backwards Trajectory)分析了形成黑河流域大气降水的水汽来源。研究发现:冷季以西风带水汽为主,局地循环水汽次之;暖季水汽来源复杂,以西风带、内陆及东南季风水汽的影响最为明显。301个降水水样的d值中,有162个d值大于+10‰,有27个d值甚至大于+20‰,这说明局地蒸散发水汽在黑河流域水汽循环中占有较大的比重,特别是在暖季;
4.利用瑞利分馏模型对西水大气降水中氢氧同位素比率进行模拟发现冷季模拟的效果要明显好于暖季,冷季模型效率达到90%以上。原因可能是:黑河流域冷季主要受西风带水汽控制,局地蒸发水汽较弱;暖季水汽来源复杂,不存在单一水汽为主的情况,且较高的温度使得蒸散发水汽对整个大气降水中氢氧同位素组成的影响较大。
The Heihe River Basin (HRB) belongs to the arid inland basin, and contains many natural landscapes, such as glacier, the Gobi, desert and the oasis and so on.In HRB, water vapor sources for precipitation are complex and the spatial and temporal distribution varies greatly. Precipitation has an important impact on the ecological environment, and studying on atmospheric precipitation characteristics of HRB and water vapor sources has an important significance on a whole-depth study of water cycle characteristics of HRB, even the northwest arid zone, and guide reconstruction and rehabilitation of the ecological environment in arid areas. Using of environmental isotopes to analysis precipitation of large-scale basin and water vapor sources has its advantages. Water samples of atmospheric precipitation were collected from six sampling points in the upper reach and middle reach of HRB during October 2002 to September 2004. Based on laboratory analysis of samples and data analysis, the spatial and temporal distribution of hydrogen and oxygen isotopes in atmospheric precipitation and the relationship between it and several of impact factors were studied and discussed deeply. Combined both Backwards Trajectory and stable isotope value in precipitation, water vapor sources of Heihe river basin were studied, at the same time,hydrogen and oxygen isotope in precipitation of Xishui were simulated according to meteorological data of Xishui Station from 2002 to 2003. Based on the comprehensive study and analysis, results obtained were as follows:
1.δ18O in precipitation of every precipitation event values and monthly weighted average values showed that:there is clearly the "temperature effect" inδ18O andδD of precipitation in HRB. The linear relationship betweenδ18O and temperature had a certain relationship with the altitude of sampling area, that was, the altitude higher, the slope greater, andδ18O/ΔT gradient greater from downstream to upstream. "Precipitation Effect" was not observed by monthly weighted average values ofδ18O in precipitation, but it was existent in successive precipitation events value ofδ18O. The monthly weighted average values ofδ18O in precipitation ranged from the lower reaches of the Gobi desert to the mountains is-2.2‰to -9.2‰, with the increase of altitude of sampling points,δ18O showed a downward trend. "Altitude Effect" is observed vividly between Yingluoxia and Xishui, the altitude of them is 1690m and 2569m, respectively. By analyzing weighted average values ofδ18O in precipitation of the Xishui, Yingluoxia, Ganzhou, Pingchuan and Zhengyixia, the height gradient ofδ18O is -0.47‰/100 m, which can be obtained. Through simulating changes ofδ18O value over time, seasonal changes ofδ18O values in precipitation of each sampling point showed apparent sine wave trend.
2. The local meteoric water line (LMWL) equation of HRB was established byδD=7.82δ18O+7.63 and the correlation coefficient r= 0.99. The slope of precipitation line was less than the slope of the global meteoric water line, which is 8. This was consistent with dry conditions and strong evaporation conditions in HRB.
3. The distribution and influencing factor of d-excess in precipitation of the basin were studied; water vapor sources which formed the atmospheric precipitation in HRB were analyzed combined with Backwards Trajectory. Results obtained as follows:water vapor of westerlies was dominant in cold seasons, followed by local circulation of water vapor; water vapor sources in warm seasons were very complex, and water vapor of westerlies, inland and southeast monsoon were the most pronounced, d-excess values of 162 samples were greater than +10‰, and the values of 27 samples were even greater than +20‰in 301 water samples of the precipitation, which showed water vapor of local evapotranspiration was in a large proportion in water vapor cycle of HRB, especially in the warm seasons.
4. Hydrogen and oxygen isotope values in the atmospheric precipitation of Xishui were simulated by Rayleigh fractionation model and the simulation results in cold season were significantly better than in the warm season, and the efficiency of model for cool season was over 90%. This probably because that Heihe River Basin was mainly controlled and affected by water vapor of westerlies, and local evaporation was weak in cold season. Water vapor sources in warm seasons were complex, and there was no case that single water vapor was dominant. Hydrogen isotope values in the whole atmospheric precipitation were great influenced by water vapor of evapo transpiration owing to the higher temperature.
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