|Alternative Title||Environmental hydrochemistry study in a permafrost catchment based on hydrological processes|
|Place of Conferral||北京|
|Keyword||多年冻土 水文过程 氢氧同位素 水化学 河流溶解有/无机碳 河流总氮|
多年冻土区小流域尺度上的水文过程一直是寒区水文学研究的热点领域，而降水入渗过程、坡面产流过程、河道汇流过程也是地球化学元素和地表生源物质迁移的重要通道。因此，从水循环和物质迁移的角度出发研究青藏高原多年冻土区小流域环境水化学特征，对于认识气候变暖背景下冻土区水资源保护、水环境的演化机理和生物地球化学元素迁移具有重要意义。本论文选取青藏高原多年冻土区典型流域—风火山小流域为研究区，充分利用流域内已有的大气-植被-土壤等水、热观测体系，以野外试验和室内分析相结合的手段，系统研究了不同水体的同位素特征、流域水文过程及水化学演化机制，揭示了以多年冻土为基础的风火山流域河流碳、氮输出特征及控制因素。取得的主要结论如下：（1）风火山流域不同水体氢氧同位素特征表明：大气降水中氢氧同位素δD和δ18O平均值分别为-54.18‰和-9.24‰，温度效应和降水量效应明显，风火山地区局地大气降水线方程为：δD=9.53δ18O+33.82，降水气团受海洋性气团和陆地局地气团的交替影响，后向轨迹图模拟显示风火山地区5月、6月、10月份降水气团来源于西风带的远程输送，7-9月份大部分的降水气团来自印度次大陆和孟加拉湾，我国东部海岸和内陆地区的局地气团也会对降水产生重要影响；河水中δD和δ18O的平均值分别为-62.73‰和-9.35‰，随植被盖度的增加，风火山流域及其各个支流中δ18O含量表现出一定的上升趋势，说明植被分布是造成风火山流域河水同位素空间差异的主要原因；地表覆被类型和盖度对土壤水中氢氧同位素具有重要影响，高寒草甸土壤水中δ18O大于高寒沼泽，植被盖度越低，土壤水中δ18O越高；利用氘盈余质量平衡法，对风火山流域年径流量做径流分割，结果显示34.4%的河水是由降水直接贡献的，地下水对河水的贡献比例为65.6%，这与实际观测结果是一致的。（2）风火山流域不同水体水化学时空特征及控制因素的研究表明：风火山地区天然降水中总溶解固体（TDS）浓度平均值为30.36 mg l-1，优势阳离子为Ca2+和NH4+，HCO3-在阴离子中占绝对优势，水化学类型为HCO3--Ca2+-NH4+，降水形态（降雨或降雪）对离子浓度没有显著影响，各组分离子表现出较强的时间变异特征，降水中的HCO3-、SO42-和Ca2+主要来源于矿物气溶胶，NO3-（和部分SO42-）主要来源于大气中的酸性气溶胶，NH4+主要来源于高原上牛羊粪便的氨挥发；河水中TDS浓度平均值为1419.1 mg l-1，优势阳离子为Na+ 和Ca2+，优势阴离子为SO42-和Cl-，水化学类型为SO42-Cl--Na+-Ca2+型，从5月-10月河水中阳离子Ca2+和Mg2+的比重增加，（Na++K+）比重在降低，对阴离子而言SO42-和（CO32-+HCO3-）所占的比例在增加，而Cl-所占比例在降低，河水水化学类型有向SO42--HCO3--Ca2+-Mg2+类型转变的趋势。Gibbs图分析说明风火山流域的河流水化学组成受岩石风化和蒸发结晶的双重影响，而蒸发结晶作用比岩石风化对河水化学组分的影响更大；风火山流域地下水位埋深受植被类型和盖度的影响表现出明显的季节差异，植被盖度越小，地下水位埋深越大，高寒草甸的地下水位埋深要明显大于沼泽草甸；高寒草甸地下水化学类型为HCO3--Cl--Na+型，沼泽草甸地下水化学类型为SO42--HCO3--Na+-Ca2+-Mg2+型，说明沼泽草甸地下水的矿化程度要明显高于高寒草甸，Gibbs图分析说明流域地下水组成主要受岩石风化和蒸发结晶的双重影响，而且以岩石风化为主。（3）对风火山流域河流溶解碳输出特征及影响因素的研究表明：流域内年径流输出量为161.40 mm，大约有400.10×106 g 溶解无机碳（DIC）和81.73×106 g 溶解有机碳（DOC）从陆地生态系统搬运到河流系统中；河水中DIC和DOC的输出浓度分别为17.75和4.05 mg l-1，输出通量分别为3.56 g C m-2 a-1和0.73 g C m-2 a-1；径流量与河流DIC和DOC的输出浓度和通量均表现为明显的正相关；植被盖度越大，DIC和DOC浓度越大，而通量却由于植被的截留蓄水作用而减少；多年冻土活动层的融化深度与DIC和DOC的浓度和通量表现为显著的正相关关系，说明活动层的融化过程控制着DIC和DOC的释放；风火山流域河流溶解碳的输出量只占陆地生态系统碳排放的2.79%，说明青藏高原多年冻土区通过大气交换的垂直方向上碳的输出量远远大于通过河流搬运的水平方向上的碳的输出量。（4）对风火山流域河流总氮（TN）输出特征及影响因素的研究表明：2014年大约有31.13×106 g N从陆地生态系统进入河流系统，TN平均浓度为1.99 mg l-1，通量为0.28 g N m-2 a-1；其中风火山2#支流为整个流域河流氮素的主要贡献者，3#支流可以代表整个流域的平均水平；TN的浓度与河水径流量表现为负相关，以稀释作用为主；随着植被盖度的增加，河流氮素的浓度和通量均表现为下降的趋势，说明河流中氮素的输出是植被固定、径流搬运综合作用的结果；在0-60 cm土层内，河流总氮的输出浓度随活动层融化深度的增加而增加，当融化深度超过70 cm，活动层融化深度对河流总氮浓度的影响不明显，河流总氮的输出通量却与活动层融化深度表现为显著的指数正相关，主要原因在于融化过程深刻影响着河流的水文过程。;
Hydrological processes study at a permafrost catchment scale had been a hot area of cold-region hydrology research, and atmospheric precipitation, the processes of surface runoff generation and river flow route were important channels of migrating of geochemical elements and biogenic substances. Therefore, studying the environmental hydrochemistry in a permafrost catchment of the Qinghai-Tibet Plateau was of great significant to understand the water resources protection, aquatic environment evolution mechanism, biogeochemical element transport, especially under the background of climatic warming. This paper selected Fenghuoshan catchment as the object, which has established water-heat observation system of “atmosphere-vegetation-soil”. This paper studied characteristics of hydrogen and oxygen isotopes and water geochemistry in different water bodies and hydrological processes of Fenghuoshan catchment, and revealed export characteristics of DIC, DOC and TN and their control factors. Main conclusions of this paper are as follows:(1) The characteristics of stable isotopes in different water bodies of Fenghuoshan catchment showed that: the mean value of δD and δ18O in precipitation were -54.18‰ and -9.24‰, respectively. The composition of stable isotopes was significant effected by temperature and precipitation. The equation of local meteoric water line was δD = 9.53δ18O + 33.82, and the precipitation air-mass were affected alternately by the terrestrial air-mass and oceanic air-mass. The backward trajectories analysis presented that the precipitation air-mass of Fenghuoshan catchment came from remote delivery of westerly zone in May, June and October, while from Indian sub-continent and the Bay of Bengal in July to September and those air-mass from east coast and inland areas also had a significant impact on precipitation. The mean value of δD和δ18O in river water were -62.73 ‰ and-9.35 ‰, respectively. With the increase of vegetation coverage, the δ18O in river water showed an increase trend in the Fenghuoshan catchment and its sub-catchments, indicating that vegetation was main reason which led to the differences in spatial distribution of hydrogen and oxygen isotopes. The δ18O in soil water also impacted by types and coverage of land-cover, the lower vegetation cover, the higher δ18O; and the δ18O in soil water of alpine meadow were higher than that of alpine swamp. The hydrograph separation were applied to assess the contributions of precipitation and groundwater to runoff, the results showed that 34.4% of the runoff from precipitation, and the contribution ratio of groundwater to runoff was 65.6%, which was consistent with field observation data.(2) The characteristics of water geochemistry and control factors of the different water bodies of Fenghuoshan catchment showed that: the mean TDS concentrations of precipitation was 30.36 mg l-1, Ca2+ and NH4+ were the dominant cations, HCO3- was the dominant anion and water geochemistry type of precipitation was HCO3--Ca2+-NH4+. Precipitation form (rain or snow) had no significant effect on the ions concentration, and major ions concentrations showed strong temporal variations. Main source of HCO3-, SO42- and Ca2+ were mineral aerosols, NO3- (and partial SO42-) were mainly from atmospheric acid aerosols, and NH4+ were mainly from ammonia volatilization of dung. The mean TDS concentration of river water was 1419.1 mg l-1, Na+ and Ca2+ were the dominant cations, SO42- and Cl- was the dominant anion and water geochemistry type of precipitation was SO42-Cl--Na+-Ca2+. The proportion of Ca2+ and Mg2+ increased with that of Na++K+ decreased for cations, and the proportion of SO42- and (CO32-+HCO3-) increased with that of Cl- decreased for anions from May to October, indicating that the water geochemistry types tended to SO42--HCO3--Ca2+-Mg2+. Gibbs diagram analysis showed that the water geochemistry composition of river water was affected by both rock weathering and evaporative crystallization, and evaporation crystallization greater impact on water geochemistry composition than rock weathering. The water level of groundwater showed significant seasonal differences under different land-cover types and coverage, the smaller vegetation coverage, the greater depth of groundwater table, the groundwater depth of alpine meadow was significant greater than that of alpine swamp. The groundwater geochemistry types were HCO3--Cl--Na+ for alpine meadow and SO42--HCO3--Na+-Ca2+-Mg2+ for alpine swamp, indicating that the mineralization of alpine swamp groundwater was higher than that of alpine meadow. Gibbs diagram analysis showed that the water geochemistry composition of groundwater also was affected by both rock weathering and evaporative crystallization, but mainly by rock weathering.(3) Study on export of riverine dissolved carbons and its influencing factors of Fenghuoshan catchment showed that approximately161.40 mm surface runoff, 400.10 × 106 g DIC and 81.73 × 106 g DOC were transported from terrestrial ecosystems to aquatic system in 2014. The DIC and DOC concentrations were 17.75 and 4.05 mg l-1, and the fluxes were 3.56 g C m-2 a-1 and 0.73 g C m-2 a-1, respectively. The relationship between both concentrations and fluxes of DIC and DOC and runoff showed a positive correlation; the concentrations of DIC and DOC increased with vegetation coverage, while the fluxes of them decreased due to vegetation effect in reducing surface runoff generation. The relationship between both concentrations and fluxes of DIC and DOC and the thawing depth of permafrost showed a significant positive correlation, indicating that the release of DIC and DOC were controlled by the thawing process of the active layer. The export of riverine dissolved carbons accounted for only 2.79% of emission carbon in terrestrial ecosystems of Fenghuoshan catchment, indicating that the export carbon through the atmosphere exchange in the vertical direction was far greater than that by river transport in the horizontal direction at permafrost region of Qinghai-Tibet Plateau.(4) Study on export of riverine total nitrogen and its influencing factors of Fenghuoshan catchment showed that approximately 31.13×106 g DOC were transported from terrestrial ecosystems to aquatic system in 2014. The concentrations and fluxes of TN were 1.99 mg l-1 and 0.28 g N m-2 a-1, respectively. The 2# sub-catchment was main contributor of TN to entire the Fenghuoshan catchment, while 3# sub-catchment could represent the mean level of the entire catchment. The concentrations of TN and discharge showed a negative correlation. The concentrations and fluxes both decreased with the vegetation coverage increased, indicating that the export of TN as a result of biological nitrogen fixation and runoff transportation. The TN concentrations increased with the increasing of active layer thawing depth of 0-60 cm, while the effect of active layer thawing on TN concentrations were not obvious when the thawing depth reached over 70 cm. However, the fluxes of TN showed a significant positive correlation with the active layer depth, mainly due to the thawing process of active layer profoundly impacted on the hydrological processes.
|毛天旭. 基于水文过程的多年冻土区流域环境水化学特征研究[D]. 北京. 中国科学院大学,2016.|
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