摘要:中亚咸海流域的资源和环境问题一直是全球关注的热点。为揭示咸海流域水环境变化及其成因，本文利用2019年中下游流域21个采样点水质数据，通过主成分、聚类和秩次等多元统计分析方法相结合，分析了水体的基础理化属性、氮磷营养元素、其他元素和离子等20种指标的空间分布特征，辨识了该流域主要水质类型、空间分布及成因，探索了不同空间尺度土地覆被类型对水质类型的影响。结果表明：1）电导率、总溶解性固体物质和离子浓度越往河流下游数值越高，其中在咸海最大；不同形态磷元素浓度在阿姆河中游较高，硝态氮浓度在锡尔河较高；碳元素浓度也在阿姆河，特别是阿姆河下游三角洲地区较高。2）所有采样点水体根据水质指标相似性聚类可划分为3种综合性水质类型。第1类型为水质指标浓度均偏低的水体，分布在锡尔河中游和咸海；第2类型为氮磷营养元素浓度偏高的水体，分布在阿姆河中下游；第3类为碳元素和离子浓度均偏高的水体，分布在咸海。第1和2类型水体的元素浓度主要受荒漠地区岩石风化过程控制，阴阳离子来源于硅酸盐岩和蒸发岩的风化；第3类型水体主要受气候干燥导致的蒸发-结晶过程控制，阴阳离子除来源于硅酸盐岩和蒸发岩的风化外，还受碳酸盐岩风化影响。3）当各采样点缓冲区半径从0.5 km增加到10 km时，对第1类型水质浓度变化影响显著的土地覆被类型逐渐从荒地变为水域、灌木、草地和农田与植被混合，其中水域的影响最大；第2类型水质浓度变化与土地覆被类型无显著性关系；对第3类型水质浓度变化影响显著的土地覆被类型从水域变为水域、农田与植被混合等，其中水域的影响最大。因此，咸海流域水质浓度变化与当地干旱缺水和蒸发剧烈等气象条件以及土地覆被类型有密切关系。为改善咸海中下游流域水环境状况，在增加中下游河道流量和咸海的补给，减弱下游和咸海蒸发-结晶作用的同时，还需加大河岸带的植被修复和退耕还林还草，特别是在阿姆河和锡尔河中下游农业区、咸海等地区。
关键词:水质类型/
空间尺度/
土地覆被/
多元统计分析/
咸海流域
Abstract:Water resources and environmental issues in the Aral Sea Basin of Central Asia are global concerns. In this study, the water quality variables (i.e., basic physical and chemical attributes, different forms of nutrients, other elements, cations, and anions) from 21 sampling sites in the middle and lower reaches of Aral Sea Basin were measured in 2019 to explore water environmental variations and their causes. Spatial variation in 20 water quality variables was investigated, and the representative water quality types, spatial differences, and their causes were identified via multivariate analysis methods (i.e., principal component analysis and cluster analysis). Furthermore, the effects of land cover on the spatial variation in water quality types were explored. The results showed that: 1) the values of electronic conductivity (EC) and total dissolved solids (TDS) increased from the middle to the lower reaches, and the highest values were in the Aral Sea. This indicates that the concentrations of anions and cations increased from the middle to the lower reaches. For the nutrient variables, high phosphorous concentrations were in the middle reaches of Amu Darya, and high nitrate-nitrogen concentrations were in the Syr Darya. For the different forms of carbon, the highest concentrations were in the Amu Darya, particularly in the delta area of lower reaches. 2) The water quality at all sampling sites can be divided into three water quality types according to the similarity classification of water quality variables. The first type had low concentrations for most water quality variables, which were distributed in the middle reaches of Syr Darya and the Aral Sea. The second type had high concentrations of different forms of nitrogen and phosphorus, which were distributed in the middle and lower reaches of Amu Darya. The third type had high concentrations of carbon, anions, and cations, which were distributed in the Aral Sea. The water quality concentrations of the first and second types were mainly due to rock weathering processes on bare land, and the anions and cations were mainly derived from the weathering of silicates and evaporites. The concentrations of the third type were mainly due to the evaporation and crystallization processes of a dry climate, and the anions and cations were mainly derived from the weathering of silicates and evaporites, which may also be affected by carbonate weathering. 3) With an increase in the buffer zone radius for each sampling point (0.5 km to 10 km), the significant land cover changed from bare land to water, shrubland, grassland, mixed farmland, and vegetation for the first water quality type; the most significant land cover was water. There were no significant relationships between the second water quality type and land cover. For the third water quality type, the significant land cover changed from water to water, mixed farmland, and vegetation—the most significant land cover was water. Therefore, spatial variation in the water quality variables was mainly affected by the local climate conditions (i.e., climatic drought and intensive evapotranspiration) and the mainland cover types (i.e., bare land, water, farmland, grassland, and urban). To improve the water environmental conditions in the middle and lower reaches of Aral Sea Basin, stream flow should be increased to recharge the Aral Sea and weaken the evaporation and crystallization processes in the lower reaches of Aral Sea. Vegetation restoration and a return of farmland to forest and grassland should also be strengthened in the riparian zone, particularly in the middle and lower reaches of Amu Darya and Syr Darya and the Aral Sea.
Key words:Water quality types/
Spatial scale/
Land cover/
Multivariate analysis/
Aral Sea Bain
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