汉江中下游水质时空变异与驱动因素识别

Temporal and Spatial Variations in Water Quality of Hanjiang River and Its Influencing Factors in Recent Years

汉江是南水北调中线工程主要水源区和影响区,近年来汉江中下游水生态环境质量退化严重,河流水华频繁暴发,科学识别水环境质量时空变异及主要驱动因素成为优化上游调水工程重要管理需求.本研究基于近年汉江水文水质多源数据,综合利用Daniel趋势检验、MK突变分析、K-means聚类、空间相异度和冗余分析等数学方法,系统揭示了汉江中下游水质时空变异特征及关键影响因素.结果表明:(1)近年汉江干流水质总体较好,总体处于Ⅱ类,中下游部分断面水质为Ⅲ类;多项水质指标多年来总体较好,但总磷和总氮负荷较高,近年10个断面总磷和总氮平均浓度分别处于0.028~0.263 mg·L^(-1)和0.630~1.852 mg·L^(-1)水平;(2)时间变化上,Daniel和MK突变结果发现2004~2018年宗关站总磷和总氮年变化趋势不显著,其它多项水质指标也无明显年变化趋势;枯水期主要水质指标总氮、氨氮和五日生化需氧量总体大于丰水期,化学需氧量不同点位呈现出不同的丰枯变化规律,总磷丰水期浓度下降不明显;(3)空间分布上,K-means聚类和空间相异度结果发现不同断面水质指标变化趋势差异较大,10个水质监测断面总体上可以聚类为3类,上游水质指标最好,中游次之,下游较差;值得注意的是,下游小河等断面多项水质指标均趋于改善,可能与近年来实施的控源减排和清澈养殖等保护行动和措施有关;(4)在总体较高的氮磷负荷条件下,流量和水温是影响汉江中下游3个分区河流水质指标的重要因素,其中流量在上、下游对水质贡献率较大,水温则在中游贡献率最大.

Hanjiang River is the main water source and influence area of the Middle Route of the South to North Water Transfer Project. In recent years,the water quality and ecological environment in the middle and lower reaches of the Hanjiang River has become seriously degraded and water blooms occur frequently. Scientific identification of the temporal and spatial variations in water environment quality( and the main driving factors) has become an important management requirement for optimizing the upstream water transfer project. The temporal and spatial variations and influencing factors of water quality in the Hanjiang River basin were systematically analyzed,based on multi-source data and using the Daniel trend test,Mann Kendall test,K-means cluster analysis,dissimilarity analysis,and redundancy analysis. Results showed that:(1) in recent years,the main stream of the Hanjiang River had generally good water quality,which was generally classified as class Ⅱ of GB 3838-2002,while the water quality of some sections in the middle and lower reaches was classified as class Ⅲ. However,the total phosphorus( TP) and total nitrogen( TN) load was relatively high,with 10 stations in the Hanjiang River basin showing averaged concentrations of 0. 028-0. 263 mg·L^(-1) and 0. 630-1. 852 mg·L^(-1),respectively,during 2014-2018.(2) From 2004 to 2018,TP and TN at Zongguan station did not show significant variation,and other water quality indexes did not exhibit any regular patterns. The concentrations of TN,NH^(+)_(4)-N,and BOD5 in the dry season were higher than those in the wet season. In the wet season. the permanganate index showed different variation patterns,while TP concentration did not decrease significantly.(3) Different sections showed obvious differences in the variation of water quality indexes. However,the ten stations can be clustered into three categories: the upstream stations showed the best water quality,followed by the middle reaches,and the downstream stations showed the worst. The water quality at Xiaohe station improved significantly over the study period,which may be related to protection measures implemented in recent years,such as source control,emission reduction,and removal of aquaculture.(4) Water discharge and temperature were important factors affecting the water quality of the three regions in Hanjiang River. According to redundancy analysis,the contribution of discharge to water quality in the upstream and downstream areas was much larger,while the contribution of water temperature was greatest in the middle reaches of the river.