关键词: 星云湖流域, 入湖河流, 水质, 土地利用, 季节变化

Abstract:

The land use types determine the distribution pattern of pollutant “sources” within the plateau lake basin, while seasonal variations become the key driving force of pollutant “sinks” by regulating hydrological processes such as rainfall. The two factors jointly dominate the water quality changes of rivers flowing into the lake through the “source and sink” processes. However, the impact of land use and seasonal changes on the spatiotemporal variation of water quality in the main rivers flowing into the lake in the Xingyun Lake basin remains unclear. Xingyun Lake is one of the nine plateau lakes in Yunnan Province. Since 2002, with the expansion of urban areas and the intensive development of agriculture, the water quality of the lake has gradually deteriorated, consistently falling below the Class V water quality standards for surface water (GB3838). Among these factors, domestic sewage discharge and agricultural non-point source pollution have become the primary contributors to the deterioration of the water quality of Xingyun Lake. Rivers serve as crucial passageways linking land-based pollution sources to lakes. Changes in nutrients (such as nitrogen, phosphorus, etc.) in river water not only reflect land use changes on the land, but also directly affect the degree of eutrophication in lakes through their transport processes. Therefore, understanding the patterns of river water quality changes is crucial for revealing the spatial and temporal characteristics of water pollution and identifying key driving factors. This not only helps to understand the health status of rivers flowing into lakes, but also provides a scientific basis for precisely formulating pollution control measures and protecting the water quality of rivers and lakes. This study takes the Xingyun Lake basin as the research object, collecting water samples from 23 sections at the upper, middle, and lower reaches of eight major rivers flowing into the lake every month. Combining 30 m DEM, remote sensing image interpretation, and the SWAT model, the basins where the eight rivers were located were divided. The characteristics of spatiotemporal variation in chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), and total phosphorus (TP) concentrations in river water during the rainy season (from June to November) and dry season (from December to May of the follow year) were systematically analyzed, and the relationship between spatiotemporal variation of water quality, land use, and seasonal hydrological factors was elucidated. The results showed that there were significant spatial and temporal differences in water quality among the 8 main rivers flowing into the lake. Among these rivers, the Dajie River and Xiaojie River exhibited the most severe pollution levels, with COD, TN, NH₄⁺-N, and TP concentrations of 45.8 and 40.7 mg·L⁻¹, 14.1 and 10.8 mg·L⁻¹, 6.42 and 4.59 mg·L⁻¹, and 1.01 and 0.54 mg·L⁻¹, respectively. Following these two rivers, the Jiuzhou River, Dongda River, Xida River, and Yucun River showed moderate pollution levels, whereas the Xuehe River and Luoshipu River had the lowest concentrations of the four pollutants, indicating relatively good water quality. At the cross-sections along the upper, middle, and lower reaches of the 8 rivers, the middle reaches were the pollution hotspots. Compared with the upper and lower reaches, the COD (37.5 mg·L⁻¹) in the middle reaches has increased by 30.3% and 32.4%, TN (9.64 mg·L⁻¹) has increased by 38.1% and 42.2%, NH₄⁺-N (5.24 mg·L⁻¹) has increased by 72.4% and 89.2%, and TP (0.66 mg·L⁻¹) has increased by 37.5% and 15.8%, respectively. The seasonal effect on river water quality was significant, with rainfall and river water temperature being the key factors influencing the seasonal variation of water quality in major rivers flowing into the lake. During the rainy season, increased rainfall led to a dilution effect on river water, while higher water temperature promoted microbial activity and pollutant degradation, collectively resulting in reduced concentrations of the four pollutants. In contrast, during the dry season, limited rainfall reduced the runoff volume, weakening the dilution capacity, and lower water temperature inhibited pollutant decomposition, leading to the accumulation of pollutants. Consequently, the average concentrations of COD (36.20 mg·L⁻¹), TN (9.90 mg·L⁻¹), NH₄⁺-N (4.10 mg·L⁻¹), and TP (0.63 mg·L⁻¹) during the dry season increased by 12.4%, 15.7%, 78.3%, and 14.8%, respectively, compared to those during the rainy season. Correlation analysis demonstrated that the concentrations of COD, nitrogen, and phosphorus were positively correlated with the area proportions of construction land and cultivated land in the basin. Among the eight main rivers flowing into the lake, the water quality of the Dajie River (with 84.4% of its basin area covered by cultivated land and construction land) and Xiaojie River (with 84.42% of its basin area covered by cultivated land and construction land), which were dominated by a mix of construction land and cultivated land, was the worst. The water quality throughout the year was classified as inferior V category according to GB 3838 standards for surface water. The next worst was the Yucun River basin, which was dominated by cultivated land (with a cultivated land area ratio of 62.4%). The water quality was only classified as inferior V category in November and December. In contrast, the Luoshipu River (with a forest and grassland area ratio of 73.2%) and Xuehe River basin (with a forest and grassland area ratio of 53.6%), dominated by forest and grassland, maintained good water quality, with surface water classified as Class II-IV from April to October. Therefore, the management of agricultural non-point source pollution in plateau lake basins requires differentiated countermeasures based on the land use characteristics of different basins. It is necessary to strengthen the water conservation function of forest and grassland-dominated basins, prioritize the management of agricultural non-point source pollution in cultivated land-dominated basins, improve sewage collection and treatment facilities in construction land-dominated basins, and vigorously promote the reuse of reclaimed water.

Key words: Xingyun lake basin, river flowing into lake, water quality, land use, seasonal variation