Using Dual Isotopes of Nitrate to Identify the Source of Nitrogen Pollution in the Songhua River Basin, Northeast China

Nitrogen pollution is a severe problem in the Songhua River Basin (SHR) in China. Samples were collected from 36 sections of the SHR during the high, low, and flat seasons of the river, and the main sources of nitrogen in the water were qualitatively analyzed with isotope data for nitrogen and oxygen of nitrate. The contribution rates of each major pollution source were quantitatively analyzed using the Iso Source mass balance model. The results from these experiments indicate that the values for δ15N-NO3 and δ18O-NO3 in the flat flow season range from 1.52‰ to 14.55‰ and −14.26‰ to 2.03‰, respectively. The values for δ15N-NO3 and δ18O-NO3 in the low flow season range from 6.66‰ to 15.46‰ and −5.82‰ to 65.70‰, respectively. In the low flow season, nitrogen comes from the input of domestic and manure sewage (53%) and soil organic N (45%). The values of δ15N-NO3 and δ18O-NO3 in the high flow season range from 2.07‰ to 14.24‰ and −3.99‰ to 8.03‰, respectively. In the high flow season, nitrogen comes from soil organic nitrogen (41%), domestic and manure sewage (32%), and nitrogen fertilizer (27%), which are the main sources of nitrogen pollution in the SHR. The conclusions from the qualitative and quantitative analysis of nitrogen sources in the SHR can provide a scientific basis for the source control and treatment of nitrogen pollution.

Method to identify composition and production phases of spring runoff in high-latitude mid-temperate regions: a case study in the Second Songhua River Basin, China

Simulation and forecasting of runoff play an important role in the early warning and prevention of drought and flood disasters. To improve the accuracy of spring runoff simulations, it is important to identify spring runoff production patterns under the combined effect of snow and frozen soil. Based on the theory of the hydrological cycle, three important parameters, which include surface and subsurface runoff, precipitation and temperature, were selected for this study. The trend analysis, statistical analysis and Eckhardt's recursive numerical filtering method were used to qualitatively identify the production patterns of spring runoff, the start and end dates and stage periods of the production patterns. Based on the qualitative identification results, the contribution of each production runoff to the total annual runoff and the total annual spring runoff is quantitatively assessed. The results of the study show that the spring runoff production patterns in the Second Songhua River Basin can be divided into snowmelt runoff, frozen soil conditions of snowmelt–rainfall runoff and rainfall runoff under frozen soil conditions; the snowmelt production is from 21 March, the frozen soil conditions production is from 21 April and the frozen soil ablation ended on 15 June; the shortest phases of each production pattern last 28, 20 and 18 days and the longest last 31, 26 and 24 days. This research provides the basis for improving the principles of production runoff calculation in spring runoff simulation methods.

Investigation of spatial and temporal variability of river ice phenology and thickness across Songhua River Basin, northeast China

The regional role and trends of freshwater ice are critical factors for aquatic ecosystems, climate variability, and human activities. The ice regime has been scarcely investigated in the Songhua River Basin of northeast China. Using daily ice records of 156 hydrological stations across the region, we examined the spatial variability in the river ice phenology and river ice thickness from 2010 to 2015 and explored the role of snow depth and air temperature on the ice thickness. The river ice phenology showed a latitudinal distribution and a changing direction from southeast to northwest. We identified two spatial clusters based on Moran's I spatial autocorrelation, and results showed that the completely frozen duration with high values clustered in the Xiao Hinggan Range and that with low values clustered in the Changbai Mountains at the 95 % confidence level. The maximum ice thickness over 125 cm was distributed along the ridge of the Da Hinggan Range and Changbai Mountains, and the maximum ice thickness occurred most often in February and March. In three subbasins of the Songhua River Basin, we developed six Bayesian regression models to predict ice thickness from air temperature and snow depth. The goodness of the fit (R2) for these regression models ranged from 0.80 to 0.95, and the root mean square errors ranged from 0.08 to 0.18 m. Results showed significant and positive correlations between snow cover and ice thickness when freshwater was completely frozen. Ice thickness was influenced by the cumulative air temperature of freezing through the heat loss of ice formation and decay instead of just air temperature.

Quantifying Contributions of Climate Change and Local Human Activities to Runoff Decline in the Second Songhua River Basin

In the past several decades, climate change and human activities have influenced hydrological processes, and potentially caused more frequent and extensive flood and drought risks. Therefore, identification and quantification of the driving factors of runoff variation have become a hot research area. This paper used the trend analysis method to show that runoff had a significant downward trend during the past 60 years in the Second Songhua River Basin (SSRB) of Northeast China. The upper, middle, and lower streams of five hydrological stations were selected to analyze the breakpoint of the annual runoff in the past 60 years, and the breakpoints were used to divide the entire study period into two sub-periods (1956–1974 and 1975–2015). Using the water–energy coupling balance method based on Choudhury–Yang equation, the climatic and catchment landscape elasticity coefficient of the annual runoff change was estimated, and attribution analysis of the runoff change was carried out for the Fengman Reservoir and Fuyu stations in SSRB. The change in potential evapotranspiration has a weak effect on the runoff, and change in precipitation and catchment landscape were the leading factors affecting runoff. Impacts of climate change and land cover change were accountable for the runoff decrease by 80% and 11% (Fengman), 17% and 206% (Fuyu) on average, respectively; runoff was more sensitive to climate change in Fengman, and was more sensitive to catchment landscape change in Fuyu. In Fengman, the population was small, owing to the comparatively inhospitable natural conditions, and so human activities were low. However, in Fuyu, human activities were more intensive, and so had more impact on runoff for the Lower Second Songhua River compared to the Upper Second Songhua River.