Change in Water Quality and Its Relationship to Water Quantity in Tai’an Section of the Dawen River Basin

Abstract

【Background and objective】 Quantity and quality are two indicators of water in a river basin and they are closely correlated. In general, a change in water quantity leads to a change in water quality, and the objective of this paper is to study their relationship in water at Tai’an section of the Dawen River Basin. 【Method】 Water quality data measured monthly from 2010—2019 at seven stations and flow rate at one hydrology station in the section were analysed using the Kendall test to identify the rends in the changes of chemical oxygen demand (COD) and ammonia nitrogen (NH3-N). The Copula function was used to construct two-dimensional and three-dimensional joint distributions of water quantity and quality to analyse their simultaneous variation. 【Result】 The concentrations of COD and NH3-N had both been in decline during the studied period, with the seasonal Kendall test showing that 71% of the evaluated results showing a downward trend. Spatially, the concentrations of COD and NH3-N decreased from upstream to downstream. At Dawenkou station, the concentrations of COD and NH3-N at wet seasons was reduced by 6% and 37% respectively, and monthly water quality that reached Class IV standard was increased by 10%, compared those measured in dry seasons The two-dimensional joint distribution of water quantity and quality calculated from the Frank Copula increased as the flow rate and water quality increased. When the joint probability was the same, the concentration of COD and NH3-N decreased asymptotically as the flow rate increased. The three-dimensional joint distribution of precipitation, water quantity and quality calculated from the Clayton Copula function showed that when its value was less than 0.6, the increase in the joint probability was induced predominantly by the increase in flow rate, while when the joint probability was greater than 0.6, the impact of COD and NH3-N increased. 【Conclusion】 There was a close relationship between water quantity and quality at Tai’an section of the Dawen River Basin. The Copula function was able to describe the joint distribution of water quantity and quality in the section.

Keywords

• dawen river basin
• water quality
• water quantity
• copula joint distribution