Identifying priority management intervals of discharge and TN/TP concentration with copula analysis for Miyun Reservoir inflows, North China

Abstract

The quantitative environmental management of reservoir inflows is challenging due to complex coexistence relationships between water quantity and water quality variables. Taking discharge as a representative water quantity indicator, as well as total nitrogen (TN) and total phosphorus (TP) as water quality indicators for the twin rivers (i.e., the Chaohe and Baihe rivers) which run into the Miyun Reservoir in North China, this study calculated marginal probability distributions of these indicators, and analyzed the joint probability distribution of discharge and TN/TP concentration by applying the Frank copula function. According to an analysis of various scenario combinations of discharge and TN/TP concentration, the quantitative management intervals including the priority control interval, the key attention interval and the daily maintenance interval, were identified. The results were as follows: (a) a fitting degree evaluation indicated that the Pearson-III distribution for the marginal probability distribution of discharge and the lognormal distribution for that of TN/TP concentration were feasible. Additionally, the Frank copula theory was applicable for their joint probability analysis according to the applicability analysis and goodness-of-fit test; (b) regarding to the water quality of the Miyun Reservoir inflows, it is more important to enhance the control of the Chaohe River and the monitoring of TP concentration; and (c) the TN concentration within division values of discharge (i.e., 16.59, 24.14m3/s) was tend to exceed the class III limitation of the Environmental Quality Standard for Surface Water in China, and the concentrations of TN and TP increased as the discharge increased for the two rivers. The quantitative management intervals based on copula analysis is an intuitive and effective solution for comprehensive risk management of reservoir inflows.