Modeling study of residence time and water age in Dahuofang Reservoir in China

Abstract

Understanding the dynamics of water renewal in a reservoir is essential when the transport and fate of dissolved substances are evaluated. A three-dimensional hydrodynamic model was implemented to compute average residence time and water age in Dahuofang Reservoir in China. The model was verified for a one-year time period in 2006. A simulation reproduced intraannual variation of mixing represented by the fall/winter mixing and the spring/summer stratification. The simulated variation of vertical thermal structures also matched observation. The spatially varying average residence times and age distribution were investigated through a series of numerical experiments using a passively dissolved and conservative tracer as a surrogate. Residence time estimations yield a broad range of values depending on the position. The average residence time for a tracer placed at the head of the reservoir under high-, mean-, and low flow conditions was found to be about 125, 236 and 521 days, respectively. The age simulation reveals that the age distribution is a function of the freshwater discharge. In the vertical direction, the age of the surface layers is larger than that of the bottom layers and the age difference between the surface and bottom layers decreases further downstream. The density-induced circulation plays an important role in the circulation in the reservoir, and can generate vertical age distribution in the reservoir. These findings provide useful information for understanding the transport process in Dahuofang Reservoir that can be used to assist the water quality management of the reservoir.