Residence time in rivers

Posted on October 15, 2015

A knowledge of the amount of time that water spends travelling through a catchment is essential when assessing how rivers process and remove nutrients and pollutants. A new method for predicting the residence time of water in streams uses readily available information, in the form of river flow gauging data, to integrate a standard velocity equation (Manning’s equation) along a length of channel. The method was validated using data for the River Tees in England, and then applied to 323 other U.K. rivers by drawing on information in the National River Flow Archive. The study found that the average stream residence time was less than three days, and that 50% of the total U.K. residence time was contributed by only five large, low-gradient rivers. If the source and rate of turnover for a given water quality component are known, residence time data can be used to identify which parts of the river haven’t had time to reduce the concentration. For example, because dissolved organic carbon (DOC) is largely processed by U.K. rivers within 24 hours, only parts of a catchment less than a day’s travel time from a water treatment plant would be significantly affected by DOC controls on treated effluent.

Reference: Worrall, F. et al. 2014. A method of estimating in-stream residence time of water in rivers. Journal of Hydrology 512, 274–284.