Correction of fluvial fluxes of chemical species for diurnal variation

Many river solute concentrations exhibit a diurnal cycle, caused by slower in-stream biogeochemical processes at night compared to daylight hours. So, if sample observations of such solutes are not taken across the diurnal cycle, derived estimates of solute concentrations and fluxes will be subject to bias. We developed a simple two-rate model to correct daily sampling results using estimates of in-stream residence time and day length and applied the method to the example of a monitoring point on the River Tees in northern England for dissolved organic carbon (DOC) concentration and flux. Estimates of day and night-time degradation rates combined with assessment of the daily in-stream residence time suggest that derived fluxes would be overestimated by up to 19% when diurnal variation was considered due to the highest concentrations of DOC and highest flows being in winter when day lengths are shortest. The maximum correction for diurnal differences in degradation rates was 17%. There was a significant interaction between model degradation rates and in-stream residence times but the effect of latitude on day length was insignificant. We propose a general method to estimate the correction factor and in-stream residence time based on gauging station data, which is relevant to other rivers where sufficient data are available to support the method.