Understanding the diurnal cycle in fluvial dissolved organic carbon - The interplay of in-stream residence time, day length and organic matter turnover

F. Worrall*, N. J K Howden, T. P. Burt

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

12 Citations (Scopus)

Abstract

There is increasing interest in characterising the diurnal fluctuation of stream solute concentrations because observed data series derived from spot samples may be highly subjective if such diurnal fluctuations are large. This can therefore lead to large uncertainties, bias or systematic errors in calculation of fluvial solute fluxes, depending upon the particular sampling regime. A simplistic approach would be to assume diurnal fluctuations are constant throughout the water year, but this study proposes diurnal cycles in stream water quality can only be interpreted in the context of stream residence time and changing day length. Three years of hourly dissolved organic carbon (DOC) concentration and flow data from the River Dee catchment (1674km2) were analysed, and statistical analysis of the entire record shows there is no consistent diurnal cycle in the record. From the 3-year record (1095days) there were only 96 diurnal cycles could be analysed. Cycles were quantified in terms of their: relative and absolute amplitude; duration; time to maximum concentration; asymmetry; percentile flow and in-stream residence time.The median diurnal cycle showed an amplitude that was 9.2% of the starting concentration; it was not significantly asymmetric; and occurred at the 19th percentile flow. The median DOC removal rate was 0.07. mg. C/l/hr with an inter-quartile range of 0.052-0.100. mg. C/l/hr. Results were interpreted as controlled by two, separate, zero-order kinetic rate laws, one for the day and one for the night. There was no single diurnal cycle present across the record, rather a number of different cycles controlled by the combination of in-stream residence time and exposure to contrasting light conditions. Over the 3-year period the average in-stream loss of DOC was 32%. The diurnal cycles evident in high resolution DOC data are interpretable, but require contextual information for their influence on in-stream processes to be understood or for them to be utilised.

Original languageEnglish
Pages (from-to)830-838
Number of pages9
JournalJournal of Hydrology
Volume523
DOIs
Publication statusPublished - 1 Apr 2015

Keywords

  • DOC
  • Greenhouse gases
  • Residence times
  • Solute dynamics

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