✍ Scribed by T. P. Burt; N. J. K. Howden; F. Worrall; J. J. McDonnell
No coin nor oath required. For personal study only.
High-frequency water quality sampling is all the rage, and rightly so. Kirchner et al. (2004) show how fine-scale data open up new insights into data structures. They note that new water quality sampling technologies have transformed, and will continue to transform, our view of catchment processes by allowing us to make observations at temporal resolutions that are orders of magnitude finer than before.
High-frequency measurements of chemical behaviour will certainly yield novel insights into many key questions in catchment hydrology and will compel new approaches to data analysis and modelling. This will in turn drive new approaches to field experimentation. While the Kirchner et al. (2004) commentary has been highly influential and accelerated interest in new hydrochemical signatures, we fear that one interpretation may be that there is little to be gained from studying past low-frequency data and that we may have been diverted from examining such records.
Here we explore the value of low-frequency water quality data and ask the rhetorical question: are they still useful? We do this because we fear abandonment of many low-frequency long-term datasets and a general attitudinal shift regarding the value of such records. Our purpose here is simply to redress the balance and to point out that examination of long water quality time series remains fruitful. Furthermore, as long records are required to assess long-term changes in catchment response, it would seem negligent to pay no attention to them, especially because recent work has shown that such data may contain evidence of rare events or of very rapid shifts in system behaviour (Burt, 1994; Burt et al., 2010a; Howden et al., 2010), indicate long-term trends (Howden and Burt, 2008, 2009) and set the context within which shorter records can be interpreted (Burt et al. , 2008). This last point is crucial because shortterm catchment studies (i.e. <10 years) tend to be highly influenced by inter-annual hydroclimatic variability, such that long-term trends are often obscured. However, our main point in this commentary is that we cannot afford to wait years or decades for new types of high-frequency data to accumulate when good scientific use can be made of existing sources. Before the advent of data loggers and field-deployable analytical instrumentation, high-frequency sampling cost much more time and effort in the field, and in laboratory analysis of large numbers of water samples. It is not that high-frequency data were never obtained, but that they were uncommon.
One example to make the point, that is emblematic of many such records around the world, is the 50-year record of river water nitrate concentrations and flux for the Great Ouse river in eastern England. Gauged daily flows derived from 15-min values are available from 1 January 1933 (NRFA gauge 33002; catchment area 1460 km 2 ) and, more relevant here, nitrate concentrations are available for the same site from 1957 (first sample 2 January 1957, n = 4250). Sample frequency averages 83 samples per year; however, the record benefits from very frequent sampling during the mid-1980s, with approximately weekly samples for the remainder of the record.
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