Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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Science is often driven forward by the emergence of new measurements. Whenever one makes observations at a scale, precision, or frequency that was previously unattainable, one is almost guaranteed to learn something new and interesting. Our purpose in this commentary is to argue that catchment hydrochemistry is on the verge of just such a major new advance, driven by automated, online continuous analysis for many chemical constituents in natural waters.To date, most catchment hydrochemical studies have been based on hourly or sub-hourly measurements of water fluxes, and weekly or monthly samples of rainfall and streamflow chemistry. This stark mismatch in measurement time scales springs from the measurement technologies involved. Water flux measurements are easily automated, and can be logged at any interval that is desired. Conventional laboratory measurements of water chemistry, by contrast, are time consuming and expensive, and at high sampling frequencies the sample bottles pile up fast. For this reason, high-frequency chemical monitoring has typically been restricted to intensive studies of individual storm events.That is now changing. Field-deployable autoanalysers are now a reality, and ion-specific electrodes continue to improve. These technological developments promise to provide measurements of rainfall and streamflow chemistry at hourly or sub-hourly intervals (similar to the time scales at which hydrometric data have long been available) and to provide these measurements for long spans of time, not just for intensive field campaigns associated with individual storms.These technologies are likely to transform our view of catchment processes, by allowing us to observe their hydrochemical evolution at temporal resolutions that are orders of magnitude finer than before. Continuous online measurements of pH and electrical conductivity have been available for years, and they provide a preview of the high-frequency hydrochemical behaviour that will become observable through automated online chemical analysis

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