The time it takes for rainfall to travel through a catchment and reach the stream is a fundamental hydraulic parameter that controls the retention of soluble contaminants and thus the downstream consequences of pollution episodes. Catchments with short flushing times will deliver brief, intense contaminant pulses to downstream waters, whereas catchments with longer flushing times will deliver less intense but more sustained contaminant fluxes. Here we analyse detailed time series of chloride, a natural tracer, in both rainfall and runoff from headwater catchments at Plynlimon, Wales. We show that, although the chloride concentrations in rainfall have a white noise spectrum, the chloride concentrations in streamflow exhibit fractal 1/f scaling over three orders of magnitude. The fractal fluctuations in tracer concentrations indicate that these catchments do not have characteristic flushing times. Instead, their travel times follow an approximate power-law distribution implying that they will retain a long chemical memory of past inputs. Contaminants will initially be flushed rapidly, but then low-level contamination will be delivered to streams for a surprisingly long time.

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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

To determine whether stable isotopes can be used for identifying the geographic origins of migratory bird populations, we examined the isotopic composition of hydrogen (deuterium, δD), carbon (δ 13 C), and strontium (δ 87 Sr) in tissues of a migratory passerine, the black-throated blue warbler (Dendroica caerulescens), throughout its breeding range in eastern North America. δD and δ 13 C values in feathers, which are grown in the breeding area, varied systematically along a latitudinal gradient, being highest in samples from the southern end of the species' breeding range in Georgia and lowest in southern Canada. In addition, δD decreased from east to west across the northern part of the breeding range, from New Brunswick to Michigan. δ 87 Sr ratios were highest in the Appalachian Mountains, and decreased towards the west. These patterns are consistent with geographical variation in the isotopic composition of the natural environment, i.e., with that of precipitation, plants, and soils for δD, δ 13 C, and δ 87 Sr, respectively. Preliminary analyses of the δD and δ 13 C composition of feathers collected from warblers in their Caribbean winter grounds indicate that these individuals were mostly from northern breeding populations. Furthermore, variances in isotope ratios in samples from local areas in winter tended to be larger than those in summer, suggesting that individuals from different breeding localities may mix in winter habitats. These isotope markers, therefore, have the potential for locating the breeding origins of migratory species on their winter areas, for quantifying the degree of mixing of breeding populations on migratory and wintering sites, and for documenting other aspects of the population structure migratory animals -information needed for studies of year-round ecology of these species as well as for their conservation. Combining information from several stable isotopes will help to increase the resolution for determining the geographic origins of individuals in such highly vagile populations.

Abstract. The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting.

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