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Abstract:Catchment travel time distributions reflect how precipitation from different storms is stored and mixed as it is transported to the stream. Catchment travel time distributions can be described by the mean travel time and the shape of the distribution around the mean. Whereas mean travel times have been quantified in a range of catchment studies, only rarely has the shape of the distribution been estimated. The shape of the distribution affects both the short-term and long-term catchment response to a pulse input of a soluble contaminant. Travel time distributions are usually estimated from conservative tracer concentrations in precipitation and streamflow, which are analyzed using time-domain convolution or spectral methods. Of these two approaches, spectral methods are better suited to determining the shape of the distribution. Previous spectral analyses of both rainfall and streamflow tracer time series from several catchments in Wales showed that rainfall chemistry spectra resemble white noise, whereas the stream tracer spectra in these same catchments exhibit fractal 1/f scaling over three orders of magnitude. Here we test the generality of the observed fractal scaling of streamflow chemistry, using spectral analysis of long-term tracer time series from 22 catchments in North America and Europe. We demonstrate that 1/f fractal scaling of stream chemistry is a common feature of these catchments. These observations imply that catchments typically exhibit an approximate power-law distribution of travel times, and thus retain a long memory of past inputs. The observed fractal scaling places strong constraints on possible models of catchment behavior, because it is inconsistent with the exponential travel time distributions that are predicted by simple mixing models.

There h concern that changes i n atmospheric deposition, climate, or land use have altered the biogeochemistry o f forests causing soil base-cation depletion, particularly C e The Bear Brook Watershed i n Maine (BBWM) is a paired watershed experiment with one watershed subjected to elevated N and S deposition through bimonthly additions of (NH,)aO,.Quantitative soil excavations i n 1998 measured soil pools of exchangeable base cations9 yr after treatments began. Stream sampling at the weirs on a weekly and event basin, and weekly precipitation sampling, were used for input-output estimates. The treated watershed had lower concentrations of exchangeable Ca and M g i n ail horizons, with evidence for the greater depletion i n the 0 horizon compared to underlying m i n e d soh, and i n softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment-induced baseation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha-' of exchangeable Ca and Mg, respectively, after accounting for soil m w differences between watersheds. ?his was comparable with tbe total armulative excess stream Ca and Mg export i n West Bear after 9 yr of treatment o f 55 and 11 kg ha-', respectively. Model simulations o f watershed response to treatments predicted excess soil exchangeable Ca and M g losses in the treated watershed o f 47 and 9 kg ha-', respectively. These results indicate that the response to a step-incmase i n N and S deposition during Ule first d d e of treatments in t h i s experimental forested watershed was to invoke cationexchange buffering, resulting in a net decline in soil exdungeable base cations. CCELERATED LEACHING of base cations from forest

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Existing models of the acidification of freshwaters by acid deposition span a wide spectrum of approaches. At one extreme are empirical models that relate changes in one variable, such as the acidity of precipitation or the deposition of SOT,t0 changes in another variable (the acidity or the SOii~c oncentration of freshwaters) {1-3).These empirical models, especially those of Hcnriksen, have been very useful in providing an initial understanding of the relationship between acid deposition and freshwater composition as a function of sulfur and H + deposition (2). Such models are based on the concepts of electrical neutrality and alkalinity but do not reflect biogeochemical processes or the temporal variations in the response of freshwaters to acid deposition.At the other extreme are dynamic mechanistic models. These are much more data intensive and site specific than the empirical models and may require an extraordinary amount of effort to verify and use {4).We present here a simple conceptual model that incorporates only the few key processes that we believe determine how aquatic ecosystems respond