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[1] Storm events have major implications for biogeochemical cycles at local and regional scales and they provide an excellent opportunity to study the hydro-biogeochemical functioning of catchments. However, concentration-discharge (C-Q) responses have only been studied in detail for short periods or a few selected events. In consequence, it is difficult to quantify the diversity of C-Q responses in a hydrological system and impossible to assess whether the succession of forms of C-Q responses follows a predictable sequence or not. Bearing in mind these shortfalls, the variability of dissolved organic carbon (DOC) and nitrate (NO 3 ) pulses during storms is analyzed in a detailed 4-year series from an intermittent Mediterranean stream. In this study, each DOC and NO 3 -Q response is synthesized by two descriptors that summarize its trend (DC; dilution/ flushing/no change) and shape (DR; linear/nonlinear response). We observe that C-Q responses are widely distributed along the two-dimensional DR versus DC continuum. Furthermore, the temporal succession of forms of DOC and NO 3 -Q responses follow a random pattern, and only the dynamics of the DR (NO3) descriptor show periodicity. The long-term data set reveals that it is impossible to predict with reasonable precision the full properties of DOC and NO 3 -Q responses. Thus, a ''typical'' C-Q response does not really exist at our study site, and this apparent diversity of responses has to be handled with a probabilistic approach that allows synthesis of the complexity of the hydrobiogeochemical functioning of a specific catchment.Citation: Butturini, A., M. Alvarez, S. Bernal, E. Vazquez, and F. Sabater (2008), Diversity and temporal sequences of forms of DOC and NO 3 -discharge responses in an intermittent stream: Predictable or random succession?,

River networks modify material transfer from land to ocean. Understanding the factors regulating this function for different gaseous, dissolved, and particulate constituents is critical to quantify the local and global effects of climate and land use change. We propose the River Network Saturation (RNS) concept as a generalization of how river network regulation of material fluxes declines with increasing flows due to imbalances between supply and demand at network scales. River networks have a tendency to become saturated (supply) demand) under higher flow conditions because supplies increase faster than sink processes. However, the flow thresholds under which saturation occurs depends on a variety of factors, including the inherent process rate for a given constituent and the abundance of lentic waters such as lakes, ponds, reservoirs, and fluvial wetlands within the river network. As supply increases, saturation at network scales is initially limited by previously unmet demand in downstream aquatic ecosystems. The RNS concept describes a general tendency of river network function that can be

Climate exerts a powerful influence on biological processes, but the effects of climate change on ecosystem nutrient flux and cycling are poorly resolved. Although rare, long-term records offer a unique opportunity to disentangle effects of climate from other anthropogenic influences. Here, we examine the longest and most complete record of watershed nutrient and climate dynamics available worldwide, which was collected at the Hubbard Brook Experimental Forest in the northeastern United States. We used empirical analyses and model calculations to distinguish between effects of climate change and past perturbations on the forest nitrogen (N) cycle. We find that climate alone cannot explain the occurrence of a dramatic >90% drop in watershed nitrate export over the past 46 y, despite longer growing seasons and higher soil temperatures. The strongest climate influence was an increase in soil temperature accompanied by a shift in paths of soil water flow within the watershed, but this effect explained, at best, only ∼40% of the nitrate decline. In contrast, at least 50-60% of the observed change in the N export could be explained by the long-lasting effect of forest cutting in the early 1900s on the N cycle of the soil and vegetation pools. Our analysis shows that historic events can obscure the influence of modern day stresses on the N cycle, even when analyses have the advantage of being informed by 0.5-century-long datasets. These findings raise fundamental questions about interpretations of long-term trends as a baseline for understanding how climate change influences complex ecosystems.forest ecosystems | long-term monitoring | streamwater chemistry | precipitation chemistry | nutrient cycles O ur understanding of how climate change impacts complex ecological systems depends on our conception of a baseline against which change can be judged and knowledge of how this baseline has been shaped by historical conditions. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, for example, we know that current concentrations of nitrate in watershed streams are the lowest in 46 y of measurement and that ecosystem nitrate losses have decreased by >90% over this time (Fig. 1A). If we were to take the early high nitrate period (1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976) as the historical reference, we would estimate that nitrate export has dropped by a total of ∼125 kg nitrogen (N) ha −1 during the 30 y of the decline (Fig. 1A). Such a large drop in N export is ecologically relevant and constitutes a dramatic shift in the ecosystem N cycle: from a leaky cycle that retained only ∼30% of external inputs in the high stream water nitrate period to a highly retentive cycle that currently captures ∼90% of atmospheric inputs (Methods).We adopt a watershed mass balance approach (1, 2) to examine the factor(s) responsible for this dramatic change in the forest N cycle (Fig. 2). Because climate is an overriding and powerful driver of biological process, we pay particular attention to whether the observed changes i...

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO 3 -N and NH 4 -N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km 2 ) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000-2001 was 70 kg km À1 year À1 , of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO 3 -N, DON and NH 4 -N, respectively.

Abstract. Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport–Flux–Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash–Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (Rp; the sum of the local DOC production rates in individual grid cells) and export rate (Re; the product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving Rp to its maximum but Re to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when Re peaked as the stream reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration–discharge (C–Q) relationship characterized by an unusual flushing–dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (<18 % of the streamflow). When groundwater flow increased above 18 %, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C–Q patterns switched to a flushing-only pattern with increasing DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export – temperature and hydrological flow paths, respectively – can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm events.

Abstract:A progressive perceptual understanding approach was used to identify a model structure able to represent the non-linear behaviour of the hydrological cycle in a small intermittent Mediterranean stream. The initial lumped model structure consisting of a series of four connected water tanks (LU3) progressed to a model with five tanks (LU4), and finally to a semidistributed model structure (SD4) in which spatial variability of the evapotranspiration according to the vegetation cover and to the local aspect was considered. In the final model structure, which gave the best fit (Nash-Sutcliffe efficiency index D 0Ð78), an additional tank representing the riparian zone was included (SD4-R). Results showed that the abrupt changes of the riparian water table during summer and the formation of a perched water table during the transition from dry to wet conditions were the main mechanisms leading to the non-linear hydrological behaviour. The transpiration process from the saturated zone and the spatial variability of evapotranspiration resulted in key factors successfully representing the annual water balance. The spatial and temporal validations carried out for each of the four model structures considered in this study supported the hypothesis adopted during the calibration process.

Mediterranean regions are characterised by a stream hydrology with a marked seasonal pattern and high inter-annual variability. Accordingly, soil N processes and leaching of solutes in Mediterranean regions also show a marked seasonality, occurring in pulses as soils re-wet following rain. The Integrated Nitrogen Catchment model (INCA) was applied to Fuirosos, a Mediterranean catchment located in NE Spain, using hydrological data and streamwater nitrate and ammonium concentrations collected from 1999 to 2002. This study tested the model under Mediterranean climate conditions and assessed the effect of the high inter-annual variability on the ability of INCA to simulate discharge and N fluxes. The model was calibrated for the whole three-year period and the n coefficients of determination (r 2 ) between simulated and observed data were 0.54 and 0.1 for discharge and nitrate temporal dynamics, respectively. Ammonium dynamics were simulated poorly and the linear regression between observed and simulated data was not significant statistically. To assess the effect of inter-annual variability on INCA simulations, the calibration process was run separately for two contrasting hydrological years: a dry year with a total rainfall of 525 mm and a wet year with a total of 871 mm. The coefficients of determination for the correlation between observed and simulated discharge for these two periods were 0.67 (p<0.0001) and 0.62 (p<0.0001), respectively. Nitrate temporal dynamics were not simulated as well in the dry year (r 2 = 0.13 p<0.0001) as in the wet year (r 2 = 0.56 p<0.0001). Annual nitrate balances were similar to those estimated from observations. Results suggest that, in Mediterranean catchments, both hydrology and nitrate mobilisation are influenced strongly by soil moisture, which is highly variable within and between years; also, a single parameter set is insufficient to capture the inter-annual variability in Fuirosos. It is suggested that, when using INCA in semiarid systems such as those in Mediterranean regions, certain of the parameters currently fixed in INCA (e.g. base flow index or drainage area) be treated as variables dependent on soil moisture deficit.