[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?,

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.

The forms, rotational patterns and trends of hysteretic loops of dissolved organic carbon (DOC) and nitrate (NO 3 ) were investigated in three headwater Mediterranean streams during one autumn-winter period using two biogeochemical descriptors summarizing the changes in solute concentrations (DC) and the overall dynamics of each hysteretic loop (DR). The study had two aims: (1) to examine whether the variability of solute hysteretic loops monitored in different streams during cold seasons followed a consistent and recurring pattern, (2) to identify hydrological parameters which could potentially influence features of the DOC and NO 3 hysteresis. Relationships between hysteresis features and hydrological parameters in the studied streams were explored using multivariate redundancy analysis (RDA). Both DOC and NO 3 typically increased in concentration during storm events, although hysteretic loops did not obey any consistent pattern across the three streams. The rotational patterns of DOC and NO 3 hysteresis ranged widely, from clockwise to counterclockwise. Storm hydrographs and also the magnitude of antecedent storm events were explanatory of the DOC and NO 3 concentration changes across the study sites. However, the detailed hydrological information did not offer a satisfactory explanation of the entire DOC and NO 3 concentration dynamics during the storm events.

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.

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