Calibration of the INCA model in a Mediterranean forested catchment: the effect of hydrological inter-annual variability in an intermittent stream

Abstract. 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 determinion (r²) 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 (r² = 0.13 p<0.0001) as in the wet year (r² = 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.

Keywords: environmental modelling, intermittent stream, Mediterranean climate, Fuirosos, hydrology, nitrate