The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in a small forested watershed

The relationship between stream discharge and the concentration of differen t solutes reflects the hydrological processes occurring in a catchment, but is also strongly affected by antecedent moisture conditions. In this study , the effects of antecedent conditions were examined for a small forested w atershed near Montreal (Quebec) by comparing data from sequences of fall ev ents for two years of contrasting antecedent moisture conditions. The relat ionship between discharge and stream water chemistry was also quantified us ing crosscorrelation analysis. Results show that concentrations of dissolve d organic carbon, nitrate and calcium increased during storm events with dr y antecedent conditions but that the relationships were weaker under wet co nditions. For both years, concentrations of sulfate, silicon and sodium and the pH were negatively correlated with discharge (cross-correlations from -0.20 to -0.53). With dry antecedent conditions, there was a general decrea se in the concentration of all solutes with time, whereas concentrations re mained at about the same level under wet conditions. The concentration-disc harge relationships for sulfate shifted frequently from higher concentratio ns during the rising limb to higher concentrations during the falling limb from one event to another as a result of changing antecedent conditions. Al though the contrast in antecedent moisture conditions between the two years was moderate in comparison with that reported in other studies (25% dryer than average in 1995 and 12% wetter than average in 1996), the hydrochemist ry of the stream was markedly different. Copyright (C) 1999 John Wiley & on s, Ltd.