THREE-COMPONENT MODEL OF RUNOFF GENERATION, LYSINA CATCHMENT, CZECH REPUBLIC E BUZEK, J. HRUSKA and P. KRAM* Czech Geological Survey, Kldrov 3, 118 O0 Prague 1, Czech Republic (Received August 5, 1994; accepted August 18, 1994) Abstract. The forested Lysina catchment is situated in an area very susceptible to acid deposition. The streamwater is characterized by extremely high concentrations of total dissolved A1 (volume weighted mean 66/zmol L -1) and H + (average pH = 3.87). In a simple two-component model, the surface runoff component contributes only 6% of runoff in winter and 4% of runoff in summer. During flood episodes, the direct runoff contributes up to 20% of streamflow. There is a strong positive correlation between stream acidity and stream discharge. The observed exponential increase in streamwater acidity with discharge during high flow periods cannot be explained by the simple two-component model. A three-component model used for hydrograph separation is based on chemical and 180 analysis of precipitation, soil water and runoff. It incorporates a soil water component along with groundwater and rainfall components in streamwater generation. Dissociated organic acids leached during the flow of water through the uppermost soil horizon help to balance an apparent anion deficit. The apparent anion deficit was found to increase exponentially with flow rate. Low variability in streamwater 6180 corresponds to a high contribution of indirect components (i.e., soil and ground water) in the runoff. The soil water contribution to indirect runoff calculated from the apparent anion deficit of streamwater, varied from 0 at base flow up to 80% during floods. On average, 40% of the streamwater is derived from soil water (from 3 ! to 39% in winter and from 47 to 54% in summer). 1. Introduction Highly acidified streamwaters are typical of forested ecosystems impacted by acidic atmospheric deposition in Central and Northern Europe. The concentration of H + often increases with streamflow to dramatically change streamwater chemistry. High concentrations of inorganic monomeric A1 are a common result of the sudden drop in pH during storm events (Gunn and Keller, 1984; Henriksen et al., 1984; Driscoll, 1984; Driscoll etal., 1984). Acidic episodes may be caused by an increase in organic acidity rather than by an increase in mineral acids or a decrease in base cations (Bishop et al., 1990). In such a case, the major portion of runoff water must have moved laterally through the uppermost, organic-rich soil horizon by translatory flow. This concept contradicts to the common conceptual models in which surface runoff, or even Hortonian overland flow, play a dominant role in flood generation (Horton, 1933; Erichsen and Nordseth, 1984), and it cannot be easily reconciled with models derived from isotopic tracer studies which have demonstrated a significant role of groundwater in flood generation (Sklash and Farvolden, 1979; Stichler and Herrmann, 1982; Rodhe, 1987). A simple two- component model of direct (surface) and indirect (subsurface) flow does not account * Present address: Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13244, U.S.A. Water, Air and Soil Pollution 79: 391-408, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.