Implications of Catchment Structure for Time-Varying Hydrological and Hydrochemical Response in a Forested Headwater Catchment

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Catchment structure, both surface topography and subsurface permeability, is an important determinant of catchment response to precipitation events. Stream water chemistry is in turn influenced by time-varying flow paths that characterize hydrological response. Although much progress has been made in recent years in understanding the flow paths and how chemical composition of waters reflect these paths, the impact of different simplifying assumptions underlying current models have not been explored systematically. In particular, the dynamics of a subsurface stormflow zone above a regional water table have not been addressed fully even though water in the vadose zone plays a major role in determining stream water chemistry. An explicit accounting of soil moisture must be part of an adequate quantification of catchment hydrochemical response. The goal of the study is to develop a quantitative and predictive physically-based model of the hydrological response of a saprolite-granite catchment and to explore the importance of surface topography and subsurface structure in determining catchment hydrochemical response.