Collaborative Research: Landform controls on hydrologic flowpaths and pedogenesis explain solute retention and export from pedon to catchment scales

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Headwater catchments are inherently complex. The soils, subsoils, and geomorphic properties exhibit heterogeneity at different scales and stream chemistry draining these areas typically varies from one catchment to another in space and time. Yet these headwater catchments comprise the majority of the landscape and are responsible for setting the quality of water at a regional scale. The project is aimed at explaining the spatial and temporal variation in stream water chemistry at the headwater catchment scale using a hydropedological framework, i.e. the combined study of hydrology and soil development. This framework provides a functional basis for discretizing the catchment into similar regions that can be integrated to explain catchment runoff and water quality. Chemical reactions related to pedogenesis (soil development) that operate at scales from the pedon to hillslope record the geochemical signature reflective of the dominant hydrologic flowpaths and regulate the chemical quality of water draining hillslope soil sequences, ultimately setting stream chemistry. The way water chemically evolves along flowpaths in the landscape as it travels to the stream is strongly influenced by the soils through which it passes. In a small headwater catchment at the Hubbard Brook Experimental Forest, four subcatchments that have contrasting stream chemistry representative of forested streams throughout northern New England will be studied to examine how distinct patterns of soil development can be used to interpret sources of solutes in stream water. Flow pathways are predicted from landform shape, subsoil type (hydrologic restriction zones) and soil development sequences determined by soil extraction chemistry. Along these pathways, artificial tracer experiments, geochemical patterns, and isotopic geochemical tracers will be used to predict the patterns and processes of solute transport that generates streamflow in each subcatchment and forms the integrated response of the entire catchment. The overall goal of the project is to develop a predictive model of landform control on hydrologic flowpaths and pedogensis that explains solute retention and export from pedon to hillslope to catchment scales. The project will demonstrate how hydrology strongly influences soil development and soil chemistry, and in turn, controls stream water quality in headwater catchments. Understanding the linkages between hydrology and soil development can provide valuable information for managing forests and stream water quality. Feedbacks between soils and hydrology that lead to predictable landscape patterns of soil chemistry have implications for understanding spatial gradients in site productivity and suitability for species with differing habitat requirements or chemical sensitivity. Tools are needed that identify and predict these gradients that can ultimately provide guidance for land management and silvicultural decision making. Better integration among soil science, hydrology, and biogeochemistry will provide the conceptual leap needed by the hydrologic community to be able to better predict and explain temporal and spatial variability of stream water quality and understand water sources contributing to streamflow.