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.
Understanding the linkages between hydrology and how soil develops can provide valuable information for managing forests and stream water quality, particularly in headwater systems. Soils are essential in determining the chemistry of headwaters. All precipitation eventually filters through the soil and dissolves minerals and organic materials, which end up in streams and rivers, where humans depend on freshwater resources. However, there are fundamental knowledge gaps in hydrology and soil science about how water flows through a watershed and how elements are retained or released from a watershed. The structure and organization of soils in a watershed is partly controlled by the water flow pathways and wetting/drying regimes in soils. Feedbacks between soils and hydrological processes lead to predictable landscape patterns of soil chemistry, which have implications for understanding spatial variation in site productivity and suitability for species with differing habitat requirements or chemical sensitivity. This interaction between soil development processes and hydrological processes is the focus of an emerging new study area called hydropedology. Our hydropedologic study of Watershed 3 at Hubbard Brook Experimental Forest was designed to enhance our understanding of the interactions between topography, groundwater and soil formation at that site, and how those variables determined soil composition across the watershed. We showed that there is much spatial variability in soil composition, but that variability can be quantified and predicted based on topographic characteristics, such as local slope, landform curvature, and distance to stream or bedrock outcrops. These soil spatial patterns were also related to shallow groundwater occurrence and frequency and whether water in the soil was primarily flowing downslope or vertically into the ground. Taken together, this hydropedological approach provided insight into streamflow generation and chemical patterns at this site. For example, the structure and organization of soils in Watershed 3 at Hubbard Brook was critical in understanding the sources of dissolved organic carbon that contributed to streamflow or in identifying hotspot locations of denitrification (i.e., a gaseous loss of nitrogen to the atmosphere). Therefore, under changing climate or disturbance, we now have better insight into areas of a watershed that may be sensitive to change and critical in controlling element cycling. These examples illustrate the importance of hydropedological variation in regulating key ecological processes in watersheds and forming a basis for predicting stream water chemical variation more broadly.
