Problem

One of the concerns associated with the prospect of global warming is its potential consequences in the hydrologic regime.  Changes in the amount and seasonality of precipitation may have far-reaching implications to the patterns of water and nutrient movement in terrestrial ecosystems.  The current state of knowledge on the interrelations and transfers of water, energy, and chemical compounds is insufficient to adequately model these processes, thus hampering predictive efforts of the true environmental impacts of global warming.  A better understanding of Water, Energy, and Biogeochemical Budgets (WEBB) is needed over a range of scales, in a representative cross section of global ecosystems.

Objectives

• Investigate and describe the linkages between soil hydrology and solute transport in the soil zone through detailed accounting of water and chemical flux at points on a hillside.
• Apply carbon, oxygen, and strontium isotopes to trace the movement of water and solutes.
• Assess the sensitivity of trace gas budgets to changing land use and climate.
• Determine the partitioning of hydrologic pathways as basin size increases.
• Investigate the relation of radiation balance, aspect, and topography on snowmelt rates in a forest environment, and the streamflow response.
• Investigate hydrologic and geochemical interactions between hillslope and the riparian zone.

 

At the Sleepers River Research Watershed in Danville, Vermont, we have implemented a nested and paired watershed design.  We are conducting research at sites with a long historical record (more than 40 years) so that contemporary data can be evaluated in a historical context.  Most importantly, several high quality long-term streamflow and meteorological data sets are continuing, which may be important in assessing global change.  The latter include complete records of snow depth and water equivalent since 1959 and ground frost depth since 1983.

Solute budgets are calculated using the watershed mass-balance approach.  For water and chemicals this requires measuring inputs of water and solutes in precipitation, and outputs of water and solutes in streamflow.  Differences between inputs and outputs indicate whether the watershed is net producer or consumer of a solute, and thus forms a boundary condition from which ecosystem hypotheses are generated.  For example, if more nitrogen enters the watershed in precipitation than leaves in steamflow, one might hypothesize that nitrogen is a limiting nutrient being sequestered by the trees.  More detailed research is ongoing in the watershed to directly assess biochemical processes.  To continue the nitrogen example, tracking of nitrate and ammonium in the precipitation, soil water, and shallow and deep groundwater are tracked in detail and results show  where in the system nitrate is being taken up. This in turn indicates what part of the biosphere is responsible (trees, soil bacteria), or if N is being lost through abiotic processes.

The study was designed with watersheds that are nested (smaller watersheds in larger watersheds) and paired (otherwise similar watersheds with a single important difference, such as land use) to allow USGS to investigate how watershed processes change as scale and land use change.

 A central approach in our biogeochemical research is the use of isotopes to trace  the source of water and solutes.  Oxygen isotope ratios in water indicate whether streamflow is generated from new water (rain or snowmelt from the current event) or from displaced groundwater.  This information sets constraints on the flow paths that water takes to the stream, and helps our understanding of solute sources and movement.  Other isotopes, such as strontium, lead, and carbon, provide additional  information about sources of solutes within the watershed.
 

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