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.
- 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.
of saturated areas is a key step in the
process at Sleepers River
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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