GIS and Modeling in Ecological Studies: Analysis of Beaver Pond Impacts on Runoff and its Quality
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GIS and Modeling in Ecological Studies: Analysis of Beaver Pond Impacts on Runoff and its Quality
Published Date
1994-02
Publisher
University of Minnesota Duluth
Type
Technical Report
Abstract
The ARC/INFO GRID module was used to derive watershed variables
for input to AGNPS, a cell-based runoff model that estimates
water volume, peak flow, eroded and delivered sediment, chemical
oxygen demand, and nutrient export from watersheds (Young et al.
1987). The boundary of a 534 ha watershed in Voyageurs National
Park was hand-digitized from 1:24,000 topographic maps, and used
to clip elevation data from a 7~ minute U.S.G.S. Digital
Elevation Model (DEM) with 30 m mesh-point spacing. ARC/INFO
GRID was used to generate slope, slope shape, and field slope
length for each of the 90x90 m cells used to subdivide the
watershed. A surface runoff network was then generated using the
FLOWDIRECTION, FLOWACCUMULATION, and STREAMLINE hydrologic
modeling tools in ARC/INFO GRID. Each of the 90x90 m cells was
uniquely numbered, and receiving cell numbers were derived for
each source cell based on FLOWDIRECTION results. A 1:24,000 land
cover map (Allen et al. 1993) was digitized and gridded to derive
Manning's roughness coefficient, surface condition constant, and
chemical oxygen demand factor for each cell. Detailed soil maps
have never been made for the wilderness study site used, so land
cover classes were coupled with information about soil series
from nearby mapped sites to estimate soil texture, soil
erodibility factor, and hydrologic group (to derive SCS curve
numbers). The drainage area for each beaver impoundment in the
watershed was derived from a digital database of subwatersheds.
All variables were exported from ARC/INFO into the Microsoft
Excel spreadsheet program, which was used to generate a data file
in the appropriate format for AGNPS.
The methodology was applied to the 534 ha, third order stream
watershed to determine the influence of beaver ponds on water
quality and quantity. Beavers influence runoff by: 1)
constructing dams that retard the flow of water, 2) creating
ponds that promote sediment deposition and increase phosphorus
retention, and 3) changing forest land cover to water and wetland
vegetation. We ran the model for a range of storms with average
24-hour rainfalls equivalent to a 1 yr, 2 yr, 5 yr, 10 yr, 25 yr,
50 yr, and 100 yr storm, based on National Weather Service
records for the region. Model runs for the beaver-impounded
landscape ("with ponds") were compared with those for the same
watershed without the influence of beaver ("no ponds"), based on
historical vegetation and adjacent forest types.
The ''with ponds" scenario resulted in slightly increased water
flow at the mouth of the watershed. This is because, assuming
that the pond is full, 100% of the rain that falls onto it will
flow off of it. This caused a 10% increase in runoff at the
lowest rainfall intensity, but only a 1% difference during the
100 yr storm. The runoff contribution of individual cells changed
relatively little between the two scenarios, but there were
easily discernable differences in accumulated runoff per cell
ii:
with distance downstream. Sediment deposition in the beaver
ponds also had an effect that accumulated downstream, so that the
''with ponds" scenario yielded 7 to 12% less sediment than the ''no
ponds" scenario, an effect that increased with storm intensity.
The model predicted a 4% decrease in watershed nitrogen output
with ponds for a 1 yr storm, but there was no effect for storms
with a 10 to 50 yr frequency, and a net increase for a 100 yr
storm. This implies that while beaver ponds may retain N during
low-intensity storms, there may be a flushing of that retained N
during high-intensity storms. This pattern is visible on GIS maps
for "with ponds" scenario with low intensity storm: higher
nitrogen concentrations were observed at the locations, where no
ponds were situated, and nitrogen content in runoff had
remarkable alterations in cells adjacent to ponds. Chemical
oxygen demand (COD) showed the largest effect of any of the
parameters predicted: the presence of beaver ponds was
associated with a 10 to 17% reduction of COD, depending on the
storm intensity. This is because a forest has a lot more primary
productivity than a pond, and therefore contributes more organic
matter (and therefore COD) to the system.
This model-based approach provided insight into the landscape
scale influence of beaver ponds that could not have been derived
using conventional field techniques. The modeling was done at a
spatial level of detail that would have been impractical using
manual data entry to AGNPS. Automating the derivation and
interchange of variables with a GIS made this research possible.
Description
Voyageurs National Park, Minnesota, case study.
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Funding information
University of Minnesota Duluth, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811; Research funded by the National Science Foundation.
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Nawrocki, Tatiana; Johnston, Carol A; Sales, James. (1994). GIS and Modeling in Ecological Studies: Analysis of Beaver Pond Impacts on Runoff and its Quality. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/189322.
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