HELENA MITASOVA, LUBOS MITAS
University of Illinois at Urbana-Champaign
Supported by the SERDP project: "Terrain Modeling and Soil Erosion Simulation".
We illustrate the estimation of topographic potential for erosion and deposition using the elevation data provided at 30m horizontal resolution and 1m vertical resolution , for the following subarea of a selected installation
Soil detachment by water flow in complex terrain can be estimated as a function of erodibility, water flow rate and slope. This index is also sometimes used as a modified LS factor for computation of erosion risk based on USLE/RUSLE (Mitasova et al. 1997). Topographic potential for net erosion and deposition is estimated by the USPED model (Mitasova et al. 1996). Even with rather crude elevation data, it is possible to identify the areas with high topographic risk for soil detachment and erosion/deposition, as illustrated by the following sequence of maps showing the intermediate steps and results of computations (click on images to see full size picture):
original 30m DEM
slope from the original DEM.
Note the impact of plateaus in low elevation areas on the slope pattern
(artificial steeper slopes along 1m contours), highlighted by red
arrows. These plateaus are due to the insufficient vertical resolution
of 1m.
upslope contributing area as
a measure of steady state water flow computed from the original 30m DEM
by r.flow. Pits and pleateaus in the DEM cause problems in flowtracing
with almost no flow generated in lower elevation areas and disrupted
flow in valleys.
modified LS factor predicts
well some areas of high potential for soil detachment, especially in
hilly parts of the region. However, it does not predict the location of
areas with concentrated flow with high potential for rilling and gully
formation . Also, in lower elevation areas the artificial pattern of
increased erosion is preedicted along 1m contours.
net erosion/deposition map
computed by the USPED model predicts high net erosion in hilly regions
and on slopes along the streams. It also shows that a significant
portion of the material eroded from hillslopes is deposited before it
could reach the main streams. However, this map lacks the prediction of
high erosion due to concentrated flow in valleys and shows artificial
waves of erosion and deposition along the 1m contours in flatter areas.
The given DEM was reinterpolated to 10m horizontal and 0.01m vertical resolution using the RST method (Mitasova et al. 1995, Mitas and Mitasova, in press). Simultaneously with interpolation, maps of slope, aspect, profile and tangential curvatures were generated (click on images to get a full size picture):
Note, that in the slope map the artificial pattern of steeper slopes along 1m contours disappeared.
steady state water flow
computed by r.flow from the smoothed 10m DEM shows potential for
channel formation in valleys and predicts water flow also in low
elevation region. Flow in the two main streams is not adequatelly
described because of lack of data, but it can be incorporated if stream
data are available.
modified LS factors computed
from the 10m DEM using different exponents for the water flow term
(p=0.6 and p=1.5). Value of this exponent is still a subject of
research and discussion in erosion research community. The first result
puts more weight on the influence of slope while the second result puts
more weight on the influence of water flow. Because the exponent
depends on the conditions of water flow in a particular area it should
be calibrated. For this area, if you know the situation there, the map
which is closer to the observed pattern should be used.
topographic potential for net
erosion/deposition computed from the 10m DEM using the USPED model
(Mitasova et al. 1996). Similarly as for the result of 30m DEM, the
model shows high erosion in hilly area and along main streams and
deposition in concave areas. It also indicates high erosion in areas
with concentrated flow which could reach the main streams. The
artificial pattern of erosion/deposition along contours is not present.
Impact of soil and cover will be incorporated when the land use/cover/soil categories or at least a rough estimate of C-factor are available.
Estimation of net erosion deposition by process based distributed model SIMWE can be performed after we get data on land cover.
Mitasova, H., J. Hofierka, M. Zlocha, and R. L. Iverson, 1996, Modeling topographic potential for erosion and deposition using GIS, Int. Journal of Geographical Information Science, 10(5), 629-641.
Mitasova, H., J. Hofierka, M. Zlocha, and R. L. Iverson, 1997, Reply to a comment. Int. Journal of Geographical Information Science, Vol. 11, No. 6
Mitas, L., Mitasova, H., 1997, Spatial Interpolation. In: P.Longley, M.F. Goodchild, D.J. Maguire, D.W.Rhind (Eds.), Geographical Information Systems: Principles, Techniques, Management and Applications (in press).
Mitasova, H., Mitas, L., Brown, W.M., Gerdes, D.P., Kosinovsky, I., 1995, Modeling spatially and temporally distributed phenomena: New methods and tools for GRASS GIS. International Journal of GIS, 9 (4), special issue on integration of Environmental modeling and GIS, p. 443-446.
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