Geoscience Reference
In-Depth Information
and maximum horizontal slope length augmented by values provided by the USDA
Natural Resources Conservation Service.
7.4.4 Cropping and Management (C) Factor
The cropping and management factor values express the ratio of soil loss under a
specific cropping and management system to the expected soil loss under a clean-till,
continuous fallow system. The
C
factor represents the effect of land use on erosion.
It is the single factor most easily changed and is the factor most often considered in
developing a conservation plan. The land cover values used in this study were based
on LULC data derived from Landsat TM imagery (see above) using coefficients
provided by the USDA Natural Resources Conservation Service.
7.4.5 Conservation Practice (P) Factor
The conservation practice factor represents the reduction in soil erosion due to
conservation measures such as contour farming, strip cropping and terracing. In
the RWB, no information on the extent of conservation practices was available.
Consequently, a
P
factor value equal to one was used. However, it was demon-
strated that for future studies, using the DOQQs as a background image, terraces
and contour strips could be on-screen digitized and used to modify
P
factor
values.
7.4.6 RUSLE Implementation
Estimated soil loss in tons/acre/year was calculated in raster mode using ArcGIS
software. The RUSLE estimates were mapped using a threshold value of soil loss
greater than 5 tons/acre/year, as this value is considered indicative of locales that
require additional conservation practices (Fig.
7.2
). Output for selected basins was
also depicted on DOQQs (Fig.
7.3
). In ArcGIS, the wetland basin maps were over-
laid with land ownership information to aid RWBJV staff in identifying fields and
landowners who may require assistance to address conservation needs.
The RUSLE modeling exercise proved useful for demonstrating how GIS and
remotely-sensed imagery can be used in concert to quickly produce informa-
tion important for wetlands management. This effort had the additional benefit of
increasing partners' awareness of possible shortcomings in both geospatial data and
methods. For example, the estimates of soil loss are clearly related to the LULC
existing in a specific year and to the annual average precipitation used in calculat-
ing the estimates. The LULC data in the RWB project did not reflect the impacts of
conservation practices, since those data were not available. On the other hand, the
utility of remote sensing for updating LULC data, and for future development of
data on conservation practices was evident.
