Fundamentals of Engineering Design
Sources of aerial photography include the geographic information clearinghouses described above
as well as local or state government planning agencies, forest products companies, and federal agencies
such as the U.S. Army Corps of Engineers, Natural Resources Conservation Service (formerly SCS),
Farm Services Agency, U.S. Forest Service, Bureau of Land Management, etc. If the desired land use
/ land cover data are not available in an appropriate digital form, the techniques described for determining
watershed area may be used in conjunction with maps or photos to determine the relative proportions of
various land uses within the watershed boundary.
From a practical engineering hydrology standpoint, representative land use information is essential
to determining appropriate parameters to describe runoff-infiltration processes. The most common forms
of such parameters are the SCS Curve Number (CN) and the Rational "C" runoff coefficient. If a lumped
parameter model such as HEC-1 is used, an area weighted curve number or runoff coefficient is usually
computed for the entire watershed or, if the watershed has been discretized into sub-watersheds, each sub-
watershed. If a distributed-parameter hydrologic model is used to simulate watershed hydrology, a
parameter representing runoff-infiltration processes associated with land use and soil type will be required
for each analysis cell. Export coefficients or Universal Soil Loss Equation parameters associated with
sediment delivery from different land covers / uses and conservation practices also provide critical
information in understanding and predicting watershed response.
Soil physical properties are also important determinants of runoff-infiltration processes. Soil pore
space is a giant reservoir that provides the primary buffering of precipitation delivered irregularly to the
surface of the Earth. As with land use, some knowledge of soil properties in the watershed of interest is
essential to the selection of appropriate model parameters. Even in the simplest hydrologic models such
as the SCS CN approach, soil physical properties are commonly used in conjunction with land use / land
cover information to select parameters describing runoff-infiltration processes.
Physical, chemical, and biological properties of various soil mapping units are readily available in
the form of Natural Resources Conservation Service (NRCS) County Soil Surveys. Depending on the
level of detail required in a particular hydrologic analysis, a few or all of the following data provided by
NRCS may prove useful:
Soil maps (usually 1:24,000 scale)
Soil texture of each mapping unit
Soil permeability (usually overestimated in county soil surveys, particularly for non-agricultural soils)
Soil Hydrologic Groups (A, B, C, D)
As described above, digital soil layers (DSLs) are available for many parts of the country.
Although DSLs may only contain the soil mapping unit name in addition to the basic GIS descriptors, each
polygon or cell in a DSL may be linked to additional "lookup tables" so that the full range of soil physical
properties described in a county soil survey may be utilized in hydrologic analyses.