Garden with Insight v1.0 Help: Soil params watershed group
Watershed area for water flow: The area of the
watershed through which water flows for this soil patch. Each soil patch exists in its own simulated
watershed independent of all other soil patches, with no interaction between soil patches or watersheds.
WSA - The watershed drainage area (ha)
Usually WSA is small because EPIC assumes homogeneous soils and management.
Watershed channel depth for water flow: The depth of the main channel
through which water flows in the soil patch's watershed. Allowable range is from 0.01 to 5.0 meters.
Watershed channel length for water flow: The length of the main channel through which water
flows in the soil patch's watershed. Allowable range is from 0.05 to 5.0 kilometers. For a watershed of less
than 20 hectares this parameter is not critical.
CHL - Distance from outlet to most distant point on watershed (1)
The channel length is the distance along the channel from the outlet to the most distant point on the
watershed. Often in small areas (approx. 1 ha) there is no defined channel. In such cases the length is
measured along a concentrated flow path or it can simply be estimated from the length-width ratio of the
watershed. For areas < or =20 ha, the channel length measurement is not critical.
Watershed channel slope for water flow:
The slope of the main channel through which water flows in the soil patch's watershed, determined by
height/distance. Allowable range is from 0.0001 to 4.0. For small watersheds this slope is not critical.
CHS - Mainstem channel slope (1)
The average channel slope is computed by dividing the difference in elevation between the watershed
outlet and the most distant point by CHL. For small areas this measurement is not critical because CHL
and CHS are only used in estimating the watershed time of concentration. The dominant portion of the
time of concentration is involved with overland rather than channel flow in small watersheds.
Field length for wind erosion: The length of the area (field) to be considered
for wind erosion in this soil patch. Allowable range is from zero to four kilometers. Used in calculation of
wind erosion.
FL - Field length (7) (1) (km)
If wind erosion is to be considered (variable ACW > 0.), field dimensions and orientation must be
specified. If wind erosion is simulated for specific sites, FL, FW, and ANG can be measured easily.
However, hypothetical sites are often used in long-term simulations associated with large-scale decision-
making. In such cases, values of FL, FW, and ANG should be chosen to represent typical field
configurations of the area. Efforts to match field dimensions and drianage area are not necessary. The
field dimensions are used only to estimate wind erosion, with the exception that FL is used to estimate
water erosion from furrow irrigation. Thus, the simulation site may be a small area (1 ha) in a field of 1 x
0.5 km. It should be noted that the change in simulated wind erosion is not large for any FL > 0.3 km.
Therefore, the estimation of inputs for FL and FW is not usually critical for fields with areas greater that
about 10 ha. When fields larger that 10 ha are stripcropped, however, the estimation of FW becomes more
important. To evaluate the effect of stripcropping, FW is estimated as the average width of the strips.
Field orientation (from due North) for wind erosion: The compass orientation
faced by the main slope of the soil patch, in degrees (or radians) from
due North. For example, a soil patch with a South-facing slope would have an orientation of 180 degrees.
Field width for wind erosion: The width of the area (field) to be considered for wind erosion in
this soil patch. Allowable range is from zero to two kilometers. Used in calculation of wind erosion.
Watershed slope length for water flow: The length of the
slope along which water flows in the soil patch's watershed. Allowable range is from 10.0 to 150.0 meters.
SPLG - Slope length (m)
The watershed slope length can be estimated by field measurement as described by Wischmeier and Smith
(1978) or from topographic maps using the Contour-Extreme Point Method (Williams and Berndt 1977).
See MLRA-Soils tables if you have an SCS workshop notebook for English estimates by RCA soil.
Watershed slope steepness for water flow:
The steepness of the slope along which water flows in the soil patch's watershed. Equals the slope height
(delta Y) divided by the slope length (delta X). Ranges from 0.0001 to 0.50.
S - Slope steepness
The average watershed slope can be estimated from field measurment or by using the Grid-Contour
Method (Williams and Berndt 1977). See MLRA-Soils tables if you have an SCS worksop notebook for
English estimates by RCA soil.
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