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 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.  

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. 

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 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. 

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.