5.3.6 Converting USLE to Water Erosion Rate


     The average annual soil loss, Aloss, calculated by the USLE is in units related to a soil mass basis (i.e., t ha-1 yr-1). The source-term release module requires the volumetric water erosion rate, E, which has units related to a soil volume basis (i.e., cm3 cm-2 yr-1). Therefore, E, is calculated by the conversion expression



where E is the volumetric water erosion rate (cm yr-1).

(In Equation 5.55, the factor 1x10-2 converts t ha-1 yr-1 to g cm-2 yr-1.)

Table 5.12. P-Factor Values for Various Practice Conditions (after Ports 1973,
as reported by Novotny and Chester 1981)

Erosion Control Practice
Pfact
Surface Condition with No Cover
Compact, smooth, scraped with bulldozer or scraped up and down hill
1.30
Same as above, except raked with bulldozer and root-raked up and down hill
1.20
Compact, smooth scraped with bulldozer or scraped across the slope
1.20
Same as above, raked with bulldozer and root-raked across slope
0.90
Loose, asa disked plow layer
1.00
Rough irregular surface, equipment tracks in all directions
0.90
Loose with rough surface 0.3 m depth
0.80
Loose with smooth surface 0.3 m depth
0.90
Structures
Small sediment basins:
0.09 basins/ha
0.50
0.13 basins/ha
0.30
Downstream sediment basins:
With chemical flocculants
0.10
Without chemical flocculants
0.20
Erosion control structures: 
Normal rate usage 
0.50
High rate usage
0.40
Strip building
0.75