Environmental Engineering FOURTH EDITION

Nonpoint Some Water Pollution 237

Figure 11-1. Average annual rainfall erosivity indices (R) for the USA.

LS = topographic factor,

C = vegetation cover factor, and

P = erosion control practice factor.

The rainfall erosivity index (R) is calculated by summing the product of rainfall energy
during an individual storm event (E), measured in foot-tons per acre per inch of
rainfall, and the maximum 30-min rainfall intensity (Z), measured in inches per hour,
for a specific period of time such as one year or a season:

(11.2)

Figure 11-1 shows average rainfall erosivity indices for the USA.
The soil erodibility factor, K, is the average soil loss, in tons per acre, per
100 ft-tons per acre of rainfall erosivity standardized for a plot of land with 9% slope,
72.6 ft long, under continuous cultivation (Fig. 11-2). The topographic factor, LS,
combines both the slopelength and slope-steepness factors and reflects the effect of
both length of flow and steepness of slope on soil loss (Fig. 11-3). These two factors
together account for the capability of runoff to detach and transport soil material. The
vegetation cover factor (C) and the erosion-control practice factor (P) adjust for the
influences of different vegetation types and erosion-control techniques (Tables 11-3
and 11-4). Both are calculated as ratios compared to the soil quantity eroded from
clean-tiled soil under identical slope and rainfall conditions. Local Soil Conservation
Service offices keep records of regional values for C and P.