Page 93 - Soil Degradation, Conservation and Remediation

80 3 Soil Erosion by Water

installations. Off-site effects of water erosion include burrowing of crops and

households, siltation on lands, sedimentation of reservoirs, floods, eutrophication of
water bodies, pollution of land and water, and lowering of water quality. Sediment
which reaches streams or watercourses can clog drainage ditches and stream

channels, cover fish spawning grounds, and reduce downstream water quality.
Pesticides and fertilizers, frequently transported along with the eroding soil, can
contaminate or pollute downstream water sources and recreational areas. Because
of the potential seriousness of some of the off-site impacts, the control of “non-point”
pollution from agricultural land has become of increasing importance.

3.3.9 Soil Loss Equations (USLE and RUSLE)

An equation for the estimation of the amount of soil loss due to water erosion was
proposed by Wischmeier and Smith ( 1965 ). They estimated soil loss due to sheet
and rill erosion from farm-level experimental data on various soils at many different
locations in the USA using the same standard conditions. Erosion plots were 22.6 m
long on 9 % slopes and were subjected to the same soil management practices. This
equation has been applied satisfactorily in many other areas than USA and, through
revisions of the factors included in the equation (Wischmeier and Smith 1978 ), has
become universally accepted. It is known as the Universal Soil Loss Equation
(USLE). The USLE was further revised to give the Revised Universal Soil Loss
Equation (RUSLE). In 1987 and early 1988, Porter wrote a computer program that
would do the computations for RUSLE. It was built using the existing R factor data-
base, supplemented with a climate database needed to estimate residue decomposi-
tion (monthly temperature and precipitation). It used a time varying soil erodibility
value, based upon the standard USLE K value and temperature, the standard LS
value for the USLE, a subfactor approach based upon the work of Laflen et al.

( 1985 , 1990 ), which was built in part from the subfactor approach of Wischmeier. P
factors were based upon the existing approaches used in the USLE (Lafl en and
Moldenhauer 2003 ). The RUSLE computes sheet and rill erosion from rainfall
and the associated runoff on landscape scale. It incorporates data from rangeland
and other research sites in the USA to significantly improve erosion estimates on

untilled lands. It can be used to compute soil loss on areas where signifi cant over-

land flow occurs but is not designed for lands where no overland flow occurs, such
as undisturbed forestlands. The soil loss is an average erosion rate for the landscape
profile. RUSLE uses the same factorial approach employed by the USLE:

A = R × K × LS × C × P
A – the potential long-term average annual soil loss in tons per acre per year
−1
−1
(× 2.24 Mg ha year ). This is the amount that is compared to the “tolerable soil
loss” limits.
R – the rainfall and runoff factor. The greater the intensity and duration of the
rainfall, the higher the erosion potential.