Empirical methods for soil loss estimation

• The estimation of soil loss that occurs in some areas, under specific conditions of action of erosion factors and agricultural land use, can be done using empirical models that are described in a summarized way in this text.
• Among the various existing methodologies for quantitatively assessing soil erosion, those that best fit the availability of data in the study areas can be chosen given their adaptability to regions with conditions close to those in these study areas.
• Despite this, the available data may be insufficient to apply these models according to standardized methods, which can motivate the need to make some adjustments and adaptations in the process of determining erosion factors.

a) Universal Soil Loss Equation (USLE):

• SLE is an empirical erosion model designed to estimate long-term mean soil losses from runoff in field areas, crops and specific cultivation methods, and which can still be useful for construction areas and other non-agricultural conditions, but which does not estimate deposition nor calculate sediment production by erosion in ravines, banks and riverbeds.

Mathematically,

A=RKLSCP

where, when in metric units:

A = loss of soil (t/ha∙year),

R = rain erosivity factor (MJ.mm/ha∙h),

K = factor of soil erodibility (t∙h/MJ∙mm),

L = slope length factor (dimensionless),

S = slope factor (dimensionless),

C = land cover factor and cultural operations (dimensionless),

P = factor of conservative practices (dimensionless).

b) Revised Universal Soil Loss Equation (RUSLE)

• The RUSLE is an empirical model that conserves the basic structure of the USLE, with regard to the main equation, the same being the factors that determine soil losses through erosion, with the difference that these are calculated according to different procedures, involving treatment per computer.
• RUSLE is a powerful tool for planning and conservation, for inventorying erosion rates in vast areas, and for estimating sediment production, which can become a sediment harvest in watersheds, and can be used on farmland, pastures, disturbed forests, construction sites, and other areas where runoff occurs as a result of greater rainfall than infiltration.

Mathematically,

em=0.29[1−0.72exp(−0.05Im)]

in which:

em is the maximum unit energy when the intensity tends to infinity, in MJ∙ha−1,

Im the maximum rainfall intensity in mm∙h−1.

c) Soil Loss Estimation Model for Southern Africa (SLEMSA)

• his methodology is considered to be particularly appropriate for countries that, although urgently in need of measures to combat erosion, have resource limitations to support the expenditure of research programs, and was designed to estimate the average annual soil loss due to sheet erosion that occurs on arable land between two adjacent ridge.

Mathematically,

K=exp((0.4681+0.7663F)lnEp+2.884−8.1209F) .

Where, E_p= Precipitation energy

F= Soil erodibility

d) Rainfall factor(R)

• Potential ability of a rain to cause erosion.

Mathematically,

R= EI30

Where,

R: erosivity or rainfall factor

E: kinetic energy of rainstorm (916+331+log I)

I30: maximum 30 min rainfall intensity (mm/hr)

– raingauge,

e) Soil erodibility factor(K) (soil texture, structure, %OM, soil permeability)

• The factor denotes the case with which a soil can be eroded from a standard plot.
• The standard plot is chosen as 1/100th an acre i.e 72.6 ft long 6 ft wide with 9% slope. value of k is found by,

A=RK

(all other factors like C, S, P and L values = 1 (unprotected fallow land)

K ranges from 0.1 to 1.1.

0.1 = non-erodible soil

1 = easily erodible soil

f) Length or slope length factor(L)

• The length of slope in m when raised to 0.5 power of it is called slope length factor.

Mathematically,

L=I^1/2

Where I represent length of slope

g) Slope gradient factor (S)

• The percentage of the slope, when raised to 1.4th power, it is called slope gradient factor or slope steepness factor.
• This means that the amount of erosion is assumed to increase as 1.4th power of the % steepness of the slope.

Mathematically,

S= s 1.4

where, S: slope gradient factor

s: percentage of slope

h) Crop management factor (C)

• It is the soil loss from cropping system to the loss from continuous fallow soil.
• Value ranges from 0.004 to 1.
• 1 for land in continuous fallow and the value of C is equal to 0.004 for good growth of permanent pasture.
• Indicates that continuous fallow losses soil about 250 times as fast as permanent pasture.

Value of C can be determined by

i) row crops (corn, cotton, tobacco, potato, soyabean) (higher erosion hazard)

ii) small grain crops (rice, wheat, barley, oat ; close spacing and rapid growth, less erosion but permits erosion)

iii) forage crops (pasture, legumes, close growing, extensive root system, good soil str., permeability, controls erosion)

i) Soil conservation practice (P)

• It is the ratio between amount of soil loss with the conservation practice i.e. contouring, stripping, terracing etc. to the soil without conservation practice.
• Value is equal to 1 till a special practices such as contouring or contour strip cropping is not used.