Regional Sheet and

Rill Soil Erosion

Prediction with the

Revised Universal

Soil Loss Equation

(RUSLE) – GIS

Interface

by Jacek Blaszczynski

Physical Scientist, National Science

and Technology Center

Background

An ecological issue of great importance to the United States and the

world is degradation of rangelands,

the major symptom of which is

increase in the rate of water-caused

soil erosion. Livestock grazing and

drought can combine to reduce the

grass cover, compact the soil causing increases in runoff and erosion,

and create a patchy distribution of

water and nutrients. Intensified

grazing during periods of moderate

drought can lead to a decrease in

grass cover that normally supports

a uniform distribution of moisture

and nutrients throughout the soil

profile. Bare patches, which may

have developed during the drought,

become more pronounced. Topsoil

compacted by trampling allows less

rainfall to infiltrate the topsoil,

leading to greater surface runoff

and related sheet and rill erosion.

The problem of erosion is further

exacerbated by loss of organic matter in the topsoil that tends to hold

the soil particles together. If overgrazing and drought continue, the

bare patches expand, and erosion

becomes a significant problem as

rain and nutrients are carried away

from the patches to be deposited in

nearby surface depressions.

Creosote and mesquite shrubs

begin to appear in these areas of

concentrated soil, water, and nutrients. As soil around the grasses

loses its depth, nutrients, and moisture through erosion and leaching,

it can no longer sustain the grasses,

limiting their ability to regenerate. To avoid this downward spiral, management of rangeland soil

resources has to be performed in

a way that does not deplete the

resource and results in loss of soil

productivity. Technologies to estimate, manage, and contain soil

erosion are needed to help prescribe management practices that

maintain existing rangelands in an

ecologically healthy state and, if

possible, to return degraded rangelands to a more pristine condition.

For most of the world’s regions,

accurate estimates of soil erosion

are not available. An examination

by experts of the available figures

including a review carried out for

the United Nations Food and

Agriculture Organization (UNFAO)

concluded that these estimates have

little scientific merit. The overwhelming share of land reported to

the United Nations Environment

Program (UNEP) as under danger

from soil erosion and related desertification processes is rangeland.

If global climate warming proves

actual, leading to an increase in

mean surface temperature, the rates

of desertification in various regions

may sharply increase. Since a significant portion of the public lands

managed by the Bureau of Land

Management (BLM) consists of

rangelands, BLM has a vested

interest in issues related to rangeland soil erosion. Development and

application of soil erosion prediction methodologies is therefore of

utmost importance. The Revised

Universal Soil Loss Equation

(RUSLE)/Geographic Information

System interface provides such a

methodology for regional level

analysis of soil erosion. While it

is likely that application of more

complex models than RUSLE can

provide more accurate estimates

of soil erosion for specific sites,

the data requirements of these models make them difficult to use on a

regional scale. By comparison, data

for utilization of RUSLE/GIS interface can be relatively easy to obtain.

Methodology

The RUSLE/GIS methodology

permits calculation of potential soil

loss from sheet and rill erosion for

rangelands. The RUSLE equation

calculates potential erosion (A) as

follows:

A = R * K * L * S * C * P

Where R is the rainfall and runoff

factor; K is the soil erodibility factor; L is the slope-length factor; S is

the slope-steepness factor; C is the

cover and management factor; and

P is the erosion control practice

factor. In the modeling procedure,

the values for the different RUSLE

factors are assigned to their respective mapping units (i.e., a soil unit

map, a vegetation class map, and a

digital elevation model).

Values for the factors can be

obtained from reference sources or

calculated from mapped or field

collected data about the project

location. To facilitate calculation of

RUSLE factors, the user can input

relevant information into a PC

program, such as the RUSLE program prepared by BLM in 1987

upon which RUSLE/GIS interface

is based. The rainfall and runoff

factor R value is calculated from

latitude and longitude and 2yr-6hr

precipitation of the study area. The

soil erodibility K factor values are

NO. 46 DATE 04/18/01

Range

Soils/geology
obtained from soil surveys or calculated from field information that

includes percentage of silt and very

fine sand in the soil, percentage of

clay, percentage of organic matter,

soil structure, and permeability

code. The slope gradient S and

slope length L factors are calculated

from digital elevation data using

GIS hydrologic terrain analysis

capabilities and map algebra. The

C factor values are calculated using

field survey information on percent

and average height of vegetation

cover, the root mass in the top four

inches of soil, percent of rock, gravel, litter, and vegetation on the

ground surface. In calculation of

the C factor, the RUSLE program

also provides a table of surface

roughness values from which one

can pick the appropriate roughness

number. The erosion control P factor values are obtained from a table

provided by the designers of the

RUSLE. After the factor values

have been assigned or calculated for

each of the mapping units, the factor

maps are overlaid to produce a

visualization of soil erosion potential.

There has been much controversy

over the accuracy of the Universal

Soil Loss Equation and its close

cousin, the Revised Universal Soil

Loss Equation. However, even if we

regard the resultant soil-erosion

potential map purely as a relative

evaluation based on known interactions and influence of rainfall, soil

cohesion, terrain shape, surface vegetation cover, and erosion control

practices, such a map still provides

us with useful information. A

preliminary run of the procedure

based on existing data should provide us with a rapid reconnaissance

level evaluation as to where we can

expect low, medium, high, and very

high erosion rates based on environmental parameters expressed by

the RUSLE factors. Although the

RUSLE model oversimplifies the

complexity of erosion processes, its

estimates can be improved to by

refining the quality of input data.

The RUSLE/GIS interface can be

used to model various possible

management and ecological health

scenarios by changing the values for

various environmental factors to

simulate different conditions, such

as seasonal change, change in precipitation, vegetation cover, terrain

and management practice.

Automated Calculation of

Terrain Factor from Digital

Elevation Models (DEMs)

At the National Science and

Technology Center, we developed

automated tools to make the

RUSLE/GIS procedure easier to

perform. LSFACTOR.AML provides three methods of calculation

of the RUSLE terrain factor from

DEM data. The products are three

terrain factor maps that represent

various methodologies.

Benefits and Uses

Interfacing GIS geographic analysis capabilities with the Revised

Universal Soil Loss Erosion provides the resource specialist with a

tool to quickly visualize the likely

sheet and rill soil erosion potential

(soil detachment potential but not

transport and deposition) based on

several major environmental parameters for large areas. This permits

regional scale studies by quickly processing large amounts of information

from data sources that are easy to

obtain from various government

agencies.

The interface also permits modeling

of various management alternatives

and changing environmental conditions in space and time. This allows

for evaluation of the possible effects

of each management practice on

the stability of the soil resource.

Because the procedure relies on

standardized methodology, it also

allows for comparison of different

geographic areas. Furthermore, it

provides maps of various resolutions depending on the resolution

of the input data. The RUSLE/GIS

interface can serve as core methodology for further GIS hydrologic and

geomorphologic applications such

as analysis of watershed condition,

sediment loading of streams and

rivers, non-point source pollution,

and the effects of hazardous wastes

in soils on water quality, etc.

Contact

Jacek Blaszczynski

National Science and Technology

Center, BLM

Denver Federal Center

Building 50, PO Box 25047

Denver, CO, 80225-0047

phone (303) 236-5263

fax (303) 236-3508

email: Jacek_Blaszczynski@blm.gov 

Range

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Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

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