Soil Loss Modeling: USLE Factors

Empirical Soil Loss Modeling

• The topic focuses on principles, development, and applications related to modeling soil loss.

Universal Soil Loss Equation (USLE)

• A = R * K * LS * C * P
• A represents potential mean annual soil loss.
• R Factor is the rainfall factor.
• K Factor is the soil erodibility factor.
• LS Factor is the topographic factor.
• C Factor is the land cover factor.
• P Factor describes conservation practices like terracing.

USLE-C Factor

• The C factor represents soil cover and management practices.
• It accounts for the influence of vegetation cover, land use, and management practices on soil erosion.
• It is a ratio of soil loss from a particular field compared to the loss under a standard treatment of cultivated bare fallow.
• Higher C values mean greater vulnerability to erosion.
• Lower C values reflect better soil conservation practices.
• Factors that Influence C:
  • Crop type
  • Tillage practices
  • Residue management
  • Conservation measures (contour farming, terracing)
  • Rainfall timing

Derivation of the C Factor

• Methodology involves empirical derivation based on field observations and research studies.
• Data Collection: Researchers collect field data on erosion rates under various land uses and management practices.
• Database Compilation: Observations are compiled into databases, enabling assignment of C values for different scenarios.
• Geographic Variability: C values can vary geographically and are often region-specific.

USLE-C Factor Common Values

• Cropland with Conventional Tillage: C = 0.3 - 0.5
• Cropland with Conservation Tillage: C = 0.1 - 0.3
• Grassland with Good Cover: C = 0.05 - 0.1
• Urban Areas with Impervious Surfaces: C = 0.9 - 1.0
• Land management decisions significantly influence soil erosion rates.

USLE-P Factor

• The P factor is the conservation practice factor.
• It compares the soil loss with conservation practices to a field with no such practices.
• P accounts for the effectiveness of erosion control measures like contour farming and terracing.
• Erosion control practices reduce slope and rainfall intensity impact on soil erosion.
• Studies assess the effectiveness of support practices in reducing soil erosion under varying conditions.
• Observations inform the assignment of P values, showing the effectiveness of erosion control measures.
• Higher P values indicate more effective erosion control practices and reduced soil erosion rates.

USLE-P Factor equation

• Wener's equation (1981) was chosen for its simplicity
• Calculates relationship between slope and support practice factor: P = 0.2 + 0.03 × S
  • P is the support practice factor, ranging from 0 to 1.
  • S is the slope expressed as a percentage.

Conservation Practices

• Terracing involves building level steps on a slope to reduce water runoff speed and increase infiltration.
• Contour farming involves planting crops along the contour lines of a slope to slow runoff.
• Conservation tillage reduces tillage amount to maintain crop residues on the surface.
• Grassed waterways are channels that convey runoff without causing soil erosion, often lined with vegetation.
• Filter strips are vegetation areas along fields or waterways that filter out sediment and pollutants.

USLE P Factor Values

• Contour Farming: P = 0.5 - 0.8
• Terracing: P = 0.6 - 0.9
• Grassed Waterways: P = 0.7 - 0.9

South Africa USLE Example

• Consider a hypothetical scenario in South Africa where deriving the Universal Soil Loss Equation (USLE) for a specific agricultural field.
• The field has a slope gradient of 8%, a soil erosion plot length of 50 meters, and experiences rainfall energy of 80 MJ mm ha-1 h-1 yr-1.
• The soil type is sandy loam, and the crop management factor (C) is estimated to be 0.3 and has moderate conservation practices.
• Calculating the soil erosion rate using the USLE equation for this scenario.