R-Factor (flow and surface runoff factor)

Often the soil erosion is calculated by transferring the (amount of) precipitation of the nearest weather station to the whole area. Since the relief intensity around "Ebergötzen" is relatively high the R-factor should be regionalised.
The following formula being made for Lower Saxony was taken to calculate the R-factor:
R = -12.98 + 0.0783 * N
Whereas:
R = R-factor
N = precipitation (mm / year)

The image shows significant differences within the R-factor (38 - 73). Especially the western upland of the "Göttinger Wald" shows high values. Even in the farming areas the values differ from 38 to 61.

Regionalising precipitation

Calculating the precipitation per annum is based on the precipitation data set of Lower Saxony (grid cell size: 200 m). The data was calculated by Jürgen Böhner (Geographical Institute - University Göttingen). The data set is based on: 46 climatic tables (weather stations) of Lower Saxony, ECHAM3-T42 e. g. ECHAM3-T108 climatic model data, circulation data (850 h Pa-level), DTM (grid cell size:200 m), calculating lee and luff position (considering the horizon exaggeration).
The values (grid cell size: 200 m) were transferred into the DTM (grid cell size: 12,5 m) of the area of "Ebergötzen" using the hypsometrical trend.

K-factor (soil erosivity factor)

Due to the USLE the K-factor is calculate as follows:
K = 2.77 * 10**-6 * M**1.14 * (12 - OS) + 0.043 * (A - 2) + 0.033 * (4 - D)
Whereas:
M = (% silt and fine sand) * (% silt + % sand)
OS = % of organic material (with OS > 4%: OS = 4)
A = aggregate class
D = permeability class

Only the laboratory can supply exact values of OS, A, D and fine sand. But it is possible to approximate these values by suitabe regression functions.

The regionalisation of the K-factor is based on regionalised silt and sand shares (more details, see: "soil/e_soil_physics/soil_texture").

LS-factor (topographical factor) (1)

Here (1) the LS-factor was calculated only within the fields supposing no surface run off being beyond (mainly above) the fields.

Modifying the LS-Factor

Modifying the LS-factor is mainly taking the catchment area (being devided by the grid cell size of the DTM) instead of the slope length (compare MOORE & WILSON 1992)(see: "Literature").

Problem: "Erosive slope length"

The USLE is only valid in the parts of the slope where erosion takes place, that means from the position where erosion starts to the position where accumulation starts. Since no field data is available in many cases the erosivity of a slope length is estimated by exclusion of the accumulating area. Various authors (MOORE & BURCH 1986)(see: "Literature") developed several erosion/accumulation indices or deposition models.
We also want to solve the problem "erosive slope length". At present the L-factor is calculated for the entire slope length. Subsequently the erosion rate is low in areas where accumulation takes place. The methods in process shall also account for the irregular slope gradients of slope sections.

Comment 1: At present the borders (as field paths ad ditches) of the erodible slope lengths are not considered.

Comment 2: Explanations concerning the calculation of base values of the LS-factor can be looked up in "Products" / "Relief" / "Local Terrain Factors ", catchword "slope gradient" and "Products" / "Relief" / "Complex Terrain Factors ", catchword "Catchment Area".

LS-Factor (topographical factor) (2)

The LS-factor (2) was calculated as "whole area is field". That means the runoff coming off everywhere. The remaining conditions are the same as in (1) (see above). As mentioned before the erosion hindrance (paths and ditches) are not digitalised in the Ebergötzen area. Therefore the erosible slope lengths (and catchment areas) are clearly overestimated in this scenario.

(1) Calculated soil erosion of cultivated areas
Scenario: crop: maize (corn)

P-factor = 1.0; C-factor = 0.277
The image shows the soil erosion estimated with USLE supposing only the erosible crop "maize" is cultivated.
It is obvious that the erosion rate (like in reality) is continually changing due to the method we developed. In contrast to the tracing field boundaries caused by the vector oriented calculation methods (because the K-factor is estimated homogeneous in the entire field).
Pointing to the overestimation of the soil erosion because the erosion hindrances and borders to accumulation areas are not yet considered (see: Comments to the LS-factor).

(2) Calculated soil erosion of cultivated areas
Scenario: crop: winter wheat

P-factor = 1.0; c-factor = 0.036
Only difference to image (1) is the C-factor. The erosion rate is lower than above taking the factor 7.69 (=0.277 / 0.036).

(3) Calculated soil erosion of cultivated areas
Scenario: Entire area is cultivated, without contour ploughing, crop: maize (corn)

This scenario shows the worst case. Because of the partly extreme steep slopes, the very large slope lengths and the erosible crop maize the erosion rate would be extremely high under these (luckily fictive) conditions.

Comment: Especially in the central part of the map (Schweckenhäuser Wiesen) a flight of the calculated values is obvious amidst the bluish parts. This is due to errors in the DTM (even the used high quality DTM is not perfect): the DTM based on the digitalised isohypses has vacillations of the altitudes (parallel the isohypses) and the derived slope gradients. Since the slope gradient is more important within the L-factor than the slope length this effect is even intensified (see above: image LS-factor (2) "whole area is field").