Soil erosion by water describes the process of soil destruction and washing away by precipitation. As a result, the fertile top soil layer is lost, along with its humus content, the nutrients and water stored there. This process threatens the fertility of the soil and therefore people's food security. Factors such as soil type, vegetation cover and slope inclination, but above all agricultural use, influence the amount of eroded soil. Several indicators have been developed for the various aspects:
The main indicator "H Avoided water erosion" describes the modelled average annual mass of soil substance whose erosion is prevented by the existing land cover. The indicator is determined from the difference between an assumed (hypothetical) erosion rate without vegetation and the modelled actual erosion (N1) under the current vegetation.
The side indicator "N1 Current water erosion" describes the modelled average annual mass of soil substance lost through erosion. H is the difference between an assumed (hypothetical) erosion rate without vegetation and the modelled current erosion (N1) under the existing land cover.
Landscape elements such as rows of trees, hedges and edges can reduce erosion. The side indicator "N2 Avoided water erosion through small structures" describes the special contribution of these elements to erosion prevention and is also included in the indicators H and N1.
Erosion can additionally be reduced through conservation tillage or organic farming on arable land. Additional information is thus provided by the side indicator "N3 Proportion of organic farming on arable land", which is not included in the other indicators.
The mitigation of water erosion is relevant on all inclined surfaces on which vegetation cover leads to a reduction in the assumed erosion rates. Higher values mean that the vegetation cover leads to a reduction in the modelled soil erosion rates. In flatter and therefore less water erosion-prone areas, the indicator value is low.
In 2015, German ecosystems avoided 16.7 Mg of soil erosion by water per hectare in 2015, which adds up to half a billion Mg in absolute terms for Germany. The highest avoidance rates are found in Baden-Württemberg and Bavaria, which have larger proportions of high and low mountain ranges (due to the forests and grassland there). Federal states in the lowlands have a low natural erosion disposition because there is comparatively less erosion to avoid. The performance gradually decreased during the period from 2012 to 2015.
Main indicator H "Avoided water erosion"
H = A (hypothetically without soil cover) - A (with current soil cover = N1)
Side indicators N1 Current water erosion and N2 Avoided water erosion through small structures
The universal soil loss erosion equation (USLE) is used to calculate erosion in its German version:
(ABAG, Schwertmann et al. 1990)
A = K × R × L S × C × P
A: Average soil erosion; factors:
K: Erodibility factor (estimated from soil overview map BÜK)
R: Regenerosivity factor (calculated from DWD precipitation data)
L S: Topography factors (slope length L, slope inclination S, calculated from DHM)
C: Cultivation factor (calculated from data from the agricultural statistics of the federal states)
P: Erosion control measures (not evaluated)