EROSION THROUGH WATER AND WIND ARE DIFFERENT PROCESSES IN PRINCIPLE, THREE STEPS CAN BE DISTINGUISHED: - MOBILISATION - TRANSPORT - DEPOSITION
- HYDRATATION - HYDROLYSIS = PROTOLYSIS - OXIDATION - COMPLEXATION IN THREE STEPS: - SOLUBILISATION -TRANSPORT - PRECIPITATION AND FIXATION LEACHING OCCURS THROUGH DIFFERENT PHYSICO-CHEMICAL AND BIOLOGICAL PROCESSES:
WATER EROSION: USLE = Universal Soil Loss Equation A = R. K. L. S. C. P A = soil loss ( g. m -2 ; t. ha-1) R = rainfall erosivity (kinetic energy of rain) K = soil erodibility (texture, structure etc.) L = length of the slope S = slope degree (inclination) C = influence of land use (type of agriculture, forestry etc. P = measures against erosion (e.g. terracing, countour stripping etc.
FORMS OF EROSION: - sheet erosion - rill erosion - gully erosion - tunnel erosion - harvest erosion (see pictures)
MEASUREMENT OF SOIL DEGRADATION - Recent (actual) erosion Measurement on erosion plots (on site) Hydrological measurements in small watersheds by gauges Problems: - not all eroded soil material reaches the waterways, but is deposited close to the erosion site; - extrapolation of results from some m² to large areas is difficult or impossible; - because soil formation is a process of thousands of years, as well as soil degradation, actual measurements are not very indicative for long time periods
- HISTORICAL (PAST) EROSION might have been quite different to the actual one, because of different climate conditions (climate variations in historical times): - topomorphic approaches: measurement of lake sediments or sediments deposited downhill in lower slope positions or in alluvial plains; - measurement of isotopes; - luminescence measurements; - time markers, such as volcanic depositions of known age in sediment layers; Exact results are in general difficult to obtain. - RESULTS OF LEACHING PROCESSES mass balances based on known processes, e.g. limestone weathering and deposition of calcrete
Estimations (assessment) of soil degradation by erosion and leaching are based on models, extrapolating local results of measurements onto larger areas, taking into consideration factors such as climatic influence, landscape, soil type, vegetation cover, land use and others (see USLE). There are some dozen models available for different physio-geographic regions of the world.
CONCLUSIONS 1. Soil degradation is causing loss of soil material, counterbalancing soil formation through weathering of rock parent material; 2. Soil losses occur in solid (erosion) and liquid (leaching) form; 3. Soil losses on one site mean at the same time deposition of the lost material on other sites - through aquatic/hydraulic transport and deposition; - through wind transport and deposition; - through precipitation after mobilisation and leaching; 4. Land and soil surfaces without protective vegetation cover like deserts were always sources of soil material, whereas others were sinks, gaining soil material, see e.g. Sahara dust in South and North America