Soil erosion – causative factors, extent and prevention
Soil degradation in Namibia is not a spectacular phenomenon. It is inconspicuous and insidious. It manifests itself as nutrient depletion, hardpan formation, and surface sealing. Wind, sheet, rill and gully erosion do occur but are not perceived as major problems compared to the inherently poor physical and chemical properties of Namibia’s soils and the overwhelming aridity of the country. Therefore, very few people realize that erosion is a problem in Namibia. (Coetzee 1999).
However, erosion does occur. During the past few years’ fields work for the Vegetation Survey project (Strohbach & Sheuyange 1999), observations have been made about the type and severity of erosion. Although the data do not yet cover the whole country, the results give a good indication of just how widespread the problem of erosion is (Table 1). Wind erosion is especially a problem in the southern Kalahari, whilst sheet erosion has been observed widespread. Table 1: Number of sites with observed erosion.
Slight sheet erosion 323 39.2%
Moderate sheet erosion 273 33.1%
severe sheet erosion 80 9.7%
extreme sheet erosion 8 1.0%
As can be seen from Table 1, only 6.8% of the observed sites did not show any sign of erosion, whilst the extreme cases of erosion, being rill erosion and extreme sheet erosion was only observed on 1.2% and 1.0 % of the plots, respectively. Erosion, albeit often only slight to moderate in degree, is this happening in over 90 % of our country!
In the SOTER methodology (FAO 1993) the following types of erosion are recognized:
· Water erosion: sheet, rill, gully and tunnel erosion
· Wind erosion, shifting sand
· Water, wind and salt deposition
Sheet erosion is the erosion of the surface soil layers over a large area. This type of erosion is the start of erosion and often not easily recognized, as no telltale rills are formed. The next step is rill erosion were small channels are formed. As these channels grow bigger, typical gullies are formed. Eventually, the gullies result in gully landscapes or badlands. These are often associated with Lesotho and the Eastern Cape in the southern African context, but are also found in Namibia Tunnel erosion is a phenomenon in which the topsoil is very stable, whilst the lower soil horizons are highly erodible, and often exposed. With water-logging, the subsoil layers wash away, leaving hollows/tunnels into which the topsoil sinks.
This form of erosion is also often associated with high rainfall areas and heavy frost areas. A distinction is made between straight forward wind erosion, where the topsoil is taken away by the action of wind and shifting sands, where (desert) plains are alternating covered/ uncovered by sand sheets. All types of erosion result also into deposition (or sedimentation) of the soils removed. Deposition often happens in locations fairly remote from the origin of these soils. The degrees of erosion is defined by SOTER as follows:
· Slight erosion: Some evidence of loss of surface horizons. Original biofunctions largely the intact
· Moderate erosion: Clear evidence of removal of surface horizons. Original biofunctions partly destroyed.
· Severe erosion: Surface horizons completely removed, with subsurface horizons exposed. Original biofunctions largely destroyed.
· Extreme erosion: Substantial removal of the deeper subsurface horizon (badlands). Complete destruction of original biofunctions.
Factors contributing to erosion
Water erosion occurs when raindrops hit the ground and dislodge soil particles from the soil, and then these dislodged soil particles wash away and in the process dislodge and remove further soil particles. The amount of erosion is thus a function of the following four factors: the rainfall energy, the vegetative cover, the length and steepness of the slope and the type of soil (Stocking 1987).
The rainfall energy is the energy which falling raindrops have then impacted with the soil. This energy is a product of the mass (i.e., the size) of the drop as well as the speed at impact. The higher the origin of the drop, the higher its impact speed. The bigger the drops, and thus the ‘harder’ the rainfall the event, the more energy is released to the soil (Stocking 1987).
Namibia has generally very ‘hard’ rainfall associated with our tropical thunderstorms compared to the eastern part of the subcontinent. The more rain per annum, the more energy is released onto the soil per annum. Thus, a high rainfall area has an inherently higher risk of erosion than a lower rainfall area.
(a) do not have the same mass as the original drop, and
(b) do have only a short distance to fall – thus not attaining the same terminal velocity as a raindrop falling from the clouds.
Slope length and steepness
After dislodging the soil particles from the soil surface, these particles have to be transported in order for erosion to take place. Gravity is the driving force: The steeper the slope, the faster the water can move. The faster the water moves, the more soil particles it can take along, and the more additional soil particles can be dislodged.
The steeper the countryside, the more erosion will take place. Obstructions along the slope will impede the flow of the water. Litter, branches, logs, stone, contours – all will slow down the flowing water and thus reduce the amount of soil particles the flowing water takes along. The longer the (uninterrupted) slope, the more the erosion caused by flowing water
An example: the main path to the house is 1000 m long on a gentle the slope of about 5 % (approx. 3°). As it is the main driveway, it is uncovered and most properly also compacted. If you don’t put in contours, you will lose 16.55 t/ha in an average rain year (300mm). If you put a contour every 100 m, you will lose (only) 5.23 t/ha during the same year!
Finally, the soil type, and especially the chemical and physical properties of the soil type, determine the ease with which soil particles are removed from the soil body. A major factor is an ease with which water is absorbed into the soil, as well as the bonding of the soil particles.
Erosion is commonly associated with gullies or donga’s. However, this is the ultimate product of erosion. Wind and water erosion both start off with the limited movement of the loose topsoil – with wind erosion a dune-like the ripple effect is seen, whilst water erosion is evident by flowing patterns on the soil surface.
Plants always germinate at or near the soil surface, with the roots developing below the soil surface. When these roots become exposed it is a sure sign of moderate to severe sheet erosion. At this stage, no obvious rills are visible that one would associate with serious erosion. Another sure sign of erosion is the formation of pedestals – small stones, leaves, pieces of wood, etc. protect the soil directly below from the raindrop impact. With time, the soil washes away around these objects, leaving them on little pedestals – about 1 or 2 cm, maybe up to 5 cm high.
Often plants are seen to grow on little hills. This can be either the effect of sheet erosion washing away the soil around the plant, but as often the sign of material deposited against the plant base by wind- or water erosion.
Soil erosion vs. production
The obvious effect of soil erosion is the loss of topsoil. But how does it affects the productivity of the land?
Vegetative cover is the main protection of the soil. It has also been shown that bush encroachment is a result of the reduction in grass cover (Walter 1971; Knoop & Walker 1985; Strohbach 1990). Essentially the grasses compete with trees and shrubs for soil moisture. As the grasses are relatively fast-growing, and use water from the topsoil, they can out-compete shrubs for their water supply. As soon as the grass layer is reduced, however, more water reaches the shrub roots, and bush encroachment can start.
A number of methods are available to combat erosion – especially gully erosion. However, the real challenge lies in the prevention and combating of sheet and wind erosion.
· Prevent overgrazing – the grass the cover is the best protection you can have against erosion.
· Know your soils – use sensitive soils cautiously, and as conservatively as possible!
· Then combating bush encroachment, – use the branches as barriers to soil flow. Either scatter the branches at random or pack them in lines parallel to the slope contours.
· Stabilise roads and tracks by building contours/humps.
