5. RESULTADOS Y ANÁLISIS
5.2. Análisis de varianza
MODULE 3
Unit 1 Meaning and Definition of Soil Erosion Unit 2 Surface Soil Erosion
Unit 3 Measurement of Surface Erosion
Unit 4 Preventing and Controlling of Surface Erosion Unit 5 Gully Erosion
UNIT1 MEANING AND DEFINITION OF SOIL
3.1 Meaning and Definition of Soil Erosion
Soil loss can occur through surface erosion, gully erosion and soil mass movement. Soil erosion involves the detachment and subsequent removal of soil particles and small aggregates from land surface by water or wind. This type of erosion is caused by the action of raindrop, thin film of flowing water, concentrated overland flows or the action of wind. While less serious in forested environments, surface erosion can be an important source from rangeland and agricultural croplands. Gully erosion is the detachment and movement of individual soil particles or large aggregates of soil in a well defined channel. Type of erosion is a major form of geologic erosion that can be accelerated under poor land management. Soil mass movement refers to erosion in which cohesive masses of soil and rock materials are displaced and moved down gradient by gravity. This movement can be rapid as occurs with landslide or bluff collapse or it can be slow as with soil creep and channel slumps.
All of the above erosion processes can occur singly or in combination.
People’s activities such as timber harvesting intensive livestock grazing, road construction, or row crop agriculture can accelerate the processes.
At times it is difficult to distinguish the basic types of erosion and to determine whether they are natural processes or been accelerated by poor land use practices.
3.2 Types of Soil Erosion
Sheet erosion is defined as the uniform removal of soil in thin layers from sloping land, resulting from sheet and overland flow. Raindrops detach the soil particles and the detached sediment can reduce the infiltration rate by sealing the soil pores. When the rate of rainfall exceeds the rate of infiltration of water into the soil, water starts to flow over the soil of the sloping land. At this point erosion commences and picks up the rain drop-detached particles and carries them along. The eroding and transporting power of sheet flow is a function of the rainfall intensity, infiltration rate and field slope for a given size, shape, density of soil particles and erodibility potential of the soil.
Rill erosion is the detachment and transport of soil by a concentrated flow of water in a minute channel. It is most serious where intense storm occurs on soil with high runoff producing characteristics and high erodible top soil.
Gully erosion produces channels larger than rills. These channels carry water during and after rains. The amount of sediment from the gully depends primarily on the run-off producing characteristics of watershed,
and the drainage areas; soil characteristics, alignment, size and shape of the gully and slope in the channel.
3.3 Factors Controlling Soil Erosion
Erosion by water is known to be directly controlled by a number of factors; climate, vegetation, soil properties and topography. Each factor is itself complex, and the various factors interact with one another.
Climate: The climatic term needs to be appropriately summed over the frequency distribution of storm rainfalls, but it can be seen that this approach provides a rationale for combining the effects of topography, soils and climate into a single integrated erosion forecast. Both low frequency and high frequency components of the climate are important for erosion. Low frequency events determine the seasonal cycle of the soil water balance, which provides the environment for growth of crops or natural vegetation. It may be appropriate to run a vegetation growth model (natural or crop), which then contains the potential to give a dynamic response to changed land use or climate conditions. High frequency rainfall events are clearly crucial for generating overland flow. The simplest effective tool for estimating runoff is the notion of a threshold storm size. Beneath the threshold there is little or no runoff;
above it all or a high proportion of the additional rainfall generates overland flow. The runoff threshold and proportion of subsequent runoff are simplifications of cumulative infiltration and runoff curves. Runoff Threshold is estimated from the crown cover, soil organic matter and soil texture/ structure characteristics. The threshold represents the effects of surface storage in random roughness and plough furrows, the dynamic evolution of soil crusting and moisture storage within the upper soil layers.
Vegetation: Vegetation acts on several ways, which may be dominant under different conditions, first by protecting the soil from rain splash impact and crusting, second by intercepting rainfall which is lost to evaporation and third by building up organic matter in the soil which greatly enhances the short-term dynamic storage and release of soil moisture. The combined effect of these processes is to decrease the runoff and soil erodibility. Vegetation also resists erosion by adding to surface roughness which reduces overland flow velocity, and binds the soil together with shallow root mats, particularly in grasses. However, it is recognized that vegetation is strongly influenced by land use, agricultural activity, both in cropland and by grazing, fire management etc.
Soil Properties: The most important soil property is the erodibility.
Erodibility is seen as primarily a property of the soil texture, with
highest values for fine sand and silt soils with low clay content. Factors that influenced soil erodibility are its texture, organic matter content, pH, structure, bulk density of plough layer and subsoil, aeration, porosity, parent materials aggregation and various interactions of these variables.
Topography: It is important to correctly allocate topographic and soil classes, since there is a strong correlation between high relief areas and strong rocks/soils. After a period of adjustment through erosion, erodible areas are reduced to lowlands while less erodible areas form highlands. High erosion is partly associated with the anomalies from such equilibrium landscape which are associated with recent tectonics or sea level change. More generally, the erosion of an uplifted highland area produces marginal piedmont areas where dis-equilibrium conditions of high erosion rates tend to persist longest in the landscape.
3.4 Soil Conservation Measures
The basic cause of erosion is usually inadequate soil and land management on farms grazing land and other cleared areas. Tackling the problem in these areas is therefore the key to reducing soil erosion rates and alleviating the impacts. The measures developed to combat soil erosion are known as soil conservation measures.
Techniques for controlling erosion by water
1. Mechanical measures Bench terraces
Contour bunds Tie- ridging Strip cropping
2. Biological measures Cover cropping
Mulching Afforestation Contour cultivation
Minimum and no-till cultivation
Techniques for controlling erosion by wind 1. Reduction of wind velocity
a. Vegetative measures
Cover cropping Close growing crops Sand dune stabilization b. Cultivation measures Mulching
Rotation grazing Crop rotation
Planting crops normal to prevailing winds Field and strip cropping
Primary and secondary tillage c. Mechanical measures Windbreaks
Shelter belts
Dune stabilization by brush matting or stones 2. Reduction of soil erodibility
a. Moisture conservation Mulching
Tillage
Timing seedbed preparation Irrigation
Terracing
Contour cultivation Strip cropping
b. Topsoil conditioning Correct timing of tillage Minimum tillage
Crop rotation Manuring
Chemical stabilizers
4.0 CONCLUSION
Soil erosion is an important form of land degradation and it regularly constrains rural development and exacerbates poverty by undermining the productive capacity of arable land agriculture. Localization of erosion-prone areas and quantitative estimation of soil loss rates with sufficient accuracy are of extreme importance for designing and implementing appropriate erosion control or soil and water conservation practices.
5.0 SUMMARY
In this unit, we have learnt: The meaning and definition of soil erosion.
Types of soil erosion.
Factors affecting the rate of soil erosion.
Methods and techniques of soil conservation.
6.0 TUTOR-MARKED ASSIGNMENT
1. Explain the term soil erosion.2. List and explain the types of soil erosion.
3. List and explain the factors affecting the rate of soil erosion.
4. List the techniques of controlling soil erosion by either wind or water.
7.0 REFERENCES/FURTHER READING
Brooks, K.N., Ffolliott, P.F. and Magner, J.A. (2013) Hydrology and the Management of Watersheds, USA: John Wiley and Sons Ltd.
Fernandez, C., Wu, J.Q., McCool, D.K. and Stöckle, C. O. (2003).
Estimating Water Erosion and Sediment Yield with GIS, RUSLE, and SEDD. Journal of Soil and Water Conservation 58: 128–136.
Bewket, W. and Teferi, E. (2009). Assessment of Soil Erosion Hazard and Prioritization for Treatment at the Watershed Level: Case Study in the Chemoga watershed, Blue Nile basin, Ethiopia. Land Degradation & Development 20: 609–622.
Kinnell, P. I. A. (2010). Event Soil Loss, Runoff and the Universal Soil Loss Equation family of models: A review. Journal of Hydrology 385: 384–397.
Toy, T. J., Foster, G.R. and Renard, K. G. (2002). Soil Erosion:
Processes, Prediction,
Measurement, and Control. New York: John Wiley & Sons.
Fan, J. R., Zhang, J. H., Zhong, X.H. and Liu, S. Z. (2004). Monitoring of soil erosion and assessment for contribution of sediments to rivers in a typical watershed of the upper Yangtze river basin.
Land Degradation & Development, 15: 411-421.