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II.
LITERATURE REVIEW
2.1. Erosion
Soil erosion is one form of land degradation besides soil compaction, low organic matter content, loss of soil structure, poor internal drainage, stalinization
and soil acidity problems. In particular, soil erosion is defined as: “physical removal of topsoil by various agents, including falling raindrops, water flowing
over and through the soil profile, wind velocity and gravitational pull” Lal 1990. Historically, soil erosion began with the beginning of intensive agriculture
activities, where people are removing protective vegetation cover and growing various food crops on disturbed soil surface.
In addition, some other large-scale opening of vegetation through commercial logging, preparation of timber and crop estates, and expansion of
human settlement accelerated it. Nowadays, soil erosion is almost universally recognized as serious threat to human’s well being. This is confirmed by facts of
active supports given by most governments to soil conservation programmes Hudson 1995. A general figure of rain erosion susceptibility is presented in
Figure 2.1.
10 Fig 2.1: Distribution of rain erosion throughout the world Hudson 1995
Soil erosion caused by water is a serious problem in sub humid, semiarid, and arid regions. Inadequate moisture and periodic droughts reduce the periods
when growing plants provide good soil cover and limit the quantities of plant residue produced. Erosive rainstorms are not uncommon and they are usually
concentrated within the season- when cropland is least protected Wischmeier and Smith, 1978.
This energy in the form of rainfall causes splash erosion. The potential energy for erosion is converted into kinetic energy, the energy of motion of the
running water. This kind of energy formed by runoff causes inters rill, rill, gully, and riverbank erosion.
2.2. Factors Affecting Soil Erosion
Agents of erosion are the carriers or the transport system in the movement of soil e.g. water, wind. Factors of erosion are those natural or artificial
parameters that determine the magnitude of perturbation, e.g. climate, topography, soil, vegetation and management. Erosion may not occur even when the agents
and factors are present. There are causes of erosion, due to human activities such
11 as farming practices, deforestation and cropping systems that facilitate the effects
of agents and factors of erosion and accelerate the various erosion processes Bergsma 1996; Lal 1990. The factors affecting soil erosion by water are:
2.2.1. Climatic Erosivity
Erosivity refers to the aggressively of the climate, or more precisely the energy of such climatic elements to cause erosion. Climatic factors that affect
erosivity are precipitation, wind velocity, water balance, mean annual and seasonal temperatures, etc.
2.2.2. Soil Erodibility
Erodibility is the susceptibility of soil to erosion. This is an inherent property of the soil and is influenced by soil characteristics e.g. texture,
structure, permeability, organic matter content, clay minerals and contents of iron and aluminum oxides.
2.2.3. Landforms
Erosion also affected the terrain relief through degree and length of slope, shape of slope and slope aspect. In general, the higher the slope gradient, the more
soil erosion by water occurs.
2.2.4. Human
Human activities affect soil erosion through their measures to natural resources. Human activities related to erosion are deforestation, grazing, faulty
farming system and cropping intensity. However, some activities in terms of soil conservation measures are reducing the amount of soil erosion e.g. contouring
planting
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2.3. Land degradation and land use land cover change
Land use and land cover change have become a central component in current strategies for managing natural resources and monitoring environmental
change. Since the late 1960’s, the rapid development of the concept of vegetation mapping has lead to increased studies of land use and land cover change
worldwide. Providing an accurate assessment of the extent and health of the world’s forest, grassland, and agricultural resources has become an important
priority.
2.3.1. Land use and land cover
Every parcel of land on the Earth’s surface is unique in the cover it possesses. Land use and land cover are distinct yet closely linked characteristics
of the Earth’s surface. Land use is the manner in which human beings employ the land and its resources. Examples of land use include agriculture, urban
development, grazing, logging, and mining. In contrast, land cover describes the physical state of the land surface. Land cover categories include cropland, forests,
wetlands, pasture, roads, and urban areas. The term land cover originally referred to the kind and state of vegetation, such as forest or grass cover, but it has
broadened in subsequent usage to include human structures such as buildings or pavement and other aspects of the natural environment, such as soil type,
biodiversity, and surface and groundwater. Myers, 1988 Land use change is generally conscious, volitional responses by humans or
human societies to changes in biophysical or societal conditions. It is a response indicator, therefore, reflecting how and to what extent society is responding to
meet its changing needs and goals or to adapt to changing environmental
13 conditions. This does not exclude the possibility that some land use changes may,
in turn, constitute a driving force for changes in the state of the environment. That is in the very nature of the complex causal network not a simple causal chain,
including a number of feedback loops, that is societys relationship with its environment.
As is the case for land use change, it is doubtful whether a single or aggregate measure of land condition change would be feasible. What is feasible in
principle is an estimation of the change in the different land qualities that influence the suitability of the land for one use or another, or for conservation
purpose, for example, of biodiversity and erosion for land degradation. Land qualities are discussed in FAO, 1976
.
2.4. GIS and RS in Soil Erosion Modeling and Land UseCover Change
Soil erosion is spatial phenomena, thus geo-information techniques play an important role in erosion modeling Yazidhi, 2003. While this is agreeable, the
quality of the results matches the quality of the input data used Svorin, 2003. Land use data required to run erosion model can be derived from remotely sensed
data. In a GIS environment it is possible to link data generated from remote sensing with their spatial location Mkhonta, 2000. In general, the use of geo-
information techniques offers the following advantages in erosion modeling: i
fast and cost effective estimates, ii
possibilities to investigate larger areas, iii
greater possibilities of continuous monitoring of these areas and iv
possibilities to refine the soil erosion model depending on the required output scale i.e. rough global to more precise local scale.
14 According to Yazidhi 2003, the use of digital elevation models and GIS
offers possibilities to estimate more relevant topographical parameters that are useful in soil erosion modeling.
2.4.1. Land Cover Mapping
Land cover mapping is one of the most important and typical applications of remote sensing data. Land cover corresponding to the physical condition of
ground surface, for example, forest, glass land etc., while land use reflects human activities such as the use of the land, for example, industrial zone, residential
zone, agricultural fields etc. To prepare, the land cover mapping from digital images “land cover
classification” should be done. There are two kinds of classification, i.e. supervised and unsupervised classification.
2.4.2. Supervised Classification
Supervised classification is the method used to transform multi spectral image data into thematic information classes. This procedure typically assumes
that imagery of a specific geographic is gathered in multiple regions of the electromagnetic spectrum.
In supervised classification, the identifying and location of feature classes or cover types urban, forest, water, etc are known beforehand through fieldwork,
analysis of aerial photographs, or other means. Typically, identify specific areas on the multispectral imagery that represent the desire known feature types, and
use the spectral characteristics of theses known areas to train the classification program to assign each pixel in the image to one of these classes. Multivariate
statistical parameters such as means, standard deviation, and correlation matrices
15 are calculated for each training region, and each pixel is evaluated and assigned to
the class to which it has the most likelihood of being a member according to rules of the classification method chosen.
One of the sample classification strategies that may be used is Maximum Livelihood Classifier. The maximum livelihood was adopted by using the training
samples of the landsat image and ground truth. Actually, this is one of the most popular methods of classification in remote sensing, in which a pixel with the
maximum likelihood is classified into corresponding class.
2.5. Runoff Erosion Potential with GIS Based Spatial Analysis Model Spatial Analysis
extends the basic set of discrete map features of points, lines and polygons to surfaces that represent continuous geographic space as a set
of contiguous grid cells. The consistency of this grid-based structuring provides a wealth of new analytical tools for characterizing “contextual spatial
relationships”, such as effective distance, optimal paths, visual connectivity and micro-terrain analysis. In addition, it provides a mathematicalstatistical
framework by numerically representing geographic space.
Spatial Statistics , on the other hand, extends traditional statistics on two
fronts. First, it seeks to map the variation in a data set to show where unusual responses occur, instead of focusing on a single typical response. Secondly, it can
uncover “numerical spatial relationships” within and among mapped data layers. The model assumes that erosion potential is primarily a function of terrain
steepness and water flow. Then the result is combined with the human factor. Admittedly the model is simplistic but serves as a good starting point for a spatial
analysis example in natural resources.
16 Fig 3.1: The general information of runoff erosion model based on spatial analysis
2.6. Hybrid Erosion Modeling Approach
Kategori