1. Overgrazing: overgrazing of rangeland is increasing with
increase of livestock numbers. This process causes animals to
destroy the plant cover, then exposing the soil to erosion.
2. Cultivation of marginal lands: This practice is expanding into
areas which display only a low land capability and in view of
the available precipitation, do not actually permit sustainable
cultivation.
3. Expansion of orchards areas: particularly in the uplands
4. Urbanization on agricultural lands.
5. Overexploitation of fertile soil
6. Traffic of heavy cars
7. Cleaning of soil from stones.
8. Water resources deterioration
5.2 Desertification indicators
The following are considered as indicators of desertification :
1. Loss of agricultural potential:
2. Appearance of unpalatable plants:
3. Increase in abandoned farmlands:
4. Dust storms:
5. Bare lands and Queries expansion
6. Changes in soil characteristics
7. Groundwater deterioration:
5.3 Criteria of desertification assessment
Criteria for desertification assessment must take in
consideration the local conditions of the study area and the
indicators of desertification. These indicators should be
investigated through field observations, with the help of remote
sensing tools. The results of such investigation can be
interpreted according to the type and degree of desertification.
The types of desertification considered in this study are
degradation of natural vegetation, wind erosion, water erosion
and soil salinization ( Kharin et al, 1999 or Kharin et al, 1991).
6. DESERTIFICATION MAPPING
6.1 Preparation stage of desertification mapping
The compilation of data for desertification mapping requires the
following steps:
6.1.1 Desertification Mapping Units (DMU)
The Desertification Mapping Units (DMU) were created via a
combination (overlay) of land units (geomorphology) and land
use types. The land units map describes the geographic aspects
of desertification, and land use map describes the human aspect
of the problem. The combination leads to the allocation of a
unique value (class) for each possible combination of the two
input layer values. Each class is a Desertification Mapping Unit
(DMU). Figure 6 shows the raster format of DMU layer, which
was converted to vector layer to be used in the GIS database.
6.1.2 GIS database
To produce an accurate desertification map, a GIS database was
compiled. This database contains many layers, including the
following for desertification mapping: Satellites images, Land
use, Soil types, Soil moisture, Geomorphology units, Slope
degrees, Drainage system, Natural vegetation, Vegetation index,
Expansion of irrigated lands, Rainfall distribution,
Susceptibility to wind erosion, Susceptibility to water erosion
and Desertification Mapping Units (DMUs).
IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002
6.1.3 Attribution of information
For each DMU an identifier was assigned, and to each DMU
information was attributed related to the different layers of the
GIS database (database table). Using this construction, one can
undertake a wide variety of database operations. However, more
importantly, one can interact directly with the linked-table
collection: database queries can be shown immediately on the
associated map layer, and map layer queries can be directly
linked to the data table. In addition, database field values can be
assigned or extracted from the GIS layers.
6.1.4 Decision making techniques
To assess the different types of desertification classes, several
criteria will need to be evaluated. The combination of these
criteria will form a single index of desertification. Such a
procedure is called multi-Criteria Evaluation (Voogd, 1983,
Carver, 1991). Multi-Criteria Evaluation (MCE) in our study is
achieved by the weighted linear combination (WLC) procedure,
where factors are combined by applying a weight to each
followed by a summation of the results to yield a suitability
map. Then this thresholded for each type of land degradation;
water erosion, wind erosion, vegetation degradation and
salinization.
6.2 Desertification mapping
As discussed previously, land degradation was separated into
four types: water erosion, wind erosion, vegetation degradation
and salinization. The combination of these types will give a
inclusive and general map of desertification. The following is a
description of each of these processes:
6.2.1 Water erosion
We identified three relevant factors to the evaluation of water
erosion: water erosion susceptibility (to emphasize the
vulnerability of land to erosion), increase in area farmlands
(which indicates the changes in land cover); and decreasing in
vegetation cover index. The process of MCE was used to create
a water erosion map, which has a range from 0-255, increasing
with the increase of water erosion. The result was divided into
four classes of degradation; slight, moderate, high and very
high.
6.2.2 Wind erosion
As for wind erosion, four relevant factors were identified. First
wind erosion susceptibility (to emphasize the vulnerability of
land to wind erosion). Second soil type (to classify the area in
terms of texture and grain size). Third land use (to identify the
areas exposed to wind erosion a cause of bad agriculture
practices like plowing marginal lands). Finally
geomorphological units (to classify the area in terms of
topography and wind characteristics).
6.2.3 Vegetation degradation
The first and most important factor indicating degradation of
vegetation cover, is the decline of land productivity. This is due
to many factors such as removal of top fertile soils and
reduction of organic strong decrease of productivity in irrigated
area followed by range land then rainfed agriculture area, and
etc. The second factor relevant to vegetation degradation is the
appearance of unpalatable plants in places of palatable ones.
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