6.3.3 Soil Erosion and Salinity
in the same way as vegetation degradation, the wind erosion, water erosion and salinity maps were achieved, but with different criteria and paprameters.
Figure 2: Vegetation degradation
Table 2: Vegetation degradation of West Asia
6.3.4 Synthetic map of desertification
There are many methods to integrate these results; here we adopted a procedure that gives an important result in all its systematic steps:
Step 1: As the wind erosion and water erosion are considered to have the same effect and shape, which is soil erosion, so we integrated the wind erosion and water erosion in one maps, which has originally 16 combinations of "soil erosion classes". In this step we need to take decisions to group these combinations into four classes of importance regarding soil erosion.
Step 2: The soil erosion affects directly the vegetation cover, and it is consider as main cause for vegetation degradation. Therefore in this step we integrated the result of step 1 with the vegetation degradation map obtained previously in the same manner as in step 1, and we obtained an intermediate desertification map with four classes explaining the degrees of desertification.
Step 3: Finally we integrate the result of step 2 with the salinization map, this combination gives 16 classes as a logic operation of intersection. Then these classes are grouped into the final classes of desertification (Table 3). Figure 3 show the final map of desertification in West Asia.
5.4.6 Desertification Assessment
The total studied land area is about 6.46 millions square kilometers, 33 percent are classified as sand or sand dune land, and generally located in extra arid environment, which is theoretically excluded from desertification assessment. But we included the sand area, which is under irrigation activity as part of salinization assessment.
The study shows that 55% of West Asia is affected by moderate to severe soil erosion, which represents about 3.58 millions square kilometer. This area losses of its topsoil material from 50 to 150 tones per hectare per year. The vegetation degradation statistics show that it has the same range of importance as soil erosion, in fact 50 % of land are subject to moderate to severe vegetation degradation. Here we can clearly interpret the loss of topsoil by the degradation of vegetation cover, which is important factor to prevent the removal of topsoil material. The salinization mapping shows an important distribution severe and very severe salinization in the irrigated areas and others areas, which have problems of waterlogging and increase of water table level (world atlas, 92). Table 3 shows that about 266,000 km2 are affected by severe and very severe salinization, which constitute a 75% of the potential of irrigation area in West Asia.
Figure 3: Desertification map of West Asia
Table 3: Desertification classes in West Asia
In Table 3, we have the general estimation for desertification in West Asia. It is supposed that all the area is subject of desertification as we mentioned previously, mainly by vegetation degradation process followed by water erosion process, even the very severe water erosion is lees that 5%. Wind erosion dos not affect 16%, and 22% are slightly affected by wind erosion. So we conclude that 6% of land area in West Asia is slightly desertified, 21% is moderately desertified, 31% is severely desertified and 11% is very severely desertified. Without doubt these results show the gravity of desertification problem in the study area.
Conclusion
We found a big lack in desertification mapping techniques using remote sensing and GIS. And few studies were conducted to map desertification using such tools and methodology. An Integrated Approach is utmost important for desertification study, which reflect the complexity of desertification process. So the desertification assessment in this study is the output of a combination of social factors, economic factors, climatic factors and physical factors. Development of standard methods and criteria is needed for desertification studies. Also there is some lack in the definition of desertification, and needs some improvement, where it is important to include the socio-economic factors when the delineation of area susceptible to desertification.
Finally the statistic results of this study show the gravity of desertification problems in all levels. It is supposed that all the area is subject to desertification, mainly by vegetation degradation process, where 40% of study area is severely and very severely affected by vegetation degradation, followed by wind erosion process (27% severe and very severe wind erosion). So we conclude that 6% of land area in West Asia is slightly desertified, 21% is moderately desertified, 31% is severely desertified and 11% is very severely desertified. Without doubt these results show the gravity of desertification problem in the study area.
References:
Porevdorj T, The estimation of percent green vegetation cover using AVHRR data-Application to Mongolian grassland-, 1998, Graduate School of Science and Technology/Chiba University.
Tubengen Atlas of Middle East, Tubengen University, Stuttgart, Germany, 1978-1987. 30 p.
World atlas of desertification. UNEP, 1992, 69 p.
