Key Words: Remote sensing, GIS, Desertification, Assessment, Mapping
Abstract: In the context of the constitution of environmental GIS database of west Asia, satellite remote sensing and GIS technology were applied as essential tools to address important aspects of environmental monitoring. A multi-temporal analysis of satellite derived monthly vegetation index data allows to create a land cover map of the study area. The land cover map with satellite images of NOAA AVHRR formed the basic elements of the environmental GIS database. The GIS is constructed in such way to envelop all necessary environmental layers (soil, geology, rainfall, elevation, temperature, moisture, albedo…etc). These layers were digitized and geo-coded to fit with latitude and longitude coordinates system as well as with the original satellite images representing the study area. Based on this environmental GIS database a desertification assessment and mapping was achieved. In order to analyze the desertification phenomena and other environmental parameters in the study area a layer of Environmental Mapping Units (EMUs) was prepared. The EMUs layer was formed using the land units layer, land cover classes and country area. Several desertification maps were produced, which reflect the desertification types persist in the study area: wind erosion, water erosion, vegetation degradation and salinization are the basic desertification maps, others are combinations of these basic maps. In term of statistic analysis 31% of the total land area (6.615 million km2) is considered as sand or sand dune, and not included in our analysis of desertification. About 27% has a slight to moderate desertification and 42% of the total land area is facing a severe to very severe desertification.
1. Introduction
Environment is considered as a worldwide phenomenon, which is characterized with sensitive equilibrium between its components; water, air and soil. The changes that take place in many areas of the world, as a result of human and other natural activities, could have the greatest impact on the environment.
The main goal of this research is to enable countries affected by desertification and drought to be associated with the generation of knowledge and techniques related to remote sensing and GIS, which are needed to solve their specifics problems in field of desertification control. And thus, to contribute to a better natural resources management and economic development.
2. Land cover of West Asia
The main land cover types occurring in the study area are listed in Table 1. Where we note that about 30% of the studied area (6653037 km2) are desert or desert rangeland and 32% is sand or sand dune. Followed by the permanent rangeland or marginal land, which represent 20% of the study area. The forest coverage constitutes 3.5% of the study area. The agriculture activity is divided into two main forms: irrigation activity and arable land, which represent 5.2% and 8% of the study area respectively.
3.0 Methodology
The environmental studies involved many types of parameters and variables, which are all dependant on each other, these variables could be related to water, land, atmosphere and people. So an integrated approach used remote sensing and GIS is the best methodology to take in consideration all aspects of environmental study, which is the case in this study.
Figure 1: land cover map of West Asia
Table 1: Land cover of the study area
The aim of this part of study is to construct a Geographic Information System containing environmental parameters influencing the regional and global changes, with focus on desertification/ land degradation problems in West Asia. More than 70 layers of information were collected or calculated then geo-referenced and entered into the GIS system.
These layers belong to five types of data:
- land cover, natural vegetation and ecosystems maps
- Soil types and derived soil properties (texture, organic matter, PH, Nitrogen %, …)
- Climatic data: includes rainfall (average of 50 years), potential evapotranspitation, aridity index and air temperature.
- Topographic data: digital elevation model (DEM) and derived data such slop, aspect , shaded relief, drainage network and elevation zones.
- Satellite data: 10-days composite data set of NOAA AVHRR 1 km resolution covering the period from April 1992 through March 1993 and others data on the period of 1995 and 1996. Monthly NDVI data, Average NDVI, Land surface temperature and albedo Vegetation condition Index (VCI =(NDVI - NDVIMIN)/(NDVIMAX-NDVIMIN)), etc.
- Land degradation and desertification layers (Wind erosion, vegetation degradation…)
- Socio-economic data
5. Environmental Mapping units - EMUs-
In order to construct the database for the prepared GIS layers, it is very important that this database represents a set of polygons or units describing the environmental characteristics of such units, we called this Environmental Mapping Units-EMUs-. This is called also Desertification Mapping Units (DMUs) when it is related to desertification mapping. Here we choose three factors to build up the database units: soil units, land cover classes and country area. In total we obtained 502 EMUs, which represent unique physical mapping unit, unique land cover class and belong to one country. Under the "Idrisi software" GIS and database structure, we extracted all the information from the GIS layer based on the EMUs, then information filled in to the database.
