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Using remote sensing technology and geographic information system modelling to detect sesertification
The multi-temporal coverage provided by satellite data facilitates the use of remote sensing images to monitor changes in land coverage and land usage over time. Remote sensing data and GIS technique is very critical in extracting reliable information that is important for assessing the environmental change quality in a given region. The purpose of this paper is to analyse how remote sensing (RS) and GIS has been used to monitor desertification or environmental change in different region of the world. To achieve this, an introduction covering the advancement of desertification and various definitions of desertification has been provided. Finally a literature review examining the current scientific knowledge of desertification; The previous examples and studies of how mapping and modelling based on GIS and RS detecting desertification are discussed.
Most of the great deserts of the world arisen as a result of natural processes in harsh dry environments that have occurred over long intervals of time. However, people, plants and animals have adapted to live in desert regions (UNEP, 2006). Desertification is a serious environmental problem that threatens the future of human beings. 6.1 billion hectares (i.e. 42.7%) of the Earth's surface are dry land of which 1 billion hectares are merely hyper-arid, arid, semi-arid and dry sub- humid lands desert out of the world land surface area of over 14 billion hectares that constitutes drylands, while the remaining 5.1 hectares are subject to desertification process and are expected to become desert in the future (UNEP, 1992). Most recent statistics estimates show that, annual productivity is lost from1.5 to 2.5 million hectares of irrigated land, 3.5 to 4 million hectares of rain-fed agricultural land, and about 35 million hectares of rangeland due to land degradation (UNEP, 2006).
Since the early 1920s, desertification has been identified as an environmental problem concerning environmentalists and the scientific community globally. However, only in the last five decades has the problem become a serious environmental issue due to its social and economic impacts (Xue & Shukla, 1993). Desertification has the greatest impact on developing countries, where poor agricultural practices lead to serious consequences of drought and famine (Sardinha, 2008), however, desertification also affects the developed nations due to unsustainable practices in agriculture and forestry from poor land management and weak government policies (Sardinha, 2008).
Desertification processes are not new and are not fully understood, but the features of the process of desertification are devastating for environments, economies and political stability of the countries in which they occur. As stated by the UNCCD (2007) ‘desertification, exacerbated by climate change, represents one of the greatest environmental challenges of our times’. Lack of proper action will cause many people worldwide be displaced by desertification and land degradation in the next few years (Sardinha, 2008).
Definition of Desertification
The term desertification has varied definitions. The United Nations Convention to Compact Desertification (UNCCD 1994) defined desertification broadly as land degradation in arid, semi-arid and sub-humid regions. More recent definitions refer to the processes that alter the productive deserts areas into non-productive areas due to unsustainable and poor land-management practices (El Hassan, 2004). NASA online (2012) refers to desertification as the reduction in productivity on the land that cannot be reversed when the land cannot support the same plant growth it had in the past, and the change is permanent on a human time scale.
Desertification is caused by natural occurrences, such as drought and human induced activities. These conditions result from various factors such as climatic variations and anthropogenic activities (Emagaili, 2003). Climatic variations make the fragile soils vulnerable to erosion and desertification (Emagaili, 2003). Human induced activities that result into desertification include over cultivation, overgrazing, deforestation and poor water management (UNCCD, 2004). Overgrazing animals and removal of vegetation cover for firewood collection accelerated desertification. On the other hand, overstocking of animals makes soil become compacted by hard-footed animals, hence becoming less able to soak up rain exposing the soils to agents of erosion (Emagaili, 2003). This further reduces plant growth.
Desertification is a serious environmental problem that threatens the future of human beings. Desertification usually affects the vegetation cover and increased soil erosion as the bare soil is exposed to wind and water erosion. The resulting soil degradation that commonly occurs in regions of grazing or marginal rain-fed farming leads to desertification. These create desert-like conditions that might be irreversible (Xue, 1993).
The impacts of desertification lead to serious threat to the world especially the developing countries and cause long-term problems in economic activity, human health and famine as well as food security. The consequences of desertification are diverse. For instance, it reduces the ability of land to support life, affecting biodiversity, agricultural crops and humans. The reduction in plant cover as a result of desertification results into soil erosion (Emgaili, 2003). Moreover, desertification affects the physical infrastructure, natural resources and the environment, national and global security. Desertification occurs as a result of increase in human population, intensive subsistence of agriculture practices, deforestation and overgrazing.
This literature review aims to examine the current scientific knowledge of desertification: The previous examples and studies of how mapping and modelling based on GIS and RS detecting desertification are discussed. Finally the gaps in the current knowledge and technique are highlighted.
Detection and monitoring Desertification by using remote sensing and GIS technique
There are many literature reviews discussing the usage of GIS and remote sensing, RS technologies in the field of desertification. The technique involving GIS and remote sensing provides an effective information management concerning changes in the relationship between soil, climate, water, vegetation, animals as well as men over a longer period of time on smaller and larger scales. The processes require proper measurement as well as interpretation. These could also be evaluated to a large extent with the help of remote sensing techniques and GIS.GIS and remote sensing have been applied to combat desertification in some countries around the world.
Remote sensing data is used as input into a geographic information system (GIS) for further analysis and comparison to other data. Sensors with improved resolution are able to recognize more details for better results and precision. The users can select the band or bands required for their particular needs (Abbas & Khan, 2006).
Aadvantages of using RS technology include saving time, wide coverage as satellite remote sensing provides the only source for this data when the data is required for wide regions. The multi-temporal coverage provided by satellite data facilitates the use of remote sensing images to monitor changes in land coverage and land usage over time. Some examples for several studies have been taken desertification and degradation using RS data, and show different results:
According to El-Hassan (2004), there are number of indicators which can be determined by the use of remote sensing techniques with an aim of monitoring desertification. Example of these factors include livestock size, population pressure, deforestation, soil salinity, type of building materials, cultivation of marginal land, number and distribution of water wells, migration of livestock, declining ground water level, and bush encroachment and species change. These indicators were used by Harahsheh and Tateishi (2001) in their research to monitor desertification using remote sensing and GIS in West Asia region (El-Hassan, 2004). They used satellite data set of NOAA AVHRR covering the period of April 1992 to March 1993, and another data set covering the period 1995 to 1996. The results of the research indicated that 31.3% of the West Asia was subject to severe vegetation degradation, 19.2% was subject to moderate vegetation degradation, and 8.5% and 8.8% were subject to very severe and slight vegetation degradation respectively (El-Hassan, 2004).
Collado et. al (2002), used remote sensing digital images to monitor desertification processes in Argentina (San Luis Province). The Landsat images which were used in this study were acquired in 1982 and 1992. Both images were geometrically and radio-metrically corrected and then multi-temporal techniques were used to emphasize areas experiencing greater degradation. Spectral un-mixing of the sand components, water and vegetation facilitated the analysis of areas of heterogeneous cover from satellite images. Satellite images which were obtained identified re-vegetation trends and variations in water bodies as a result of changing rainfall and land use patterns, and dune movement.
Al-Harbi (2010), used Landsat TM 5 and SPOT 5 data to monitor Tabuk region in Saudi Arabia with an aim of assessing the agricultural land usage changes through a period of twenty years (from 1988 to 2008). Field and laboratory works accompanied the Landsat TM 5 and SPOT 5 imagery to achieve fine analysis of this region. The satellite images which were analyzed indicated a considerable increase in the spatial extents of the agricultural land usage. Through the analysis, the researcher was able to deduce that the spatial extents of agricultural land usage had increased by 10 % during that period (1988 to 2008). This increase took place despite the fact that this region has a big percentage of bare soils.
This research indicates that, it is possible to use multi-temporal satellite data to understand the interchange of agricultural land usage with the surrounding bare grounds with an aim of modeling the likely environmental changes in the future for a certain region. Integrating field measurements and remote sensing, therefore, assist in acquiring critical information which can aid in monitoring the environmental changes of a given region. This research was able to demonstrate that remote sensing techniques such as Landsat TM 5 and SPOT 5 imagery can be used to monitor the agricultural land usage changes such as spatial cropping pattern, spatial extents, and spatial locations (Al-Harbi, 2010). Therefore, satellite imagery can be considered to be a powerful source of information that helps in studying vast areas with the same observation parameters, and obtain a regional perspective that is difficult to be achieved by the use of other methods such as conventional ground surveying. This research also recommended that socio-economic data of a given region should be combined with remote sensing techniques to assist in identifying the impact of the agricultural land usage on water content and soil properties (Al-Harbi, 2010).
According to Adjei (2009),remote sensing alone cannot yield reliable results concerning environmental monitoring and inventory of natural resources without the availability and possibilities of superimposing and using relative ancillary information. To remedy most of the weaknesses presented by remote sensing (RS), you have to combine it with ancillary information and spatial models in a GIS environment. Modern development in GIS is aiming at integrating remotely sensed data with data from different sources in order to improve the quality of the resulting thematic maps (Adjei, 2009).
Yanli et. al, (2011), conducted a research focusing on the environmental change indicators in China ( the northern part of Shaanxi Province). They used remote sensing and GIS software to classify Landsat TM images of 1987 and 2002 into five land cover and land use classes. The results of this research indicated that 28.4% of land had high environmental change, 34.2% had moderate, and 37.4% of land had low environmental change (Yanli et. al, 2011). General conclusion of the study was that, this region is exposed to a high risk of environmental change.
Hein & De Ridder (2006) examined the ecological assumptions underlying the use of satellite imagery to analyse degradation of the Sahel. They used the variability of the rain-use efficiency (RUE), which is often used as an indicator for the state of the vegetation cover. They pointed that satellite data not showing long-term degradation in the Sahel as premature.
Remote sensing data and GIS technique is very critical in extracting reliable information that is important for assessing the environmental change quality in a given region. This paper has highlighted desertification issue and elaborated how this problem is spreading rapidly. A relevant literature covering the usage of remote sensing (RS) and GIS usage in monitoring desertification in different regions of the world has been discussed. Although remote sensing data cannot be used alone to monitor desertification, it is of equal importance in this field. Combined with it ancillary information and spatial models in a GIS environment, it proves to be the best method to monitor desertification.
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