Objective
- To contribute to the development of a global understanding of the desertification phenomena as a basis for strategies to combat it;
- To design an approach integrating the biophysical ground observations with remote sensing to detect changes occurring at the land surface;
- To link socio-economic information with the remotely sensed land surface changes using a GIS, towards the definition of global desertification indicators. /ACH_ACH_TXT
Results
- Identification relevant land surface parameters :
from the field investigations and from the analysis of the radiometric measurements gathered in the database, it has been established that the main land features related to desertification are :
- the soil surface aspect (soil material composition, colour and structure) such as presence of sand sheets, of soil crust , of microphytes;
- vegetation abundance and phenological state.
- Spectral characterisation of degraded land :
simulation experiments and real data have shown that the classical satellite-based monitoring technique using vegetation indices, is not suited to the steppic vegetation encountered in the area. Building on the results of field work, specific indices have been developed, characterising the land surface brightness and colour. The surface colour index is a powerful tool to distinguish sand sheets or crusted soils from surfaces in good condition.
- Appropriate technique for remote sensing of desertification :
land degradation cannot be assessed from one image, only the difference with a previous state can indicate if a change has occurred and in which direction. Corrections techniques have to be carefully applied to satellite images, both for the geometry and the radiometry, to allow comparison and computation of changes. As an example, on the Tunisian site an increase of colour index values indicates a sand sheets' extension.
- Application/ Documenting land surface changes in the test areas :
- in the centre of the Nile Valley (Egypt) : between 1990 and 1994 a dramatic extension of saline and bare soil has been observed whereas urban land used increased by 60%, but mainly gained from desert area (non irrigated);
- in the rangeland (Tunisia) : following land surface changes over 7 images from 1972 to 1993 clearly showed the dramatic extension of mobile sand (sand sheets and dunes) with the development of cultivation until the mid 8O's (dryer years) and a fast recovery of the areas treated with sand fixing barriers and exclosure after the more humid recent years.
These results demonstrate the feasibility of satellite monitoring of desertification progression or decrease under the effect of actions undertaken to combat it. This is a step towards the concept of a << desertification watch >> satisfying the needs for information driven by the implementation of the CCD (<
- Training / Collaboration with universities :
During the project several students have been offered training periods in the institutes involved :
- three students from ENIT (Tunis), on spectral signature analysis, processing and modelling,
- two Ph.D.s. in collaboration with the university of Paris 6, on sand movement in oasis environment, and on assessment of radar images' potential for desertification monitoring (thesis to be defended in early 1997).
First recommendations :
- Three meetings have been organised during the project to gather all team members to exchange information and to discuss techniques used. The final project meeting will allow to summarise them and to draw the conclusions and the final recommendations.
- However, three main directions for future improvements already appeared during the project :
- the need for long term studies : in the areas investigated the inter-annual climatic variability is high, only observations over a series of several years can distinguish long term trends from humid vs. dryer years effects;
- the problem of integrating socio-economic parameters : the existing data are scarce, heterogeneous and not organised on a spatial basis. The only spatial discretisation is the administrative boundaries that have varied considerably during the last decades. A strong commitment of demographers and social scientists is needed to gather the appropriate data before being able to compare them with remotely sensed land surface changes within a GIS;
- the need for a regional approach : results obtained with different techniques are difficult to compare, only a commonly agreed set of techniques will bring a regional perspective in desertification assessment.
Follow-up
- These first recommendations have been taken into account to conceptualise a follow-up of this AVICENNE project. As an answer to the need for a desertification monitoring tool capable of delivering information for environmental management at a regional level, a new program has been designed involving partners from all countries of the southern part of the Mediterranean basin. Building upon the results of our tests in Egypt and Tunisia it has been conceived in close collaboration between ORSTOM (France) and the Space Applications Institute (Joint Research Centre of the European Commission), particularly the EMAP unit of this institute, dealing with desertification monitoring in Mediterranean countries of Europe. This has led to the preparation, in association with the Sahara and Sahel Observatory (OSS, Paris), of a new proposal regrouping 13 institutions under the acronym of CAMELEO and submitted to the INCO-DC program in September 1996.
Two real case studies have been carried out in different arid environments of Egypt and of Tunisia, following the same main steps :
- Existing data inventory :
retrieving and reformatting existing maps (topographic, geological, ecological), meteorological data reports, aerial photographs, thesis, socio-economic data, ...
- Field campaigns :
acquisition of ground data on land use, land surface condition (soil and vegetation), measurement of radiometric properties of vegetation and soil at different levels of desertification have been done on both test areas over a hundred of sites, accurately localised using the Global Positioning System (GPS) or detailed maps.
- Spectroradiometric measurement processing :
a specific software has been developed (SpecPro) to perform spectra averaging, noise correction and resampling; the final output is accurate reflectance values of the target (plant / soil) in the bands of current existing remote sensing satellites (Landsat, Spot).
- Ground data organisation :
a user-friendly data base has been developed (SpecBase, implemented under Windows environment) to handle all ground-collected information: precise localisation, soil and vegetation description, laboratory analysis, spectral reflectance as well as field photographs.
- Remote sensing images inventory and acquisition :
different images' sources have been used to build an historical archive of Landsat MSS starting in 1972, whereas Spot data were acquired to document recent years.
- Satellite data base organisation :
images from the various sources have been put under a standard format and geometrically corrected to be superimposable.
- Establishment of a GIS :
acquisition of reference maps and digitalisation under Arc/Info, update of administrative boundaries.
- Socio-economic data :
incorporating existing data into the GIS has been tested.
- Satellite data inter-calibration :
using reflectances measured on the ground on specific reference target, images recorded at different dates have been radiometrically rectified to allow inter-comparing them.
- Change detection techniques :
two techniques of change detection have been developed, one in Tunisia on rangelands with medium scale imagery (Landsat MSS), and one in Egypt on land use with the more detailed Spot imagery.
- Long term change detection :
time series of images over twenty years built over the Tunisian test have been tested for the assessment long term trends in landscape changes.
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Coordinator
75010 Paris
France
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