This research is based on spatial, integrated, multidisciplinary and multi-actors approaches, in order to assess the environmental situation related to land degradation and qualify the risk of desertification.
The present work consists on comparing different types of spatial information plans for a better understanding of the overall mechanisms of the territorial system, but also to distinguish the specific local states of this system, represented in the field by different types of landscape. The landscape typology is based on the expertise and participation of resource persons (the heads of villages), researchers from different disciplines (ecology, geography, agronomy, geology, etc.), and technical decision-makers (agents of regional stations of agricultural development). It allows the evaluation and the monitoring of desertification risks at the local scale. The landscape types are then considered as an operational level for decision making [17
Our work consists primarily to integrate, at the local scale, socio-economic and biophysical information, through the LEIS environmental modeling. This latter mobilizes the landscape as resulting from the interactive dynamics of the system that can be recreated and represented spatially following a formalization and geospatial rules. It structures the territory into SRU (spatial reference units) on which we can quantify the natural resources vulnerability levels or indicators face to multiple uses of these resources. These spatial indicators are thus constructed from spatial integration of allocation rules and practices of resource usages with the local biophysical conditions and availability of resources. The SRU come from the intersection of three geographic plans. The first one is built through remote sensing, geographic information system and field measures. It represents the biophysical characteristics (land cover, morphology, soils, etc.). This information plan expresses the functioning of ecological systems through their resource production levels and it structures the territory to homogenous units called landscape units (LU) [13
]. The second information plan represents the multiple agriculture practices at the local scale. This plan is built by the way of field surveys and expresses the strategies of local actors/farmers and their multiple uses of natural resources. It structures the territory to homogenous practices or combined practices units (CPU) [9
]. The third plan defines Climatic Units (CU), which are spatial units receiving the same average amount of rain over a period of more than thirty years, generating a well-defined production resources and specific coping strategies (specific logic of land use, production strategies, resources uses, etc.).
The SRUs have then the information of the nature, the availability and the level of uses of natural resources at the local scale. The vulnerability of resources is evaluated on these SRU by type of use (agricultural, pastoral, forest) by integration of the three information plans (LU, CPU and CU). It represents the ratio between the availability of resources (A), and the uses of these resources (U) for each type of use, but also for all types of use (Figure 2).
Clusters have been carried out in order to build three classes of indices, which can be associated with a qualifier, indicator of pressure degree of the multi-use of vegetation resources. This work is based on statistical classification methods. The three classes identified are:
• Class 1:
Low to medium risk of multi-use of resources, derived from a use which does not access 40 per cent of the total vegetation availability (U/A < 0.4);
• Class 2:
High risk of multi-use of resources, corresponding to a use which can affect 80 per cent of the available resources (0,4 ≤ U/A < 0.8);
• Class 3:
Very high risk of multi-use of resources, corresponding to a use which can exceed 80 per cent of the available resources (U/A ≥ 0.8).
Several works defined different ways and used different criteria for the delineation of land degradation and desertification risks categories, such as the example of land mapping in upper catchment of river tons in India [19
], which is based on vegetation cover, slope and erosion status used as criteria for the delineation of four major land degradation categories. Also, the work of Ouessar and al. [3
] carried out in the Jeffara region in Tunisia to assess the sensibility to desertification according to vegetation cover and soil characteristics, with the delineation of four levels of sensibility.
For our case, using the modeling of driving parameters/forces such as population growth, production needs, climatic conditions/changes, it is possible to provide, according to predefined scenarios, new pressure or vulnerability indexes established on the SRU.
In order to provide useful environmental information and improve the system resilience, the work uses another approach. It consists in applying several scenarios of climate change and human pressure to analyze the effects of each cause/driving force on the vulnerability of natural resources at the local scale. The objective is to assess the effects and to identify the major cause of vulnerability for each landscape type in order to better target the actions against desertification.
The Human and biophysical scenarios selected for the analysis of this sensitivity are as following:
Two scenarios have been proposed:
• Scenario 1
Increasing the level of mechanical intervention:
It may be due to changes in soil technical labor, farm equipment or fertilization. It can also be generated by a growing demand for agricultural labor or increased investment.
• Scenario 2
Doubling the population in the study area:
The objective here is to analyze human pressure on natural resources in case of a major population increase up to the doubling of the population.
We have tried to identify some of landscape sensitivity to desertification based on examples of changes that may affect the characteristics (attributes) of the "Climate" factor. Two scenarios were developed on the basis of two types of climate change.
• Scenario 1
Shift of the isohyets:
The assumption of a shift of the isohyets toward the west of the watershed has been taken into account in the application of the model. This type of change creates a new spatial distribution of annual rainfall in the catchment area and therefore impacts on local land uses and agricultural production.
• Scenario 2
Succession of four dry years:
Based on the investigations of the field, but also in our negotiations with development actors, we tested the impact of a sequence of four successive dry years on the vulnerability of natural resources. Most of changes that affect agricultural, pastoral and forest activities following these climatic conditions were evaluated in collaboration with development actors. Over the four years, agricultural, pastoral and forestry production can reach a decrease of 70%. Under these conditions, farmers will multiply supplemental irrigation to secure at least arboreal capital, especially the olive tree. The increase in irrigation rates during the four dry years is fixed at 66%.
Resource availability scenarios:
In this case two scenarios were developed on the basis of a possible change in the availability of resources.
• Scenario 1
Follow the current water and soil conservation policy:
The objective of this scenario is to analyze the impacts of this strategy on the risks of desertification in the study site (ex-ante analysis). Most of changes that affect agricultural, pastoral and forest activities following this condition were evaluated in collaboration with development actors. We arrived at the consensus that this condition can increase agriculture production by 20%. This increase, following the creation of new crop fields, will be at the expense of rangeland areas and the potential of natural vegetation. Then, we anticipate a joint reduction in availability of natural resources around 40%.
• Scenario 2
An example of a conservative policy:
This scenario is a proposal of a natural resource conservation planning through a restrictive intervention, affecting mainly pastoral and forestry uses of resources (grazing and uprooting of wood). This is a law that limits the use of rangelands and wood to 20%. These proposals are based on diverse experiences and knowledge of institutional actors on different land use policies and government strategies in term of natural resource management in Tunisia.
Finally, all of these changes were considered when testing predefined scenarios.