In North Africa, the majority of the oasis are constituted from private small gardens considered as familiar fishpond in which we found an important diversity of original plant: Date palm, olive, almond, fig, apricot, pomegranate, orange, vine, lucern, wheat, oats, barley, market gardening. Those particular agro-system are dispersed on the Sahara and they occupy more than 200 000 ha (Bob Brac 2002). The cited species can be found in the same field inside the oasis of the region. The oasis then can be considered as an agricultural genetic biodiversity reserve. By considering the local management of agricultural biodiversity, the region of North Africa is distinguishable by the fact that it shelters a rich flora of about 4000 vascular species in which 20 % are endemic (Bob Brac 2002). Those species are familiar and very adapted to arid climate. The Sahara of this region is an arid and very hot desert. The rainfall is les than 100 mm/year and the temperature can reach 50 °C in summer. The biodiversity of the region is judged to be a resources of species that resist to the drought and salinity (Bob Brac 2002). Those species, considered as Mediterranean, are very searched to ameliorate the agriculture product if installed in other places at worldwide level (Genetic Resources Action International 2003). The urbanisation of the oasis has appreciably participate in the evolution of the farming system. In fact, in the last two decades the oasis agriculture has been transformed from subsistence farming to market agriculture. The dates are the principals oasis source of revenue. It occupies the first rang of agriculture product exportations revenue for Algeria and it is the second for Tunisia. For the oasis the principal scourge is the bayous disease. The bayoud causes the decay several millions of trees (National Institute for Agricultural Research. Thème 1, 2005). The incidence of this disease is stressed for the economy of the small farmer. The best variety like Deglet Nour and Mejhoul are very susceptible to this disease. Elaborating models that permit to detect the eventual plant sickness and to control the vegetation sanitary at the scale of hour or day is primordial. I will give later the basic equation need for this purpose and a case study of a model established for the oasis of Tunisia (Sellami and Sifaoui, 1999). The preservation of local farming inside the zones of ancient cultivation and the protection of endemic species depend on upholding all the agrarian system inside the oasis. The aridity of the environment causes soil degradation, desertification and the risk of lost of agriculture genetic diversity. To be adapted to the resources diminution, the population exercises more pressure over the environment by intensifying the productive system and deforestation. The problem of non scientific and non controlled overexploitation of those spaces (oasis) judged as marginal and fragile exert a real menace that can cause social crisis on all the region. In fact there is a tendency towards the withdrawal of oasis and the rural depopulation (Bob Brac, 2002).
After trying conservation management of genetic sources ex situ ( a selection at laboratory level in order to ameliorate the product of plants) there is an orientation toward the conservation of genetic resources in situ in order to have plants more tolerant to local climatic conditions, to local techniques of agriculture water management and to natural diseases (Alain 2006; Bob Brac 2002, Genetic Resources Action International 2003 ). To conserve the environment in situ, many North African countries have installed many parks and protected surfaces over vast regions (Tassili Park Algeria). Those parks can not secure definitively the genetic resources because of there limits in space. In the arid zones the sources are dispersed over very large surface what makes it difficult to protect them, on both, technical and administrative sides. Added to the fact that generally there is expulsion of local population from those protected regions what generate conflicts with traditional farmers. This will influence badly later when to convince them to install those genetic resource either to try the amelioration or to reinforce the local and export markets. The best is to make an approach based on a participative management inside all the traditional oasis of the region. So every oasis will be a reserve of genetic resource and the farmer will act as a manager of the conservation operation (this Scientific guide will be very helpful to him). We will then produce a species tolerant to local conditions (salinity, solar radiation, hulmidity...) and adapted to traditional agricultural methods used. If we want to test a new technical, new nutrients, new quality of water, new system of production in the purpose to have a particular product that the market needed we can do that without any problems. The durable management of ecosystems is very difficult. To be done successfully a scientific studies and a formulation of problems must be realised. So every time we need to make experiments that are generally hard, difficult and time consuming we can apply those equations. Also, most of time we have to make a quickly decision to propose, accept or refuse a scenario of management, the set of equations that we will propose describe well the situation and give an estimation close to real values when thinking to establish models.
2.3 Agro-Pedological Constraints and Problems Linked to the Hydro-Agricultural Planning
The soils in the region are generally gypseous and saline. Depending on the dominance of the content of gypsum we can found the following class of soil: mineral brut, calci-magnesite, salsodic, sandy, gypseous, calcareous-gypseous, sandy-silty (Ben Mohamed June 2002; Bob Brac 2002; Genetic Resources Action International 2002; Kadri and Van Ranst, 2002; Minister of Environment and Durable Development, 2005; National Institute for Agricultural Research Thème 2, 2005). The soil has generally a fine texture with a high infiltration rate and feeble hydrous retention capacity. We can't forget the effect of soil deposit and soil erosion linked to wind dynamic. Due to those physical characteristics, the irrigation gift must be given frequently in small doses, a kept drainage network is inevitable and the presence of organic substance is obligatory (Sécheresse Info, 2007). Basic equations to simulate and to model those parameters for every type of soil and every species will be given later. I must signal that for the majority of the oasis in the region we found at the soil surface a continued thick gypseous layer. This is owing to water table level close to the surface. Installing local varieties of date palms, fruit trees and market gardening tolerant to salinity is necessary (Genetic Resources Action International 2002, 2003). Also modelling the efficient level of water table to reach by applying a holding back technique for every species and sort of soil, in each season and phenological stage is highly useful for agricultural water management and for ameliorating the farming productivity inside the oasis. We will write out in the following parts many formulas that can help to elaborate models. The abundant use of salt water for irrigation and the water table ascent (hydromorphy) lead to soil degradation, poor harvest in quality and quantity for the three stories, net diminution of the date palm vigour (number of green palms by foot, height and diameter of the trunk). Those phenomenon are very observed in the oasis of the region (Bob Brac 2002). Found methods to estimate the exact amount of water that the plant really need at the level of hours by considering the local conditions and the water table rising help deeply in biodiversity and soil conservation. Also thinking to calculate the amount of water needed for leaching at a small scale in space and time is needed for the growing diversity project inside the oasis. The utility of an efficient drainage system computed according to plantation architecture, season and traditional know how is more lucid. The equation that we will give can be very helpful. An other enquiry that we can note easily for several oasis of North Africa is Problems linked to the hydro-agricultural planning. In fact, soil salinity and risk forecast have not been sufficiently considered when implanting new oasis. The hydro-pedological and arrangement aspects that influence the productivity (salinity, hydromorphy, fertilisation, agricultural labour, trimming, harvest...) were neglected during all the project steps: the conception, realisation and rehabilitation (Bob Brac, 2002; INRA, thémel, théme2, théme3, 2005; Sécheresse Info, 2007). In the case of the oasis where the water table and the irrigation water are distinguished by there high salinity, we must expect to study the interaction soil-plant-atmosphere by elaborating models in order to have a success in the hydro-agricultural planning and to degage increment of the productivity in the irrigated plots existing in the oasis. Case studies elaborated for the oasis of Tunisia with the fundamental equations will be flaunted (Sellami and Sifaoui, 1999, 2007).
2.4 Importance Attached to the Oasis of North Africa: Projects Realised or Currently Under Execution
The environmentalist at international level attach too much importance to the oasis of North Africa region. It is considered for them as a genetic reserve for biodiversity (Association Network for Durable Development of Oases. 2002; Bob Brac 2002; Genetic Resources Action International 2002, 2003). We must signal that, for a question of nutritious security, there is many projects and programmes of re-establishment and rehabilitation of the agro-ecological systems of the oasis initiated by public power like Non governmental Organisation(Associative Network for Durable Development of the Oasis, 2001) and International organisation (Alain, 2006; Global Environment Facility's Small Grants Programme, 2006). Those initiatives encourage the dynamic management of local varieties that the farmers follow through the use of there farming traditional systems to surmount the local conditions constraints (soil, water, climate). The cultivators can then contribute in the selection of a diversified material adapted with different environmental conditions (resistance to parasites, tolerance to drought.). The field observation shows that the oasis farmers have conducted a selection and they have obtained tens of tolerant plants. There is an International tendency to recognize their right to be patented in order to encourage them (Bob Bac, 2002). A scientific guide as we are presenting in this work can be very helpful to all intervening. Inside the oasis of the North Africa the cultivators have common problems, challenges and occupations. In fact we can found problems linked to hydro-agricultural management, ways to minimise the consequences of irrigation with salt water, questions of irrigation water availability and methods for its rational use, constraints of soil pedology and agricultural management techniques for the oasis ecosystem. Also there is a regional request to elaborate a preventive manual for rehabilitation and valorisation actions inside the oasis (case of Rjim Maatoug oasis for Tunisia). This manual can be used to establish land-use plan and programme curriculum of water for the oasis in North Africa region. In this manual we must found a description, a diagnostic and exploitation conditions of the hydraulic systems inside the oasis. We must also propose many scenarios of agriculture water management inside every oasis. Also we must make a simulation of management and operation of irrigation and drainage networks. It is important to think to sewage system and storm water drainage, wastewater disposal and treatment, reuse of treated wastewater, protection of aquatic systems. Many specifications have been launched searching design offices to elaborate those manuals for many oasis in the region. Many ONG have started actions of rehabilitation of some oasis in North Africa, we can cite here:
1. The project of the development of the oasis of TAMEGHZA have been initiated by the Non-government Organization ASPHADT (Association pour la Promotion Humaine et l'Auto Développement á Tameghza). The project belongs to the Operational program on sustainable land management (land degradation, biodiversity) and it is on hand (Global Environment Facility's Small Grants Programme, 2006). This project has as target the rehabilitation of the oasis by intensifying the second (fruit trees) and the third (market gardening) plantation stories, realising a programme of soil improvement and valorisation of water resources and oasis products with an approach participative and finally introducing the oasis in local and regional eco-tourism tour.
2. Agricultural cooperation agreement between Tunisia and Monaco principality to valorise the Oasis of Ras El Ain Nefta signed the 07/09/2006. It concerns the preservation of natural resources of this oasis, to contend against erosion and desertification and to create a space for ecological tourism. This cooperation is a part of the National plan to protect and valorise the traditional oasis of Tozeur in order to ameliorate its productivity (Investir en Tunisie news, 08/09/2006).
3. An agricultural study effectuated by the department of the environment for the Oasis of Nefzaoua. This work aimed to evaluate the crop production constraints at all levels (soil, plants, climate, social effect) and to propose scenarios of sustainable development (Kadri et al, 1997; Kadri A. and Van Ranst E., 1998, 2002).
4. A proposition to create new oasis in Regim Maatoug and Ibn Chabbat in the purpose to increase the agricultural productivity (Minister of Environment and Durable Development, Tunisia, 2005).
5. For the oasis of Chenini Gabes a rehabilitation action started by a French ONG (ex name is CIEPAD now it named CARI) since 1993/1994 (Associative Network for Durable Development of Oases, 2001). The objective is to put a vocational training and to give a technical support for the oasis farmers concerning the agro-ecology, restoring and fertilisation of soil, water economy, biodiversity, valorisation of oasis product, oasis tourism. This action is a decentralized cooperation backed up by the CCFD, general committee of Hérault and the MAE. This development project is now under the direction of a local ONG (ASOC: association de sauvegarde de l'oasis de Chénini).
1. A project to reinforce the water natural resources and to valorise the biosphere reserve in the oasis of the south of Maroc (Alain, 2006). In this project they propose to rehabilitate the irrigation system and revive a traditional technique of drainage called "Khettarats" inside the palm grove of Ihandar (Ferkla El Oullia, Tinjdad, Errachidia, Maroc) (Association Oasis Ferkala for Environnement and heritage. 2006). Also this project has in view , to implant new oasis, to conserve those existing by valorising of oasis product, to remedy to soil salinity, running aground and erosion problems and to qualify the human potential living from the palm grove farming. The oasis valorisation program for the Morocco is supported by local and international organisations ( Department of territory planning, l'ADS, l'ORMVA/TF, Water and forest Commissionership, l'INRA, the FFEM, the Principality de Monaco, le PNUD, the general council of l'Hérault et CARI association).
2. The Oasis Area of Morocco's Ziz Valley farming is based on traditional, intensified, polyculture systems (date palm, fruit trees, olive, alfalfa, cereals & vegetables) developed in response to high population density, limited irrigation and environmental factors (drought, salinity, and high temperature)(Alain, 2006). Many experts judge that the farmers of this oasis agroecosystem, by using their traditional knowledge, and in the sharing of improved methods they have succeed to improve and conserve in situ a genetic resource well adapted to local dry conditions (INRA, thème1, thème2, thème3, 2005).
A Program of an agricultural redress of the oasis after the long drought of the years seventy that the Mauritany undergone (Cristiana, 2003). The project points to increase the agriculture productivity inside the oasis (date palms, market gardening and fruit trees) by encouraging the local diversities, to contend the desertification, soil degradation and salinity and water table descent, to ameliorate the vital conditions of the oasis farmers and to reinforce the institutions concerned with services, planning and financing the farming communities. The target of the project is 11 000 farms inside the oasis (problem of parcelling) that nourish 230 000 persons. Those farms are concentrated on five regions from twelve that the country allows ( Adrar, North of Tagant, Assaba, the two Hodhs).
By analysing the objectives of those projects we degage that the national, regional and international communities have the same challenges and occupation about the oasis. There is an agreement for the socio-economical part that the oasis play in the region of North Africa added to its leading role as a genetic biodiversity reserve at International level. We can say that there is many works to do because of the large area of the oasis and also because we have to study the impact on the agrarian-system of each activity realised inside every plot. So when extrapolating the results or scaling up to large zone we take the positive acts and avoid the negative ones. As we say "it's in the heat of battle that we learn." Our guide will be very helpful to them for both sides. Also this guide is fruitful when thinking to open out quantitative methods to understand environmental problems for the oasis ecosystem in North Africa region. These methods give the opportunity to establish and develop many indicators to study and analyse the oasis ecosystem risk at local and regional scales (Rita and Andrea, 2006; Wei-de LI et al, 2007).
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