In North Africa region, oases are among the most intensive and biotically diversified traditional agro-ecosystems in the world. Conserving local landraces and improving it in situ by using well adapted species to the local dry conditions maintains the high genetic diversity of crops in these systems and conserves water over non-adapted varieties. Also, empower people to use local knowledge and management strategies in order to improve local environments, to conserve the natural resources (water and soil) and to protect the polyculture systems in the oasis region is a road for durable development in the region that all the international experts advice to follow. The scientific guide subject of this chapter enhance the establishment of models that help in taking decision carefully and rapidly about actions to rehabilitate and to valorise the oasis product and environment with a participative spirit. In fact, after evoking the preponderant problems in the oasis of North Africa (soil and irrigation water salinity, agro-pedological constraints, water table rising, hydro-agricultural problems) I have presented the basic equations to describe the water, nutrient and salt distribution in different type of soil, under many irrigation systems that can be utilized inside the oasis and for all type of plants. Some equations presented can be used to monitor the ascent of water table at different time scales and to foresee the period to pull down its level according to phonological stage of plant. Also I have pointed out many formulations to determine the salt leaching requirement, to analyse the irrigation scheme efficiencies for the oasis and to calculate all the component of a drainage network very needed to avoid the problem of hydromorphy. I have putted on the radiative transfer equation, its application for studying the solar radiatif climate inside the oasis and to estimate the amount of solar radiation intercepted by every farming storey inside the oasis and by every plant inside the storey. I have introduced the reasoning to follow and the correspondent equation in order to deduce the productivity for the market gardening, fruit trees and date palms, to detect the eventual plant diseases before there spreads, to control phytosanitary situation of the vegetation in order to conserve the biodiversity, to test the best oasis architecture for an efficient use of resource by plants ( water, nutrients, solar radiation) and to propose the tolerant species to install, their densities and orientation. I have formulated the heat and mass transfer inside the oasis by the use of the thermal budget equation and the Ohm's law. The input parameters are the aerodynamic resistance, leaf boundary layer, stomata resistance of each sort of plant, geometrical characteristics of all the species, microclimatic factors. So we can easily determine the water really needed at hour scale hence monitoring the net irrigation requirement inside the oasis. The biomass productivity for each storey, very useful to search the best oasis architecture, can be estimated from the later formulation. Scaling up a result elaborated at leaf level to branch, from branch to plant, from plant to stand and from stand to a vast region like North Africa is an occupation for all the intervening. I have presented a general methodology and the results of its application for the traditional oasis of Tozeur (Tunisia). I have found satisfactory value for the transpiration by every storey and by all the oasis (Sellami and Sifaoui, 2003). The results of microclimatic factors measured at different levels inside the oasis of Tozeur (Tunisia) (Sellami and Sifaoui, 1998), their analysis and the empirical formulas established appear in the text. I have succeed to relate the global and net radiation intercepted, and the water transpired by every storey to the incident radiation measured above the oasis. A case study of two models established and validated for the oasis of Tozeur figures (Sellami and Sifaoui, 1999, 2008) were detailed. They treat the latent heat flux, sensible heat flux, biomass product and the solar radiative flux exchanged inside the oasis. The basic equation of the two models are those shown previously. Finally I have to signal that the content of this guide can be used in controlling and managing the oasis ecosystem risk in the research area. This can be done by discovering and monitoring many ecosystem risk indicators for the oasis. They must have clear meanings and can be compared in different oasis in North Africa region in the purpose to achieve more indicators, especially, for the interrelation between ecology and socio-economy (Rita and Andrea, 2006). I can't forget the fact that the equations presented and the models established can be integrated with new simulation models (François et al, 2007; Mike Austin 2007) which describe processes of agricultural ecosystems and predict plants species distributions in order to derive optimum management strategies like fertilising shemes and crop rotations.
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