Oasis of Tozeur

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The models established and experimentations realised for the oasis of Tozeur deal with many subject that concern the agricultural water management: solar radiation intercepted and shared between canopy species inside the oasis, plant productivity and transpiration, heat and mass transfer, biodiversity conservation by simulating the water really needed by every species and by detecting the eventual diseases that can emerge at hours scale, the optimal arrangement between plants, there orientations and densities for an optimal use of resources (Sellami and Sifaoui, 1998, 1999, 2003, 2008). Those models have been tested only at local level while the principal test of their generality and their significance in climate and climate impacts is in scaling up to large regions like North Africa region. Also the importance of the work done for the oasis of Tozeur is bright when reminding that the International Scientific Community are interested by scientific work that combine mathematical modelling with ecology and management of the environment and its natural resources and the scaling to vast regions. They give priority to the use of ecosystem theory to quantify the reaction of ecosystems to perturbations (impact of man's violent activities), to predict plant species distribution at landscape level and to analyse the dynamic biodiversity-landscape.

6.1 Results from the Measurement of Microclimatic Factors and Sap Flow Transpired

The field in which we have conducted the experimental protocol is a traditional oasis of about one hectare surrounded by others belonging to other farmers (problem of parcelling out). At the middle of this plot we have installed the 12 m mast with the apparatus to avoid the fetch effect and where the three stories exist and are not traversed by passages or soil canals. This form is repeated in the surrounded oasis. At levels 2, 5 and 12 m of the mast, we fixed three stems of about 2 m length. Every stem included a net pyrradiometer, a pyranomater, a cup anemometer, a temperature probe and a humidity probe. A net pyrradiometer placed 30 cm from the ground to determine the net radiation transmitted across all the oasis and that reaches the soil. The global radiative flux transmitted to the soil is measured by a set of six pyranometers installed arbitrarily to account of horizontal heterogeneity in the canopy structure. The average of the data loggers by those six probes represents the total global radiation transmitted across all the oasis. The probes mounted at 12 level deal with the total net and global incident radiation received above the oasis. The values recorded at that level gives an estimation of, respectively, the amount of water transpired by all the oasis and the total amount of photosynthetic active radiation used for the photosynthetic process inside the oasis. Those installed at 5 m provided measurements of net and global solar radiations transmitted across the palm storey and received by the fruit trees. The sensors placed at 2 m above ground gave us the net and global radiation transmitted through the fruit trees storey and entertained by the market gardening. The ambient air temperature, humidity and wind velocity were measured with the corresponding probes at different levels( 2, 5 and 12 m). The radiation intercepted by each farming storey in the oasis is obtained by the difference between radiation measured at the level right above (received radiation), and that measured at the level right below the storey (lost radiation). The radiation intercepted by the total oasis is the difference between radiation measured at 12 m and the radiation measured on the ground. The efficiency for the intercepted radiation is the ratio between intercepted radiation and the sum of input radiation above the farming types. The density of sap flow within the xylem of the palm grove and the fruit trees, expressed on a sapwood area basis, was monitored continuously during the experimentation period. The total sap flow in the trunk was obtained by the cumulative products of flux densities and associated cross sectional areas. The mean sap flow in a stand and the mean sap flow density are given as the arithmetic average of those measured on each trees. The canopy transpiration was estimated by multiplying the mean sap flow density and the cross sectional sapwood area of the stand per unit of the ground.

An hourly and daily evolution of all climatic parameters and of the plant transpiration (an approximation of water needed by plants) at three levels inside the oasis were realised. A quantification of the amount of global radiation, net radiation and photosynthetic active radiation (needed to determine the oasis productivity) intercepted in every storey were determined. The analysis of the profiles of net radiation, global radiation and sap flow at different level inside the oasis permits the establishment of divers relationships for the three plant storeys (Sellami and Sifaoui, 1998, 2003):

• The daily global incident radiation intercepted by date palms is equal to 14 % of the global incident radiation received above the oasis about 640 w/m2 per day from which 307 w/m2 used for photosynthesis

• The daily global incident radiation intercepted by fruit trees is equal to 19 % of the global incident radiation received above the oasis about 853 w/m2 per day from which 409 w/m2 used for photosynthesis

• The daily global incident radiation intercepted by market gardening is equal to 30 % of the global incident radiation received above the oasis about 1380 w/m2 per day from which 662 w/m2 used for photosynthesis

• The daily net radiation intercepted by date palms storey is equal to 19 % of the net radiation received above the oasis about 0.77 mm per day

• The daily net radiation intercepted by fruit trees storey is equal to 20 % of the net radiation received above the oasis about 0.81mm per day

• The daily net radiation intercepted by date palms storey is equal to 42 % of the net radiation received above the oasis about 1.66 mm per day

• The daily net radiation intercepted by all the oasis is equal to 81 % of the net radiation received above the oasis about 3.24 mm per day

• Daily transpiration, measured from sap flow method, for date palm storey represents 32 % from global radiation measured above the oasis

• Daily transpiration, measured from sap flow method, for fruit trees storey represents 21 % from global radiation measured above the oasis

• Daily transpiration, measured from sap flow method, for all the oasis represents 53 % from global radiation measured above the oasis

• Daily transpiration, measured from sap flow method, for date palm storey represents 59 % of net radiation intercepted inside the oasis and 53 % of that measured over the oasis

• Daily transpiration, measured from sap flow method, for fruit trees storey represents 37 % of net radiation intercepted inside the oasis and 33 % of that measured over the oasis

• Daily transpiration, measured from sap flow method, for all the oasis represents 96 % of net radiation intercepted inside the oasis and 86 % of that measured over the oasis.

• Hourly transpiration, measured from sap flow method, for date palm storey represents 43 % of net radiation intercepted inside the oasis plus 0.15

• Hourly transpiration, measured from sap flow method, for fruit trees storey represents 21 % of net radiation intercepted inside the oasis plus 0.15

• Hourly transpiration, measured from sap flow method, for all the oasis represents 64 % of net radiation intercepted inside the oasis plus 0.15

If we know the optimal need of the oasis plant on heat and water we can be repeating those measurement on other places of the same plot or other fields where there is different disposition of plant deduce about the best architecture to an optimal use of resource. The optimal requiring of the oasis plant on heat and water can be deduced by laboratory research or by field observation. In which oasis or in which zone of the same oasis the quality and the quantity of the product is the best to have an idea about probably the best disposition and the optimal need of temperature, radiations, humidity.

6.2 Results of Modelling inside the Traditional Oasis of Tozeur

The models that I have established for the traditional oasis of Tozeur concerns both the solar radiative exchange (Sellami and Sifaoui, 1999) and the heat and mass transfer (Sellami and Sifaoui, 2008). Considering the fact that the detailed architecture of the oasis is largely unknown and the nature of transport in oasis canopies is not fully understood, the models treat every storey of the canopy as a homogeneous turbid layer. In every vegetation layer they distinguishes between the interception of direct, diffuse and scattered radiations and they emerge the aerodynamic resistance and the stomata conductance of all the species. We have to signal that the basic equations of the two models are those presented in paragraph V and that the one is open on the other. In fact, plant growth and biomass production depend on leaf. photosynthesis, stomata conductance, solar radiation, sensible and latent heat flux partitioning.

For the model of solar radiative transfer, special attention is given to light sharing between the three crop stories and spatial variability of light transmitted through them. We had been able to determine the amount of solar radiation intercepted by each crop levels as function of time and space and to estimate the mean solar radiation that penetrates inside the vegetation from above-canopy measurements only. The calculus need the knowledge of the leaf area index and leaves transmittance and reflectance, so we have measured them. Those parameters, considered as input for our calculus can be accounted as out put of the model when we think to use it to detect the eventual diseases of plants or when to search the optimum plant densities and the best tolerant species to install. After comparing the modelled values to the experimental data, we had signaled a good behaviour of the model.

For the model of heat and mass transfer, we have succeed to determine the evolution of sensible and latent heat flux at hours scale and to quantify the biomass production inside a traditional oasis in the south of Tunisia. Temperature in and above the canopy also humidity, wind velocity, net, global and photosynthetic active radiation and sap flow within the xylem of date palm and fruit trees are included in the mathematical treatment. For validating our model, we have compared latent heat flux, sensible heat flux and biomass production predicted to respectively sap flow data, thermal budget and photosynthetic active radiation measured inside the oasis. We can say that the simulated values are on the verge of those measured. While, due to the fixed position of the sensors and the poor documentation about the detailed geometrical structure of the trees inside the oasis, we notice some difference between measurement and calculus especially for the market gardening. The quality of the two models can be better, in order to make the output more close the values measured, if we can sweep more space by mobile sensors and if we use more sophisticated method to determine the geometrical structure of the trees inside the oasis and their stomata resistances. As it is this work provides an initial attempt at modelling the penetrated radiation inside a vegetation with three production levels, a tool for analysing the light competition within intercropping and a method to describe the heat and mass transfer inside a traditional oasis for which the correspondent architecture is preponderant in North Africa region. More, the models are important to test absorption efficiency and the farming productivity in different cases of structure and row distances, to choose the best arrangement and to maintain the equilibrium of the mixture. Although the problems and challenges of those models are manifold, it is clear that more informations are needed before the goal of applying it to all the oasis of North Africa can be achieved. Those informations concern principally the architecture parameters of a representative number of oasis in the region of North Africa, microclimatic parameters inside the majority oasis, physiologic properties of the tolerant plants to local conditions. So it's a hard research work that must be done in order to establish a real walking encyclopaedia for the farming inside the oasis of the region. These later is considered as tools for the two models when used to test the best disposition between plant for an optimal use of resources by changing the parameters concerning the plants ( turbulent diffusivity, stomata resistance, rate of CO2 assimilation, leaf area index, inclination of leaf, its disposition on the branch, dimension of the trunk, distances.), to detect the indicators of possible plant diseases by monitoring the optical properties of leaves and to determine the photosynthetic active radiation needed to estimate biomass product. This analysis can be done at the scale of a storey (market gardening, fruit trees, date palm) and it can be refined at plant level inside the storey (which kind of plant to which architecture). Finally I have to evoke the fact that the two models can be parameterised largely with remote sensing data when predicting future oasis productivity on large scale. However, the diversity of plants as sinks and water-vapor sources is great, while remote sensing data cannot hope to resolve plant species and stress status. It is profitable, then, to seek regularities in individual-plant physiological behaviour. This is easy feasible inside the oasis with the two models presented.

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