The equations that I have presented above deal with the ways of distribution of water, salts and nutrients in the soil under different irrigation systems. Those irrigation systems are used or can be proposed to install when renewing the oasis. To calculate the irrigation scheme or network we must know the tolerant species to install, there densities and the amount of water really needed. Those information can be modeled for each type of soil as I will show later. The quantity of water that the irrigation network must bring inside the oasis is that necessary for plant consumption (net irrigation requirement for each sort of plant), water lost when distributed in the soil (evaporation and percolation) and water used for leaching (Brutsaert,1982). When fixed we can then propose the technique of irrigation to apply. It depends on the situation of the oasis, the geometry of the filed (form, length, width) and the socio-economic orientation. We can opt for a traditional technique by submersion for example, for a modern technique by aspersion or drip by drip and finally for a combination (Vaughn E. Hansen, 1980). We have to determine the number of nozzles to fix, diameter, length and number of pipes to utilize, the number and length of furrows to make, the surface of basin to realize. All those parameters can be calculated easily after determining the best and efficient oasis architecture.
4.1 Formulas to Analyze the Irrigation Scheme Efficiencies Inside the Oasis
4.1.1 Calculating Water Needed for Action of Salt Leaching
When irrigating with salt water, which is frequent for the oasis of the region, salts are left behind when water is taken up by plants or lost by evaporation. The consequence is that there is a gradual accumulation of these salts in the root zone as the season progresses and from one year to the next year. The level of salinity is affected by the salt content of irrigation water, by the method of irrigation, by soil drainage characteristics and by the prevailing cultural practices. The leaching requirement is the minimum amount of irrigation water that must percolate below the root zone in order to maintain soil salinity at a given level. This later must be maintained at a value for which the particular crop would not suffer an unacceptable reduction in yield level. The fraction of the total volume of irrigation water supplied, which should be used for the leaching of salts may be calculated by the following equation (Salisu et al, FAO 1992; Charles and Maurice 1981; Vaughn et al. 1980):
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