L: leaching requirement

: electrical conductivity of irrigation water 3soil : electrical conductivity of the soil saturation extract ^effi: leaching efficiency

For an accumulation rate creating intolerable salts levels in the root zone during the season, leaching has to be done concurrently with irrigation. If not, leaching can be done before or after the season.

4.1.2 Formulas to Determine the Efficiency of Irrigation Network Inside the Oasis

The gross irrigation requirement is the water which must be available at the head of the scheme. It concerns the flow or discharge, which must be received by the irrigation network. It includes the amount of water really needed by plants, the quantity that can evaporate or percolate when distributed in the soil and the water to use for leaching. Hence more water should be applied than the net irrigation requirements in order to cater for the losses. It can be estimated from the following relationship (Charles and Maurice 1981; Vaughn et al. 1980):

Esc qgir is the gross irrigation requirement qnir is the net irrigation requirements Esce is the irrigation scheme efficiency

The net irrigation requirement is the amount of water that the plant really need. It can be calculated and monitored at hour scale if we found a method to follow the farming transpiration inside the oasis. It is the subject of the coming paragraphs.

To determine irrigation scheme or network efficiency inside the oasis we must analyze the different levels at which water losses occur:

• at the level of the plant, when applying water to the soil

• at the level of the field, after water has entered the field

• at the level of the canals, during conveyance of the water between the main scheme inlet to the field offtake

The concept "efficiency" denotes that fraction of the total amount of water, which will benefit the field respectively the crop. Inside the oasis this efficiency must be calculated for all the component of an irrigation network. In fact, losses may occur due to deep percolation below the root zone and unwanted drainage (runoff) of water from the field. Deep percolation almost certainly will occur as it is nearly impossible to achieve uniform water distribution within a field and the correct rate of water application at the crop level. Field application efficiency is affected by the type of irrigation system, soil type and the skill of the farmer. It is defined as (Salisu et al FAO; Vaughn et al. 1980; Charles and Maurice 1981):

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