Esce is the Scheme efficiency Wsrz: Water stored in the root zone Wrhw: Water received at the head works
The scheme efficiency can also be calculated by:
Irrigation efficiencies formulas can be ameliorated and established for every oasis in the region through research by monitoring irrigation inside each field.
4.2 Formulas to Calculate a Drainage Network Inside the Oasis
Inside the oasis irrigation management is concerned with the management of the supply of water to the crops. The goal is to maintain optimum water supply so that neither water deficit nor water excess conditions are created. This is achieved by refilling the profile when soil moisture content has been depleted to a certain level and by draining excess water. To perform this management task effectively both the water requirements of the crop and the water storage characteristics of the soil have to be known. The drainage of the agriculture field consists at evacuate the surplus water from soils in order to prepare an optimal environment to plants roots and obtain the maximum crop product. A drainage network contains drains and collectors (primary, secondary, tertiary... it depends on the surface of the field to drain). The drains transport the water to the collectors those later conduct the water to the drainage channel after to the outlet (a lake, sea, plain). To determine the dimension of a drainage network we must calculate the optimal number of drains, their diameter, their clearance and their mean maximum length (the same for the collectors). Those parameters are deduced after calculating the distance between the drains and by using the geometry of the farming field (length and width). We will present here the important formulas used to calculate a drainage network. All the formulas established are function of the type of flow and the nature of soil. We will try to discuss all the cases.
For a permanent flow with rail cutting very permeable and when the drains repose directly over the impermeable substratum, to determine the mean distance between the drains in the case of permanent discharge, the most used formula is (Riadh Wasfi Assoufi, 2002):
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