To design micro-irrigation laterals, by an accurate, simple and quick method, The OPT-LAT model was created. The model calculates the maximum lateral length with field length in x-axis or y-axis at different lateral diameters. It can also estimate the optimal diameter that meets least cost irrigation system. Not only that, but also it can compare between optimal lateral diameter in x-axis or y-axis. The validity of OPT-LAT model was proved through the comparison between the lateral lengths

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... lated by the model and the corresponding ones calculated by Osama (2). The model has been applied on 160 m × 120 m plot area as a case study. At the x-axis direction, the lateral lengths were 58 m for different lateral diameters, the optimal internal diameter was 13.6 mm at annual least cost of 0.229 L.E/m/year and less percent of pressure head variation of 12 % at lateral diameter of 36 mm. The corresponding values at y-axis were 78 m lateral lengths, the optimal internal diameter was 13.6 mm at annual least cost of 0.239 L.E/m/year and less percent of pressure head variation of 12.04 % at lateral diameter of 36 mm. These results indicate that as the diameters of the laterals increased, the percent of the pressure head variation decreased, annual fixed cost increased and repair plus energy annual costs increased. The above results depend on energy price and pipe price according to its material type.