Traditionally, design of surface irrigation systems has been based on past experiences in an area. This method met the practical needs of the farmers as long as irrigation water was plentiful and conditions were similar. As the need for improved design and performance grew, engineers began combining past experiences with simple hydraulic relationships to develop semiempirical design procedures in the forms of guidelines, equations, and nomographs .
Recent advances in simulation modeling, computing, and user-friendly software for personal computers provide good tools for design of surface irrigation systems. However, the use of these tools lags behind the potential. Among the main reasons are the lack of required input data, the need for using information from field evaluations as design criteria or as input parameters, influences of land leveling and soil management on actual field performance, dependence of performance parameters on many farming factors including the irrigator decisions, and temporal and spatial variability of soil characteristics.
The derivation of infiltration from field evaluation data, as mentioned above, can play an essential role for design, either using simulation models or adopting optimization techniques. Simulation models are used for design in an iterative process of searching the best geometric and inflow parameters that provide optimum values for the performance parameters.
The input parameters include the infiltration characteristics; the length and the cross-sectional characteristics of the furrows, borders, or basins; the desired irrigation depth; and the roughness coefficient n. The slope, 50, can be fixed or adjusted together with the other output parameters: the inflow rate qin and the time duration of the irrigation tco. Input parameters can vary from one simulation to another until the best solution is obtained. The capabilities of simulation provide for alternative best solutions concerning different processes of water application and different irrigation management conditions.
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