ANALYSIS AND DESIGN OF BORDER IRRIGATION SYSTEMS

Abstract

Application efficiency (Ea) is the primary criterion for border irrigation design and management. The objectiveof this study is to analyze the behavior of the application efficiency function of border irrigation with respect to border length(L) and unit inlet flow rate (qo), given a target minimum application depth. The results show that the application efficiencyfunction is unimodal with respect to L and qo. Optimality conditions are derived for both the Ea(L) and Ea(qo) functions, basedon which simple rules that reduce the design and management procedure into a series of one-dimensional optimizationproblems with respect to qo are developed. The proposed procedure has a variable bounding step in which the feasible rangesof L and qo are determined. This is then followed by a step wherein alternative approximate optimum values of Ea(qo) arecalculated for each of the feasible values of L. Finally, the optimal Ea(qo) is selected from the available alternatives basedon sensitivity analysis and other locally pertinent practical criteria. In addition, the advantages and limitations ofadvance-phase and post-advance-phase inflow cutoff options and their effects on system design and management arediscussed. The distance-based (advance-phase) inflow cutoff option offers two main advantages over post-advance-phasecutoff: operational convenience, and a lower degree of sensitivity of design and management prescriptions to inaccuraciesin inflow measurements and to non-uniformities in the distribution of inlet flow over the width of the border. However, theresults of the study also show that, depending on the parameter set, there exist limiting conditions that preclude theapplicability of the distance-based cutoff criterion in border irrigation management. Even when the distance-based inflowcutoff criterion is feasible, the corresponding design and management scenario can be sub-optimal, in which case anear-optimum operation scenario can be realized only with post-advance-phase inflow cutoff.