Analysis and design of border irrigation systems

Application efficiency (E_a) is the primary criterion for border irrigation design and management. The objective of 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 (q_o), given a target minimum application depth. The results show that the application efficiency function is unimodal with respect to L and q_o. Optimality conditions are derived for both the E_a(L) and E_a(q _o) functions, based on which simple rules that reduce the design and management procedure into a series of one-dimensional optimization problems with respect to q_o are developed. The proposed procedure has a variable bounding step in which the feasible ranges of L and q_o are determined. This is then followed by a step wherein alternative approximate optimum values of E_a(q_o) are calculated for each of the feasible values of L. Finally, the optimal E_a(q_o) is selected from the available alternatives based on sensitivity analysis and other locally pertinent practical criteria. In addition, the advantages and limitations of advance-phase and post-advance-phase inflow cutoff options and their effects on system design and management are discussed. The distance-based (advance-phase) inflow cutoff option offers two main advantages over post-advance-phase cutoff: operational convenience, and a lower degree of sensitivity of design and management prescriptions to inaccuracies in inflow measurements and to non-uniformities in the distribution of inlet flow over the width of the border. However, the results of the study also show that, depending on the parameter set, there exist limiting conditions that preclude the applicability of the distance-based cutoff criterion in border irrigation management. Even when the distance-based inflow cutoff criterion is feasible, the corresponding design and management scenario can be sub-optimal, in which case a near-optimum operation scenario can be realized only with post-advance-phase inflow cutoff.