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Economic optimization of irrigation

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  • The research effort described in this report explores the whole question of optimization of irrigation system design, planning and operations. Optimum irrigation practices, in theory, may involve some degree of deficit irrigation (deliberate under-irrigation of crops), hence the focus of this research was the use of deficit irrigation under practical operating circumstances. Deficit irrigation involves a radical philosophical departure from conventional prac­tice. This technique is based on an axiom of economics; that water use should be reduced to the point at which the marginal cost of irrigation just equals the value of the last increment of yield. The implications of this fundamen­tal axiom are explored, and a case study is also presented in which the full marginal costs of irrigation are examined in detail. When a decision is made to under-irrigate a crop, the problem of irriga­tion system design becomes much more complex because there are no longer any hard and fast requirements for system performance. The designer has much more latitude and system performance is more difficult to predict. Design techni­ques for exploiting the concept of deficit irrigation are explored in a case study, and an algorithm for predicting system performance is outlined. The question of risk becomes more significant when crops are under-irriga­ted, in part because it is more difficult to predict yields. This question is discussed, and a case study dealing with risk is presented. Irrigation scheduling will also be a more important consideration under deficit irriga­tion. It may be necessary to rely on crop stress indicators rather than soil moisture indicators, soil and crop variability may be greater under deficit irrigation, uncertainty will be a greater problem. The problem of scheduling for deficit irrigation is examined, and a new approach to scheduling using mathematical filtering techniques is proposed. Field experiments were carried out as part of this project to study the relationship between irrigation frequency and crop yields, and to test a variety of models of the process of evapotranspiration under low soil mois­ture conditions. Both of these subjects were found to be significant con­siderations in irrigation optimization, and past research on both was found to be inconclusive. Results of these two avenues of field research are presented.
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