6 Summary
Designing irrigation application systems that have the potential to be operated effectively and efficiently is a complex task that requires a wide range of information and contributions from a range of professions.
This review has attempted to identify measures of irrigation performance that can provide meaningful feedback to designers during the design phase concerning the potential performance of the irrigation system. These cover measures relevant to the production goals of irrigation, and to the environmental effects of irrigation. Definition of the performance indicators lead to the identification of several factors which are the keys to achieving high levels of irrigation performance. A number of critical design decisions are related to these keys. These were summarised, along with the issues that must be addressed in the process of making these design decisions. Finally, an assessment was made of current New Zealand capacity to design irrigation systems that have the potential to perform well, economically and environmentally.
Overall there is a paucity of information relevant to the design of effective and efficient irrigation systems in New Zealand. The lack of accepted performance criteria, particularly related to the efficiency of water use, puts New Zealand agriculture in a weak position with respect to renewing, or obtaining, permits to take water under the Resource Management Act. It also represents a major oversight in the development of information about water resource management in New Zealand, considering that agriculture is the largest consumptive user of water in New Zealand, by a substantial margin.
Historically, the efficiency of irrigation has not met the expectations of planners and designers. Generally this has been considered to be a management problem – farmers are not monitoring soil water content and irrigating accordingly. However, a recent survey of farmers concerning irrigation practices revealed two important points relevant to system design:
The effectiveness of irrigation is more important to farmers than the efficiency.
Most irrigation systems do not have the flexibility to be managed to achieve high efficiency.
The significance of the first point is that farmers will increase application depths to a level which provides satisfactory production (effectiveness). If the application system is not able to apply water uniformly, the application depth acceptable to the farmer may be substantially larger than the soil water deficit. Application efficiently will then be significantly lower than anticipated by planners and designers.
The main conclusion of this review is that irrigation system design is the principal reason for lower than expected levels of efficiency. Achievement of sustainable irrigated agriculture depends on better design, as much as on improved management. This will require a combination of theoretical and field work.
Technology and expertise is available in New Zealand to increase understanding and obtain the information that is needed for sustainable irrigated agriculture.
7 References
Bright, JC (1986): Optimal control of irrigation systems: An analysis of water allocation rules. Unpublished PhD Thesis, University of Canterbury.
John, PH; Lees, DM; English, GM (1985): Application performance of travelling irrigators. Project Report No 35, NZ Agricultural Engineering Institute.
Painter, DJ and Carran, P (1978): Soil and Water, vol 14 pp 15-17, 1978.
Skaggs, RW; Millar, DE; Brooks, RH (1980): Soil Water: Part 1 – Properties. Chapter 4 in Design and Operation of Farm Irrigation Systems. American Society of Agricultural Engineers.
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