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1.4.2 Deficit Irrigation

Technical Description

Deficit or minimum irrigation is the deliberate under-irrigation of a crop. The technique is employed in areas where water is very limited. In some cases, water is only applied at the critical stages of crop growth. Applying minimum quantities of water at these critical stages sustains the crop so that it continues to develop and to yield reasonable quantities of produce - the alternative being no crop at all. For dryland farming, the technology is practised in conjunction with water harvesting. The harvested water being enough for "critical stage" applications.

Extent of Use

The technology is employed by large-scale commercial farmers in Zimbabwe and South Africa for growing winter wheat. It is successfully done using sprinkler and drip irrigation systems.

Operation and Maintenance

The operation of this technology requires minimum skills once the systems are installed. However, the installation of piping requires some training, and the maintenance of pumps requires artisans.

Level of Involvement

Trials are being carried out in Zimbabwe to establish the applicability of the technique for use in small rural irrigation schemes. It is usually farmer-managed with external technical advice.

Costs

Experiments carried out in Mutoko, Zimbabwe, show that applying 86% of the optimum irrigation water results in net returns of $100/ha. The operational cost was $130/ha.

Effectiveness of the Technology

The method is effective in sustaining crops with very little water. Reasonably high yields of winter crops have been recorded in Southern Africa, under conditions where there was little irrigation water available.

Suitability

This technology is suitable for use where water is limited (e.g., by drought) and a reduction in volume of irrigation waters is necessary.

Environmental Benefits

There are positive benefits to using this technique as it conserves water and enables plants and crops to be grown under adverse conditions.

Advantages

The technique saves on water. Because of the limited volumes of water used, there is also a resultant saving on electricity and labour. Also, if practised where the rains have failed, farmers will at least harvest something.

Disadvantages

The crop yields will progressively reduce depending on the percentage deficit applied. The yield will always be less than the optimum.

Further Development of the Technology

There is need for further research on how this technology could best be combined with water harvesting techniques.

Information Sources

English, M. and Stoutjeskdyk 1994. Consideration of Deficit Irrigation in the Communal Lands of Zimbabwe. In: R. Owen, K. Verbeek, J. Jackson and T. Steenhuis (Editors), Dambo Farming In Zimbabwe: Water Management, Cropping and Soil Potentials for Smallholder Farming in the Wetland. Conference Proceedings. University of Zimbabwe, CIIFAD. 193 p.

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