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This publication presents the results of a number of deficit irrigation studies carried out for various crops and under various ecological conditions, with a review of the impact of reduced water supplies on crop yield. The results of the studies are presented in ten contributions prepared by a team of scientists specialized in deficit irrigation. The articles were prepared at the request of the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture in close collaboration with the FAO Land and Water Development Division.

The studies present the latest research concepts and involve various practices for deficit irrigation. Both annual and perennial crops were exposed to different levels of water stress, either during a particular growth phase, throughout the whole growing season or in a combination of growth stages. The overall finding, based on the synthesis of the different contributions, is that deficit or regulated-deficit irrigation can be beneficial where appropriately applied. Substantial savings of water can be achieved with little impact on the quality and quantity of the harvested yield. However, to be successful, an intimate knowledge of crop behaviour is required, as crop response to water stress varies considerably.

The use of models can be an important tool to simulate crop water behaviour under different conditions of water supply. The yield-response-to-water functions as developed by Doorenbos and Kassam (FAO Irrigation and Drainage Paper No. 33) were tested with the FAO CROPWAT model and applied successfully to evaluate and predict the impact of deficit irrigation on crop yield. The crop parameters used in the model include the crop response factor, which estimates relative yield reductions based on the measured reduction in crop transpiration. The factor is a useful indicator for the sensitivity and tolerance of crop and crop stage to water stress. Analyses showed that crops less sensitive to stress such as cotton, maize, groundnut, wheat, sunflower and sugar beet can adapt well to deficit irrigation practices provided good management practices can be secured. For more sensitive crops such as potatoes deficit irrigation proved less economic.

A study carried out on winter wheat in the North China Plain (NCP) between 1992 and 2000 showed possible water savings of 25 - 75 percent by applying deficit irrigation at various growth stages, without significant loss of yield and profits. A dynamic model was used to calculate the net profits of the irrigation treatments. Procedures were developed to schedule irrigation applications according to the number of irrigations required. For one irrigation, the application should take place between jointing and booting; for two irrigations the applications should take place between jointing and heading and from heading to early milk stage, while with three irrigations, the applications should take place at tillering stage before over wintering, between jointing and booting and from heading to milk stage.

In deficit studies carried out in India on irrigated groundnuts, it was possible to increase field water use efficiency (WUE) and dry matter by imposing transient soil moisture-deficit stress during the vegetative phase, i.e. 20 - 45 days after sowing. Water stress applied during vegetative growth may have had a favourable effect on root growth, contributing to more effective water use from deeper layers.

While most studies were able to demonstrate the benefits of deficit irrigation, potatoes grown under sprinkler irrigation in the semi-arid environment of eastern Oregon, United States of America, did not show an economic benefit when exposed to stress. Growing four varieties of potato under various deficit irrigation treatments resulted in gross revenues declining by more than the production costs, and hence reduced profits. The results of this case study suggest that deficit irrigation of potatoes would not be a viable management option for that region under current economic conditions.

Fruit crops such as peach and pear trees and grapevines reacted favourably to deficit irrigation practices, with important water savings and improved fruit quality. In southeastern Australia, regulated deficit irrigation (RDI) of peach and pear trees increased WUE by 60 percent, with no loss in yield or reduction in vegetative vigour. In Washington State, United States of America, RDI of grapevines prior to fruit set (veraison) was effective in controlling shoot growth and pruning weights, with no significant reduction in yield. RDI applied after veraison to vines with large canopies resulted in greater water deficit stress. Wine quality improved with pre-veraison RDI applied as compared to post-veraison RDI. RDI applied at anytime resulted in better early-season lignification of canes and cold hardening of buds.

In addition to RDI, partial root zone drying (PRD) is also a promising practice for inducing stress tolerance in fruit trees. PRD is a new irrigation technique that subjects one-half of the root system to a dry or drying phase while the other half is irrigated. The wetted and dried sides of the root system alternate on a 10-14-day cycle. Both RDI and PRD systems require high management skills. Close monitoring of soil water content is recommended. Both practices improve the WUE of wine grape production. Micro-irrigation facilitates the application of RDI and PRD. Practical guidelines for using RDI were developed.

Subsurface drip irrigation (SDI) also improved the WUE of crops and reduced farming costs. An approach was developed for deficit SDI on cotton grown in arid east Texas, United States of America, to enable farmers with a limited supply of water to decide on the optimal area to plant and the best row width/pattern to apply. By applying deficit SDI, it proved more economical to use the available water resources over the entire farm, rather than to try to maximize water and yield on part of the farm. Moreover, with SDI, it proved possible to apply a large part of the water required as pre-planting irrigation, thus effectively advancing the timing of water application to the beginning of the season when more water is available.

In conclusion, with increasing scarcity and growing competition for water, there will be more widespread adoption of deficit irrigation, especially in arid and semi-arid regions. The technique has already been applied to a wide variety of crops as presented in this publication. However, as different crops and trees respond differently to water stress, it is important that the technique undergo continuous refinement and improvement, as deficit irrigation requires more sophisticated water controls, accurate water management and soil water monitoring. Advances in new irrigation technologies with more refined measuring techniques and soil water sensors will help improve knowledge and management techniques. In this regard, recent years have witnessed major advances in developing and marketing user-friendly and affordable soil water sensors, which farmers are using increasingly in their farm management strategies. With these techniques, it is then possible to identify irrigation scheduling strategies that minimize water demand with minimal impacts on yields and crop quality, leading to improved food security.

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