Supplemental irrigation (SI) or Deficit irrigation (DI) has been extensively investigated as a valuable and sustainable production strategy for a wide range of crops in dry regions. By limiting water applications to drought-sensitive growth stages, this practice aims to maximize water productivity and to stabilize – rather than maximize – yields (Geerts and Raes 2009 1, FAO 2002 2). It involves the addition of limited amounts of irrigation water to essentially rainfed crops, in order to improve and stabilize yields during times when rainfall fails to provide sufficient moisture for normal plant growth. Unlike full irrigation, the timing and amount of SI cannot be determined in advance given the natural variability in season-to-season and within season rainfall levels (Oweis and Hachum 2012). 3
Relationship to CSA
As well as achieving higher water productivity, the productivity and stability of crop production can be greatly increased through the addition of small amounts of SI at the correct time. For example, in Northern Iraq, where huge investments have been made in SI, rainfed wheat yields in 1997/98 increased from 2.2 to 4.6 t/ha with only the addition of 63 mm of SI in a season of 236 mm rainfall (Oweis and Hachum 2012). 3 SI has substantial adaptation benefits through the reduction and elimination of the short-term risk of yield losses or crop failure in rainfed crops due to water stress at critical stages, an adaptation benefit which is likely to become even more important in the future in regions where rainfed agriculture is important and where climate change projections suggest lower and more variable rainfall amounts.
Impacts and lessons learned
Given the unpredictable nature of SI scheduling, the best water delivery systems are those which can be used ‘on demand’, for instance, where farmers have access to wells or nearby water sources. In addition, in most instances, farmers will need an effective extension system which is able to advise them on the timings and amounts of SI scheduling. In many communities, the available water supply is inadequate to irrigate all of the available land. In those cases, farmers might consider the communal benefits of allowing sub-optimal yields on their individual fields, by practicing SI, so that the water saved might be used to irrigate additional land in the community (Geerts and Raes 2009 3, Oweis and Hachum 2012 3).
Geerts S, Raes, D. 2009. Deficit irrigation as an on-farm strategy to maximize crop water productivity in dry areas. Agricultural Water Management 96(9):1275–1284.http://dx.doi.org/10.1016/j.agwat.2009.04.009 Deficit irrigation (DI) has been widely investigated as a valuable and sustainable production strategy in dry regions. By limiting water applications to drought-sensitive growth stages, this practice aims to maximize water productivity and to stabilize – rather than maximize – yields. We review selected research from around the world and we summarize the advantages and disadvantages of deficit irrigation. Research results confirm that DI is successful in increasing water productivity for various crops without causing severe yield reductions. Nevertheless, a certain minimum amount of seasonal moisture must be guaranteed. DI requires precise knowledge of crop response to drought stress, as drought tolerance varies considerably by genotype and phenological stage. In developing and optimizing DI strategies, field research should therefore be combined with crop water productivity modeling.
FAO. 2002. Deficit Irrigation practices. Water Reports 22. Rome, Italy: Food and Agriculture Organization of the United Nations.ftp://ftp.fao.org/agl/aglw/docs/wr22e.pdf 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.
Oweis T, Hachum A. 2012. Supplemental irrigation: a highly efficient water-use practice. Aleppo, Syria: ICARDA.http://www.icarda.org/wli/pdfs/Books/Supplemental_Irrigation.pdf This book emphasizes the need greater for a better balance of investments in rainfed versus irrigated agriculture. We need a new governance, investment and management paradigm in which all water options in the farming system are considered. The book highlights several other aspects including water productivity, integration, and participatory research and development. In rainfed dry areas, where water (not land) is the most limiting factor, the priority should be to maximize yield per unit of water, rather than yield per unit of land. Supplemental irrigation can play a key role in increasing water productivity, and in ensuring more sustainable use of groundwater. For maximum benefit, supplemental irrigation must be part of an integrated package that includes non-water inputs, improved crop management methods and other components. Optimal supplemental irrigation regimes would be based on sound water management policies, economic evaluations (e.g. crop:water price ratios) and timely application. As past experience has shown, integrated, farmer-participatory research and development programs are the best way to introduce, test and scale out supplemental irrigation technology.