10 Supplemental Irrigation for Improved Rainfed Agriculture in WANA Region
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10 Supplemental Irrigation for Improved Rainfed Agriculture in WANA Region T. Oweis* and A. Hachum Integrated Water and Land Management Program, International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria; email: *[email protected] Introduction and Concepts that the bulk of world food will continue to come from rainfed agriculture. Historically, the focus of water resource planning Irrigation accounts for about 72% of global and management has been on blue water and 90% of developing-country water with- resources for irrigation, industry and domestic drawals. Water availability for irrigation may purposes. Water investments in rainfed agri- have to be reduced in many regions in favour of cultural areas, which are usually located at the rapidly increasing non-agricultural water uses in upstream of river basins, are lacking and mostly industry and households, as well as for environ- focus on stream flows, surface run-off genera- mental purposes. However, rainfed areas tion and rivers’ routing. In water-scarce regions, currently account for about 60% of world food the green water resource (the soil moisture in the production. Given the importance of rainfed plant root zone) makes up 85–90% of the cereal production, insufficient attention has precipitation, reflecting the significant portion of been paid to the potential of production growth the available freshwater that sustains rainfed in rainfed areas to play a significant role in meet- agriculture. However, green water use and its ing future food demand. This potential could be monitoring and management have received realized through adoption of improved manage- little attention from engineers, planners and ment options on a large scale. Farmers’ yields in policy makers. rainfed regions in the developing countries are About 80% of the world’s agricultural land is low largely due to low rainwater use efficiency rainfed, contributing to at least two-thirds of because of inappropriate soil, water, nutrient global food production. In sub-Saharan Africa and pest management options, lack of seeds of more than 95% of farmed land is rainfed. It is improved cultivars and poor crop establishment. almost 90% in Latin America, 60% in South There is a large untapped potential of rainfed Asia, 65% in East Asia, and 75% in West Asia agriculture, especially in Asia and Africa, where and North Africa (WANA) (Rockström et al., the bulk of the world’s poor live. Lack of clear 2007). Undoubtedly, irrigation plays a very and sound water policy in rainfed agriculture is important role in supplying food. However, the among the reasons for the low yield and water potential for increasing water withdrawals for productivity in these areas (Rockström et al., irrigation is considered quite limited. Despite 2007, Wani et al., 2008). the higher risks in rainfed agriculture, especially There are three primary ways to enhance in drought-prone areas, it is widely accepted rainfed agricultural production, namely: (i) to © CAB International 2009. Rainfed Agriculture: Unlocking the Potential 182 (eds S.P. Wani et al.) Rainfed Agriculture in the WANA Region 183 increase the effective rainfall use through are basically bred to withstand drought under improved water management; (ii) to increase irrigated conditions and poor production inputs, crop yields in rainfed areas through agricultural such as poor land preparation and lack of ferti- research; and (iii) through reformed policies lizers, are among other reasons for low rainfed and increased investment in rainfed areas. This production. Widespread deficiency of micro- chapter focuses on the first way, in which and secondary nutrients such as zinc, boron supplemental irrigation (SI) plays a major role and sulfur, in addition to organic carbon and in increasing water use efficiency and yields of macronutrient deficiencies, are largely holding rainfed crops. back the potential of rainfed areas (Rego et al., 2007; Sahrawat et al., 2007). Globally, 69% of all cereal area is rainfed. Rainfed environment Worldwide, rainfed cereal yield is about 2.2 t/ha, which is about 65% of the irrigated yield The climate of the rainfed dry areas (arid, semi- (3.5 t/ha). The importance of rainfed cereal arid, dry subhumid) is characterized by complex production is partly due to the dominance of climatic deficiencies, manifested as agricultural rainfed agriculture in developed countries. water scarcity for rainfed crop production. The More than 80% of the cereal area in developed rainfall amount, as well as distribution, is not in countries is rainfed. The average rainfed cereal favour of stable and satisfactory yield. Rainfall is yield in developed countries is as high as irri- highly unreliable, with strong risks of dry spells gated cereal yields in developing countries. during the crop growth season, even during Irrigation is relatively more important in cereal good rainfall years. Interannual and spatial production in developing countries, with nearly fluctuations of rainfall are high. 60% of future cereal production in developing Within the dry areas of WANA, because countries coming from irrigated areas. rainfall amounts and distribution are usually However, rainfed agriculture remains important suboptimal, moisture-stress periods often occur in developing countries as well. Rainfed yield in during one or more stages of crop growth, caus- developing countries is around 1.0–1.5 t/ha, ing very low crop yields. Variation in rainfall which is two- to fourfold less than that of the amounts and distribution from one year to achievable potential yield on commercial/ another causes substantial fluctuations in researcher-managed plots (Rockström et al., production, which can range, in the case of 2007; Wani et al., 2008). wheat for example, from 0.3 to over 2.0 t/ha. Lack of investment in rural infrastructure and This situation creates instability and negative poor water policies are among the reasons for the socio-economic impacts. Even for breeding dramatic gap between potential yields in rainfed modern varieties for rainfed areas, the high areas and the actual yields achieved by farmers. heterogeneity and erratic rainfall of rainfed Important policies should include higher priority environments make plant breeding a difficult for rainfed areas in agricultural extension services task. and access to markets, credit and input supplies. Investment in rainfed areas, policy reform and transfer of technology such as SI and water Reasons for low rainfall water productivity harvesting requires coordinating efforts among all players, including agricultural researchers, local Poverty, drought, low soil fertility and land organizations, farmers, community leaders, non- degradation are the major factors for low rainfall governmental organizations (NGOs), national productivity that are challenging the rainfed policy makers and donors. agriculture in the dry areas (Rockström et al., 2007; Wani et al., 2008). Another reason for low yields in the stressed environments of rainfed Supplemental irrigation as a response areas is soil deficiency in terms of soil infiltration and soil water-holding capacity: all the rainfall Shortage of soil moisture in the dry rainfed areas does not infiltrate and/or not all that infiltrates is often occurs during the most sensitive growth beneficially utilized. Improper cultivars, which stages (flowering and grain filling) of the crops. 184 T. Oweis and A. Hachum As a result, rainfed crop growth is poor and yield Estimated mean annual increase in production is consequently low. Supplemental irrigation, cost due to SI (including fixed and variable using a limited amount of water, if applied costs) as compared with rainfed equals US$150 during the critical crop growth stages, can result per hectare. Estimated mean increase in net in substantial improvement in yield and water profit between rainfed and SI for wheat equals productivity. Therefore, SI is an effective re- US$300 per hectare. The ratio of increase in sponse to alleviate the adverse impact of soil estimated annual net profit per hectare to esti- moisture stress during dry spells on the yield of mated difference in annual costs between rain- rainfed crops. Supplemental irrigation may be fed and SI is 200%, which is high (Oweis and defined as ‘the addition of small amounts of Hachum, 2006a). water to essentially rainfed crops during times when rainfall fails to provide sufficient moisture for normal plant growth, in order to improve Source of water for supplemental irrigation and stabilize yields’ (Oweis and Hachum, 2003). By this definition, and since rainfall is the Probably the first aspect that comes to mind major water supply source for crop growth and when planning SI for rainfed agriculture in dry production, the amount of water added by areas is the source of water for irrigation. In a SI cannot by itself support economical crop developed river basin with full irrigation for the production. In addition to yield increases, SI summer crops and rainfed for winter crops also stabilizes rainfed crop production (Oweis (such as in WANA countries characterized by and Hachum, 2003). Mediterranean climate), the same water source Unlike full irrigation, in which the crop and irrigation facilities are used for SI. One depends mainly on artificial irrigation since the good example of such a case is the North rainfall amount is very limited, the timing and Jazirah Irrigation Project in Nineva Province, amount of SI cannot be determined in advance, northern Iraq, in which 25% of the 60,000 ha owing to rainfall stochasticity. Supplemental irri- project area is cultivated under full irrigation in gation in rainfed areas is based on the following summer and 75% of the area is under rainfed three basic aspects (Oweis, 1997): wheat with SI in winter (Adary et al., 2002). The source of water for the project is the River 1. Water is applied to a rainfed crop that Tigris. would normally produce some yield without Groundwater is the most common source of irrigation. water for SI. In Syria, for example, groundwater 2. Since rainfall is the principal source of water represents 60% of all water used in irrigation.