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Rainfed agriculture plays and will continue to play a dominant role in providing food and livelihoods for an increasing world population. We describe the world's semi-arid and dry sub-humid savannah and steppe regions as global hotspots, in terms of water related constraints to food production, high prevalence of malnourishment and poverty, and rapidly increasing food demands. We argue that major water investments in agriculture are required. In these regions yield gaps are large, not due to lack of water per se, but rather due to inefficient management of water, soils, and crops. An assessment of management options indicates that knowledge exists regarding technologies, management systems, and planning methods. A key strategy is to minimise risk for dry spell induced crop failures, which requires an emphasis on water harvesting systems for supplemental irrigation. Large-scale adoption of water harvesting systems will require a paradigm shift in Integrated Water Resource Management (IWRM), in which rainfall is regarded as the entry point for the governance of freshwater, thus incorporating green water resources (sustaining rainfed agriculture and terrestrial ecosystems) and blue water resources (local runoff). The divide between rainfed and irrigated agriculture needs to be reconsidered in favor of a governance, investment, and management paradigm, which considers all water options in agricultural systems. A new focus is needed on the meso-catchment scale, as opposed to the current focus of IWRM on the basin level and the primary focus of agricultural improvements on the farmer's field. We argue that the catchment scale offers the best opportunities for water investments to build resilience in smallscale agricultural systems and to address trade-offs between water for food and other ecosystem functions and services.

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A gronomy J our n al • Volu me 101, I s sue 3 • 2 0 0 9 469 ABSTRACT Predicting yield is increasingly important to optimize irrigation under limited available water for enhanced sustainability and profi table production. Food and Agriculture Organization (FAO) of the United Nations addresses this need by providing a yield response to water simulation model (AquaCrop) with limited sophistication. In this study, AquaCrop was parameterized and tested for cotton (Gossypium hirsutum L.) under full (100%) and defi cit (40, 60, and 80% of full) irrigation regimes in the hot, dry, and windy Mediterranean environment of northern Syria. Model parameterization used the 2006 data and was straightforward within the designed user-interface, owing to the limited number of key parameters. Accurate simulation of canopy cover was central to sound prediction of evapotranspiration and biomass accumulation. Key user-input parameters for this purpose were identifi ed as the coeffi cients defi ning canopy development and the threshold soil water depletion levels for the water stress indices. Th e parameterized model was tested using data from the 2004 and 2005 seasons, resulting in accurate prediction of evapotranspiration (<13% error). Th e predicted yield values were within 10% of measurements, except in the 60 and 80% irrigation regimes in 2004, with errors up to 32%. Th e model closely predicted the trend in total soil water, but deviation existed for individual soil layers. Th is study provides fi rst estimate values for cotton parameters useful for future model testing and use.Model parameterization is site-specifi c, and thus the applicability of key calibrated parameters must to be tested under diff erent climate, soil, variety, irrigation methods, and fi eld management.

The West Asia‐North Africa (WANA) region, with a Mediterranean‐type climate, has an increasing deficit in cereal production, especially bread wheat (Triticum aestivum L.). Rainfed cropping coincides with the relatively cool, rainy winter season, usually from October to May. Cereal yields are low and variable in response to inadequate and erratic seasonal rainfall and associated management factors, such as lack of N and late sowing. In an area where water is limited, small amounts of supplemental irrigation (SI) water can make up for the deficits in seasonal rain and potentially produce satisfactory yields. This field study (1992–1993 to 1995–1996) on a deep clay soil (a Calcixerollic Xerochrept) in northern Syria was conducted for four growing seasons to assess the effects of SI (rainfed, 1/3, 2/3, and full irrigation) combined with N rate (0, 50, 100, and 150 kg ha−1) and sowing date (early, normal, and late) on one traditional (Mexipak 65) and three improved bread wheat cultivars (Cham 4, Cham 6, Gomam). Yields of rainfed wheat varied with seasonal rainfall and its distribution, with all main factors having significant effects. A delay in the sowing date from November to January consistently reduced yields and the response to both SI and N. With irrigation, crop responses were generally significant up to 100 kg N ha−1, while optimum response for rainfed conditions was with 50 kg N ha−1. An addition of only limited irrigation (1/3 full irrigation) significantly increased yields, but near maximum yields were obtained by 2/3 of full irrigation. Responses to N and SI were greatest for the higher‐yielding cultivars. Use efficiency for both water and N was greatly increased by SI. Thus, with minimum irrigation during the winter growing season combined with appropriate management, inputs, and varieties, wheat output could be substantially and consistently increased in the semiarid Mediterranean zone. Production functions developed from this dataset can help predict the effects of changing any of these parameters in other locations in the region.

Supplemental irrigation (SI) is defined as the application of a limited amount of water to rainfed crops when In West Asia and North Africa, shortage of water limits wheat precipitation fails to provide the essential moisture for (Triticum aestivum L.) production. Current irrigation practices aim at maximizing grain yield, but achieve lower return for the water normal plant growth. This practice has shown potential consumed. Maximizing water use efficiency (WUE) may be more in alleviating the adverse effects of unfavorable rain suitable in areas where water, not land, is the most limiting factor. patterns and thus improving and stabilizing crop yieldsWe examined the effects of various levels of supplemental irrigation (Perrier and Salkini, 1991;Oweis et al., 1998; Zhang (SI) (rainfed, 1/3 SI, 2/3 SI, full SI), N (0, 5, 10, 15 g N m Ϫ2 ), and and Oweis, 1999). Early studies at ICARDA showed sowing time (Nov., Dec., Jan.) on evapotranspiration (ET) and WUE that applying two or three irrigations (80-200 mm) to of wheat. WUE was calculated for rain (WUE r ), for total water (gross:wheat increased crop grain yield by 36 to 450%, and rain ϩ irrigation) (WUE g ), and for SI water only (WUE SI ). ET ranged produced similar or even higher grain yields than in fully from 246 to 328 mm for rainfed crops, with grain yield ranging from irrigated conditions (Perrier and Salkini, 1991; Oweis, 130 to 270 g m Ϫ2 and total dry matter from 380 to 1370 g m Ϫ2 . Irrigated 1994). Supplemental irrigation is widely practiced in crops had ET of 304 to 485 mm, with grain yield of 170 to 500 g m Ϫ2 . The degree to which water supply limits grain yield was indicated by Syria, and in southern and eastern Mediterranean counthe ratio of pre-to post-anthesis ET (2.1-2.4:1). The SI treatments tries. However, excessive use of water in SI because of significantly increased WUE g : from 0.77 to 0.83 to 0.92 kg m Ϫ3 in low irrigation cost and attractive gains from increased November and December sowings for 1/3 SI and from 0.77 to 0.92 yields has resulted in a decline of aquifers and deteriorakg m Ϫ3 in November sowing for 2/3 SI. The highest WUE g and WUE SItion of water quality in many areas (Ward and Smith, were achieved at 1/3 to 2/3 SI. WUE was substantially improved by 1994).applying 5 and 10 g N m Ϫ2 , with little increase for higher rates. DelayingIncreasing the portion of water used for plant transpisowing had a negative effect on WUE for both irrigation and rainfed ration through a large and early canopy can increase conditions. In this rainfed Mediterranean environment, WUE can be WUE. In Mediterranean environments, where crop cansubstantially improved by adopting deficit SI to satisfy up to 2/3 of opy development in winter is slow and rain occurs as irrigation requirements, along with early sowing and appropriate levels of N.