Water Use Efficiency of Rainfed and Irrigated Bread Wheat in a Mediterranean Environment Theib Oweis,* Heping Zhang, and Mustafa Pala ABSTRACT Supplemental irrigation (SI) is defined as the applica- tion 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 yields We 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 22 ), 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 1 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 22 and total dry matter from 380 to 1370 g m 22 . 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 22 . The degree to which water supply limits grain yield was indicated by Syria, and in southern and eastern Mediterranean coun- the 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 23 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 deteriora- kg m 23 in November sowing for 2/3 SI. The highest WUE g and WUE SI tion 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 22 , with little increase for higher rates. Delaying Increasing the portion of water used for plant transpi- sowing 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 can- substantially 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. frequent and small events, soil water evaporation may account for 30 to 60% of seasonal ET (Cooper et al., 1983; French and Schultz, 1984; Siddique et al., 1990; Zhang et al., 1998). Thus, agronomic practices that re- I n West Asia and North Africa (WANA), water duce soil water evaporation via a larger plant canopy resources are generally scarce, and agriculture’s and early ground cover and at the same time increase share of these resources is declining due to competition the crop’s ability to extract soil water may increase the from domestic and industrial sectors. In this region, a amount of water transpired and, consequently, WUE. typical Mediterranean climate prevails, with rain falling Nitrogen deficiency is another major constraint in can- mainly during the winter (and a lesser amount during opy development in the Mediterranean region (Ander- the warmer spring period); this rainy season is followed son, 1985). Crop responses to N fertilization depend by a hot, dry summer. Rainfed crop production under on the level of water availability (Pala et al., 1996). this climate thus depends strongly on both the amount Application of fertilizers not only increases plant shoot and distribution of rain. In the WANA region, amount and root growth (Brown et al., 1987), but also increases of rainfall is low and generally poorly distributed, so ET through a larger root system and greater extraction periods of water deficit occur during the grain-filling of stored water (Cooper et al., 1987a). In addition, a stage of wheat almost every year (Oweis et al., 1992). large and earlier canopy cover resulting from the appli- As a result, crop yield and water use efficiency (WUE) cation of N can reduce soil water evaporation and in- are generally low and variable. The production of 1 kg crease crop WUE (Zhang et al., 1998). of wheat (T. aestivum L.) grain under fully irrigated Under rainfed conditions, the date of the first signifi- conditions requires about 1 to 2 m 3 of irrigation water cant rain determines the sowing date. Early sowing of (Perrier and Salkini, 1991); in rainfed areas it requires appropriate cultivars is a recognized means of increasing from 1 to 3 m 3 of rainwater (Cooper et al., 1987a; Perrier wheat yields in other Mediterranean-type environ- and Salkini, 1991). Since water is the major limiting ments, such as Western Australia (Anderson and Smith, factor for agriculture in the WANA region, improving 1990; Anderson, 1992). Using a simulation model, Stap- WUE is vital for meeting the increasing food demand per and Harris (1989) estimated that wheat grain yield (Cooper et al., 1987b). International Center for Agricultural Research in the Dry areas Abbreviations: ET, evapotranspiration; SI, supplemental irrigation; (ICARDA), P.O. Box 5466, Aleppo, Syria. Received 9 June 1998. GY, grain yield; TDM, total dry matter; WANA, West Asia and North *Corresponding author (t.oweis@cgiar.org). Africa; WUE, water use efficiency. Subscripts: g, gross; r, rainfed; SI, supplemental irrigation. Published in Agron. J. 92:231–238 (2000). 231