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This study was carried out to determine the different irrigation regime effects on oxygen isotope discrimination of stem water (Δ 18 O s ) and its relationship with grain yield (GY), aboveground biomass (BM), and stomatal conductivity (g s ) of winter wheat. For this purpose a field experiment was conducted between 2009 and 2011 in a semiarid region of Turkey under rainfed (RF), deficit (DI), moderate (MI), and full irrigation (FI) conditions. The highest GY and BM (4.5 t ha -1 and 14.5 t ha -1 ) were measured from the FI treatment. However, the difference between the yield of DI and FI treatments was insignificant. It might be recommended that the DI treatment be used for wheat irrigation to increase water-use efficiency for semiarid climate conditions. Average Δ 18 O s values were obtained as 35.36‰, 34.56‰, 34.35‰, and 33.18‰ for RF, DI, MI, and FI treatments, respectively. Applied irrigation water and GY were found to be strongly negatively related to Δ 18 O s , so it may be a useful predictor of yield in irrigation programs. Effects of water regime on yield probably reflected the correlation with the isotopic composition of transpiration. A negative relationship was obtained between average Δ 18 O s and g s at a significant level for which the correlation coefficient was 0.68. However, the relationship was positive between GY and g s .

Agriculture is the main consumer (75%-80%) of available water resources in many countries (Baris and Karadag, 2007). Generally, crop productivity where there is sufficient soil water is higher than in dry soil conditions (Misra et al., 2010). In semiarid regions such as Central Anatolia in Turkey water scarcity is a serious problem for sustainable crop production (Oweis and Ilbeyi, 2001). Efficient use of water by plants plays a crucial role especially in arid regions. Regulation of water productivity is particularly important in arid ecosystems where plants are sporadically exposed to water stress (Tanner and Sinclair, 1983). As reported by Molden et al. (2003), productivity of irrigation water can be evaluated at the plant, field, farm, system, and basin level. The irrigation water productivity at the field level is the ratio between evapotranspiration and total diverted irrigation water for crop production (Kijne et al., 2003).In recent decades important progress has been made using isotopic techniques of water management in agriculture (Heng et al., 2005). Oxygen, hydrogen, carbon, and nitrogen abundance measurements in soil, water, and plant components can be useful in identifying the sources of water and nutrients used by plants (Bazza, 1993;IAEA, 2006). Several studies have shown that carbon isotope discrimination is highly correlated with plant water status (Xu et al., 2007;Misra et al., 2010;Wahbi and Shaaban, 2011).Two parameters are currently used to characterize carbon isotope ratio in plants: carbon isotope composition (δ) and carbon isotope discrimination (∆). Carbon isotope composition is calculated as δ 13 C(∆) =

Non-storage summer dry onion crop is among the top 10 agricultural commodities in New Mexico (NM), USA. In 2000, NM was ranked the second in the nation as the leading state in growing summer dry onion. According to USDA statistical records, onion production or sales in NM was valued at $91.4 million. Mesilla Valley is one of the major onion-producing regions of NM. Due to many years of drought in the region and the concern for climate change, irrigation managers and decision makers are interested in quantifying water use or evapotranspiration (ET) and the number of acreage of onion crop grown in the Valley. This information can then be used for managing the scarce water resources of the region. Plant phenology, Landsat-8 satellite data, and USDA crop data were used to identify onion crops in the Valley (area of about 47,000 ha) and to determine their consumptive water use or ET using remote sensing Regional ET Estimation Model (REEM) from 2014 through 2016. Time series of NDVI clearly identified Fall and Spring-season onion crops in the Valley. REEM estimated Springseason onion crop maximum ET of 973 mm in 2015 and 975 mm in 2016 during the growing season. These values compared reasonably well to ET estimates of 894 and 955 mm for the same periods (i.e. 2015 and 2016) using FAO-56 crop coefficient based method. The methodology presented could be used in other regions to identify onion crops and their consumptive water use.

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