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Abstract: Aegilops cylindrica Host (jointed goatgrass) is an exotic invasive weed in the western United States that causes economic loss due to competition with winter wheat (Triticum aestivum L.) and contamination of wheat grain. Further, because Ae. cylindrica is a wild relative of wheat, interspecific hybridization is a possible mechanism for the introgression of advantageous genes from wheat to the weedy species. The risk of introgression of traits conferring environmental stress tolerance, such as drought tolerance, from transgenic wheat has not been studied and is of high concern because it could increase the weed''s competitiveness with wheat, as well as its ability to invade novel habitats. Genetic diversity has been reported among western U.S. Ae. cylindrica accessions, however, studies to determine whether genetic diversity relates to phenotypic diversity have been limited. Here, 30 accessions of Ae. cylindrica from the western U.S. were evaluated over two years at two Colorado locations: Haxtun in 2008–2009 and Fort Collins in 2009–2010. Days to flowering, plant height, number of tillers produced per plant, number of spikelets per spike, and number of spikelets per plant were evaluated under rain-fed and irrigated conditions. Results indicate significant (P <0.01) variation among accessions for each of the traits for both years. Additionally, in 2009–2010 there was a significant (P <0.01) effect of irrigation on all traits, except days to flowering. Similarly, all traits except days to flowering showed a significant effect of the interaction of accession with treatment (P <0.05) in 2009–2010. The study shows that variation exists in traits related to competitive ability and fitness. Our data also suggest differences among accessions in sensitivity to water availability. This observation needs to be confirmed in additional environments. Implications of our results for evaluation of transgene introgression are discussed. [Copyright &y& Elsevier]

Virtual water is the volume of water used to produce a commodity or service. Hitherto, most virtual water ‘trade’ studies have focused on its potential contribution to saving water, especially in water short regions. Very little, however, has been said about the opportunity cost of the associated water. The present research critically evaluates the strategic importance of green water (soil water originating from rainfall) in relation to international commodity trade. Besides having a lower opportunity cost, the use of green water for the production of crops has generally less negative environmental externalities than the use of blue water (irrigation with water abstracted from ground or surface water systems). Although it is widely known that major grain exporters – the USA, Canada, France, Australia and Argentina – produce grain in highly productive rain-fed conditions, green water volumes in exports have rarely been estimated. The present study corroborates that green water is by far the largest share of virtual water in maize, soybean and wheat exports from its main exporting countries (USA, Canada, Australia and Argentina) during the period 2000–2004. Insofar virtual water is ‘traded’ towards water-scarce nations that heavily depend on their blue water resources, green virtual-water ‘trade’ related to these commodities plays a role in ensuring water and water-dependent food security and avoiding further potential damage to the water environments in both importing and exporting countries. This potential of international green virtual-water ‘trade’, however, is constrained by factors such as technology, the potential for further increases in the productivity of soil and irrigation water, the level of socio-economic development, national food policies and international trade agreements. [Copyright &y& Elsevier]

• We quantitatively map the sensitivity of agriculture to drought stress. • A strong spatial correspondence between drought sensitivity and irrigation exists. • The sensitivity is higher in arid/semi-arid regions and lower in humid regions. • We identify the sub-seasonal pattern of agricultural sensitivity to drought stress. • The non-irrigated crops are more sensitive to drought than the irrigated crops. Drought is a devastating natural hazard posing great threats to agriculture. Identifying the spatial pattern of agricultural sensitivity to drought can provide scientific information for decision-makers to prepare droughts, allocate resources, and mitigate impacts. Here, we use long-term state- and county-level crop data for the 10 major crops: corn grain, soybeans, hay, spring wheat, winter wheat, cotton, corn silage, sorghum, barley, and rice in the United States from 1950 to 2016. First, we perform a correlation analysis between crop yield anomalies and two drought indices (Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Precipitation Index (SPI)) to identify the sub-seasonal pattern of agricultural sensitivity to drought stress. SPEI performs better than SPI. For most crops, the sensitivity to drought increases in the early period, peaks at the critical months, and then declines. July is the most critical month for crop growth for most crops. Among all crops, soybean and corn grain are most sensitive to drought. Second, we develop an Agriculture Drought Sensitivity Index (ADSI) to quantitatively measure the sensitivity of agriculture to drought stress based on the statistical relationship between the ten major crops and SPEI. We demonstrate that there exists a very strong spatial correspondence between higher sensitivity to drought and the lower percentage of acres irrigated, and vice versa. Also, for those regions with limited irrigation, the sensitivity is higher in arid/semi-arid regions and lower in humid regions in summer. Third, given the importance of irrigation, an analysis of covariance (ANCOVA) shows that the irrigated crop yields have much higher long-run mean yields than non-irrigated crop yields. Fourth, to investigate how irrigation affects drought sensitivity, a panel data regression model shows that the responses of crop growth to drought are non-linear for all crops. Non-irrigated crops are more sensitive to droughts than the irrigated crops, particularly in severe drought conditions. This provides quantitative incentive to use irrigation as an important adaptation and coping strategy to mitigate the drought impacts on agriculture in the US. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Quantified irrigation's role in stabilizing U.S. agricultural yields. • Irrigation-Induced Reduction in Yield Variability (IIRYV) computed for nine crops. • Derived from 220,000 historical records, mean U.S IIRYV was 41 % (range: 0–90 %). • Maize and cotton demonstrated the highest and lowest IIRYV. • IIRYV increased during 1950–2015 for most crops, albeit crop and space variation. Irrigation contributes to enhance and sustain agricultural production in the U.S. across all aridity regimes, via mitigation against interannual environmental variability. Currently, a quantitative understanding of the role of irrigation in stabilizing agricultural yields is lacking, limiting the realization of irrigation-adoption impacts. Here we use >220,000 historical (1950–2015) county-year irrigated and rainfed yield records for the top-nine U.S. crops [maize (Zea mays L.), soybean (Glycine max (L.) Merr.), spring wheat (Triticum aestivum L.), winter wheat (Triticum aestivum L.), alfalfa (Medicago sativa L.), sorghum (Sorghum bicolor (L.) Moench), cotton (Gossypium hirsutum), barley (Hordeum vulgare L.) and oats (Avena sativa)]. To comprehensively quantify the "Irrigation-Induced Reduction in Crop Yield Variability" (IIRYV, defined as the percent reduction in crop yield variability when irrigated with respect to rainfed yield variability). Averaged across all crops, national-level IIRYV was 41 %, which varied from 0 to 90 % across various regions and crops. IIRYV was highly crop-specific, and maize and cotton crops demonstrated the highest and least magnitudes. IIRYV substantially varied spatially within the constituent growing regions for each crop, and thus national scale assessments masked significant spatial differences. IIRYV was subject to interannual (temporal) variability; however, in general, IIRYV increased over time (1950–2015) for most crops. These findings will help in the evaluation of the contribution of historical irrigation development policy and adoption in the U.S in mitigating external shocks (variability) in U.S. agricultural productivity. The demonstration of spatial and temporal dynamics in IIRYV will aid in irrigation-water allocations and adoption by prioritizing irrigation development for vulnerable crops and regions. [ABSTRACT FROM AUTHOR]