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Water tables are dropping by approximately one meter annually throughout the North China Plain mainly due to water withdrawals for irrigating winter wheat year after year. In order to examine whether the drawdown can be reduced we calculate the net water use for an 11 year field experiment from 2003 to 2013 where six irrigated crops (winter wheat, summer maize, cotton, peanuts, sweet potato, ryegrass) were grown in different crop rotations in the North China Plain. As part of this experiment moisture contents were measured each at 20 cm intervals in the top 1.8 m. Recharge and net water use were calculated based on these moisture measurement. Results showed that winter wheat and ryegrass had the least recharge with an average of 27 mm/year and 39 mm/year, respectively; cotton had the most recharge with an average of 211 mm/year) followed by peanuts with 118 mm/year, sweet potato with 76 mm/year, and summer maize with 44 mm/year. Recharge depended on the amount of irrigation water pumped from the aquifer and was therefore a poor indicator of future groundwater decline. Instead net water use (recharge minus irrigation) was found to be a good indicator for the decline of the water table. The smallest amount of net (ground water) used was cotton with an average of 14 mm/year, followed by peanut with 32 mm/year, summer maize with 71 mm/year, sweet potato with 74 mm/year. Winter wheat and ryegrass had the greatest net water use with the average of 198 mm/year and 111 mm/year, respectively. Our calculations showed that any single crop would use less water than the prevalent winter wheat summer maize rotation. This growing one crop instead of two will reduce the decline of groundwater and in some rain rich years increase the ground water level, but will result in less income for the farmers. [ABSTRACT FROM AUTHOR]

Highlights: [•] ET c of summer maize showed a significant downward trend during 1960–2009 in the HHH farming region. [•] The variation of ET aw did not drop significantly due to yearly fluctuation of the effective rainfall in the growing season. [•] ET c was higher in the eastern regions and lower in the western in the HHH farming region. [•] The decrease of solar radiation contributed most to the decrease of the summer maize ET c . [Copyright &y& Elsevier]

• Cotton ET c showed a significant declining trend during the past five decades in North China Plain. • Rainfall-crop compatibility index in cotton growing season reached 75% with applied irrigation of 122 mm. • The spatially maximum cotton ET c shifted from southern in 1965-1975 to northeastern then western by 2006-2016. • The decline in solar radiation, sunshine hours and wind speed were responsible for the decline of cotton ET c. Climate change directly affects crop evapotranspiration and irrigation demand. The temporal and spatial variation of crop evapotranspiration is crucial to irrigation schedule planning and water resource management. Using 52 years of climate data from 43 stations in the North China Plain, this study examined trends and spatial distributions of the cotton evapotranspiration (ET c) and irrigation requirement (I wr) and revealed the impact of climate variation on cotton ET c change. We found that, annual ET c of cotton in the growing season showed a significant declining trend, which decreased from 606 mm yr−1 in 1965-1975 to 551 mm yr−1 in 2006-2016. Annual effective rainfall (R e) and I wr of cotton's growing season did not show consistency over the 52 year period, which averaged at 451 mm (441∼ 466 mm) and 122 mm (105 ∼ 145 mm), respectively. The 75% of cotton ET c was supplied by rainfall and 25% by irrigation water. This is much better than for winter wheat where 60% must be supplied by irrigation. The location of the spatial maximum cotton ET c shifted during the past five decades. From 1965 to 1975, maximum ET c values were found in the southern part and then shifted to the northeastern part from 1976 to 1985. During the period of 1986 to 2005, maximum ET c occurred in the middle area and shifted to the western part in 2006-2016. During the past five decades, solar radiation, sunshine hours and wind speed decreased significantly resulting in a significant decrease in ET c at −1.1 mm yr−1. This study highlights the indispensable information of water requirements for future irrigation scheduling of cotton in the North China Plain. [ABSTRACT FROM AUTHOR]

Over-irrigation to ensure high crop yields in North China Plain (NCP) has led to a sharp groundwater table decline over the past decades. We conducted a scenario analysis from1980 to 2017 with 12 irrigation strategies from T1W0M0 to T12W3M2 (T is treatment and numbers of irrigation times in wheat (W) and maize (M) seasons, 75 mm each) for the dominant winter wheat-summer maize rotation using the soil-water-atmosphere-plant (SWAP) model in the NCP. After model calibration and validation with 4-year experimental data, we analyzed the long-term simulated annual crop yield, actual evapotranspiration (ET a), water productivity (WP), groundwater table change and economic benefit to the different irrigation practices. The optimized irrigation strategy is based on an integral analysis for different precipitation year-types. Results showed that annual yield and WP increased with increased irrigation until these reached a plateau. Treatment T4W2M1 was the turning point with an annual yield of 15335 kg·ha-1, significantly higher than T2W1M1 (26 %) and T1W0M0 (63 %), but not significantly different from 4 to 5 irrigations in wet and normal years. ET a increased with increased irrigation from 456 to 644 mm yr-1. The annual WP followed a similar trend as annual crop yield for all irrigation schedules. T4W2M1 had an annual WP of 2.47 kg cm-3, significantly higher by 15 % than T2W1M1 and 24 % higher than T1W0M0, with no significant difference seen with 4–5 irrigations. The rainfed treatment mitigated groundwater table decline the best, even reversing the trend during the past four decades to 0.11 m yr-1 rise. This was followed by two irrigation treatments (−0.30 m yr-1), three irrigations (−0.55 m yr-1), four irrigations (−0.72 m yr-1), and five irrigations (−0.86 m yr-1). The latter corresponds to typical farmer practice. Using the productivity indicators, including irrigation WP, marginal benefit and economic index, T4W2M1 showed the best performance. Therefore, we recommend T4W2M1 (two irrigations at pre-wintering and jointing stages for wheat and one irrigation at maize sowing stage) as the best irrigation strategy for the wheat-maize rotation to mitigate groundwater table decline, and sustain grain yields and water productivity in the NCP; another anthesis irrigation for winter wheat is recommended during extreme dry years to sustain grain yield. • Twelve irrigation strategies for wheat-maize from 1980 to 2017 were evaluated. • Three targeted irrigations per year mitigated the groundwater table decline by 40 %. • 225 mm total for three irrigations per year is best to increase water productivity. [ABSTRACT FROM AUTHOR]

Reduced irrigation generally induces the tomato fruit yield reduction but enhance the fruit quality. Elevated CO 2 (eCO 2) injection is a common technique for alleviating water stress while eCO 2 could inevitably lead to the element concentration dilution in the plant organs. Inoculated with photosynthetic bacteria (PSB) are confirmed to form a symbiotic relationship with crop roots, thereby promoting root development, and improving crop root nutrient and water uptake capacity. Yet, it remains unknown whether PSB could mitigate the adversely effect of nutrient dilution under eCO 2 condition and the yield decline under reduced irrigation. To investigate that, two water treatments, including reduced irrigation (RI, plants were irrigated to 75% field water capacity) and full irrigation (FI, plants were irrigated to 90% field water capacity) of tomato plants, were grown under normal ambient CO 2 (aCO 2 , 400 ppm) and elevated CO 2 (eCO 2 , 800 ppm) conditions, respectively. And photosynthetic bacteria were inoculated in half plants. The results showed that inoculation of PSB improved the effectiveness of CO 2 stimulation on biomass and yield in tomato plants grown under reduced irrigation conditions. The stimulation effects have been associated with increased advantages linked to their root system, we have observed that CO 2 and PSB inoculation promoted the root development and antioxidant abilities. These consequently increased nitrogen uptake efficiency under eCO 2 and reduced irrigation conditions, and increased nitrogen accumulation in various organs. Furthermore, eCO 2 and inoculation with PSB individually or in combination ameliorated yield penalties under reduced irrigation conditions. Combined elevated CO 2 and inoculation with PSB resulted in the highest fruit quality. Collectively, the responses of tomato fruit yield and quality to water-reduced irrigation regimes are further enhanced by the combination of eCO 2 and PSB inoculation, and this would be efficacious in greenhouse tomato cultivation and future agriculture production. • The combined effect of PSB and eCO 2 on cherry tomato treated with reduced irrigation were studied. • The combination of PSB and eCO 2 compensated the negative effect on tomato root limitation induced by low water treatments. • The responses of tomato fruit yield and quality to reduced irrigation are enhanced by combination of PSB and eCO 2. [ABSTRACT FROM AUTHOR]

• Quantified responses of various crop types' WUE under drought intensity. • Quantified natural and management factors of WUE variation under drought. • Crop selection according to drought resistance is effective to alleviate drought. • Soil and fertilizer are effective practices for drought mitigation besides irrigation. Agriculture's need to feed the growing population is challenged by the decline in available water resources especially in the context of global climate change. Improving crop water use efficiency (WUE) will help to safeguard the environmental sustainability of food production in dryland areas. However, the impact of drought on crop WUE varies—a comprehensive and quantitative understanding of relevant factors is needed to support evidence-based management decisions. Here, we used a meta-analysis of global drylands (81 studies with 836 paired observations) to evaluate the response of various crop WUEs to drought based on various natural and human induced factors. Our results showed that responses of crop types' WUE varied under drought intensity, which probably a reference for crop selection in drylands. Cotton could be one of crop selection reference in hyper-arid or arid regions, and legume is not recommended for drylands without irrigation. Cereal crop are suitable for growing in semi-arid or dry sub-humid areas. Moreover, soil improvement and fertilizer management are the effective methods to alleviate drought stress, in addition to irrigation. Soil with medium-texture, 1.3–1.4 g cm−3 bulk density and 15–20 g kg−1 organic matter is beneficial for improving WUE under drought. Fertilizer should be carefully considered and not exceed 200 kg ha−1 for wheat in drylands. Water deficit can improve crop WUE, but should not exceed 40% of the full irrigation amount. Field management practices, such as mulching and weed control, can help alleviate drought by regulating WUE to some extent. This study evaluated the responses of various crop WUEs to drought and highlighted the factors contributing to and/or decreasing crop WUE from natural factor and management practices, which provides a basis for agricultural drought mitigation strategies under future climate change in dryland areas. [ABSTRACT FROM AUTHOR]