Climate change led to increased temperature and variable rainfall, which may pose great threats to both agricultural productions and environmental impacts. In this study, we aim to explore how changing climate and its extremes in the 20th and 21st-century influence system water use efficiency (sWUE) of a corn-soybean cropping rotation in a humid sub-tropic environment, and how much can cover crops mitigate these impacts. Different from the traditional yield-focused water use efficiency (WUE), sWUE addresses both production and environmental quality goals by considering grain yields and all major system water losses (evapotranspiration, runoff, and drainage). A calibrated crop simulation model, Root Zone Water Quality Model version 2 (RZWQM2), was applied to simulate grain yields and all major system water losses. The model was forced by daily climate data from in situ observations during 1956–2015 and 10 downscaled and bias corrected General Circulation Model (GCMs) projections under the representative concentration pathways (RCP) 4.5 and 8.5 scenarios during 2020–2079. The results showed that, under the historical baseline and the future RCP4.5 and RCP8.5 scenarios, due to the growth of cover crops, the sWUE for corn were improved by 1.7%, 2.6% and 2.3%, respectively (p-value < 0.001), and for soybean by 0.7% (p = 0.06), 1.0% (p-value < 0.001) and 0.9% (p-value < 0.001). Soil evaporation, as the largest source of water loss from the cropping system, was significantly decreased by 1.7%, 2.6% and 2.3% during the corn growing season, and by 0.7%, 1.0% and 0.9% during the soybean growing season. The annual drainage was decreased by 38 mm, 53 mm and 67 mm, under the baseline, RCP4.5 and RCP8.5, respectively. With the incorporation of wheat cover crops, the correlations of temperature or precipitation extremes with grain yields and major water losses were mostly decreased, suggesting that growing cover crop is an effective means to mitigate the impact of climate extremes on sWUE of a corn-soybean cropping.

气候变化导致气温升高和降雨量多变，这可能对农业生产和环境影响构成巨大威胁。在这项研究中，我们旨在探讨 20 世纪和 21 世纪气候变化及其极端情况如何影响潮湿亚热带环境中玉米 - 大豆轮作的系统用水效率（sWUE），以及多少可以覆盖作物减轻这些影响。与传统的以产量为重点的用水效率 (WUE) 不同，sWUE 通过考虑粮食产量和所有主要系统水分损失（蒸散、径流和排水）来解决生产和环境质量目标。一个校准的作物模拟模型，根区水质模型版本 2 (RZWQM2)，被应用于模拟谷物产量和所有主要系统的水分损失。该模型由来自 1956-2015 年现场观测的每日气候数据以及 2020-2079 年期间代表性浓度路径 (RCP) 4.5 和 8.5 情景下的 10 次缩小和偏差校正的一般环流模型 (GCM) 预测强制执行。结果表明，在历史基线和未来 RCP4.5 和 RCP8.5 情景下，由于覆盖作物的生长，玉米的 sWUE 分别提高了 1.7%、2.6% 和 2.3%（p 值< 0.001)，大豆分别为 0.7% (p = 0.06)、1.0% (p-value < 0.001) 和 0.9% (p-value < 0.001)。土壤蒸发作为种植系统水分流失的最大来源，在玉米生长季显着减少1.7%、2.6%和2.3%，在大豆生长季显着减少0.7%、1.0%和0.9%。年排水量减少38毫米，基线下分别为 53 毫米和 67 毫米，RCP4.5 和 RCP8.5。随着小麦覆盖作物的加入，温度或降水极端事件与粮食产量和主要水分损失的相关性大多降低，表明种植覆盖作物是减轻气候极端事件对玉米大豆作物 sWUE 影响的有效手段.