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Summary: In this paper, the Genetic Algorithms (GA) and Bayesian Model Averaging (BMA) were used to simultaneously conduct calibration and uncertainty analysis for the Soil and Water Assessment Tool (SWAT). In this combined method, several SWAT models with different structures are first selected; next GA is used to calibrate each model using observed streamflow data; finally, BMA is applied to combine the ensemble predictions and provide uncertainty interval estimation. This method was tested in two contrasting basins, the Little River Experimental Basin in Georgia, USA, and the Yellow River Headwater Basin in China. The results obtained in the two case studies show that this combined method can provide deterministic predictions better than or comparable to the best calibrated model using GA. The 66.7% and 90% uncertainty intervals estimated by this method were analyzed. The differences between the percentage of coverage of observations and the corresponding expected coverage percentage are within 10% for both calibration and validation periods in these two test basins. This combined methodology provides a practical and flexible tool to attain reliable deterministic simulation and uncertainty analysis of SWAT. [Copyright &y& Elsevier]

[Display omitted] • The crop parameters of spring maize in the SWAT model are calibrated. • An index is designed to assess different cropping systems and irrigation schemes. • MCI and irrigation should be reduced to stop groundwater drawdown in recent periods. • Agricultural systems are optimized at the subbasin scale during future periods. • The future climate favors grain production with groundwater balance. The global crisis of groundwater depletion is becoming a challenge in irrigation agriculture. How to optimize the agricultural system amidst climate change is crucial for groundwater and food security around the world. In this study, the multiple cropping index (MCI) reducing strategy and irrigation-limiting strategy were assessed using the SWAT-GW model in the piedmont plain of the Taihang Mountains of North China Plain (NCP). Model calibration and validation were conducted using field data for spring maize (SPM) with a target NRMSE below 25%, alongside previously calibrated parameters for winter wheat (WW), summer maize (SUM), evapotranspiration, and groundwater levels. Historical simulations (1993–2012) indicated that a two-year-three-crop cropping system (WW-SUM → SPM) with 82 mm of irrigation quota (3H2Y_O, "double-reducing strategy") maintained shallow groundwater levels while accepting a 27% yield reduction. Future simulation (2030–2049) suggest that increased groundwater recharge due to climate change might allow shallow groundwater balance through the irrigation-limiting strategy alone ("single-reducing strategy"), with crop yields expected to rise by 5.8%–6.9% for wheat and 1.2%–2.3% for maize. By 2070–2089, enhanced precipitation could lead to sufficient groundwater recharge, enabling equilibrium under a double-cropping system (WW-SUM) with full irrigation scheme ("non-reducing strategy") in most areas. Wheat yields may increase by 9.7%–10.5%, but maize yields could fall by 1.1%–10.9%. This study could provide detailed data for groundwater recovery planning in China's grain-producing regions, and offer a methodological framework for the trade-off between water scarcity and food security for other over-exploited groundwater areas worldwide. [ABSTRACT FROM AUTHOR]

Promoting appropriate water-saving irrigation is an important pathway to alleviate groundwater overexploitation in irrigated agriculture. Whether an alternative irrigation method is feasible relies on quantitative assessments of its impacts on crop yield and hydrological cycle, especially in the groundwater overdraft area that faces a dilemma between water scarcity and food security. Using a groundwater-module-improved SWAT (Soil & Water Assessment Tool) model, five GCMs (General Circulation Models) projections of CMIP6 (Coupled Model Intercomparison Project 6), and a linear programming algorithm, sprinkler irrigation was simulated, evaluated, and optimized in a representative global groundwater mining location, the piedmont plain of the North China Plain. During the historical period (1993–2012), the results showed that, in general, sprinkler irrigation had advantages in benefiting crop transpiration of winter wheat and reducing percolation, hence improving water use efficiency with reference to traditional surface irrigation. Specifically, compared to traditional surface irrigation schemes, winter wheat yields increased by 12%− 14% with a 225 mm irrigation quota; they were stable with a 150 mm irrigation quota; but they decreased by 4% with a 75 mm irrigation quota. The shallow groundwater tables would decline at an average rate of 1.29 m/yr, 0.83 m/yr, and 0.35 m/yr, respectively, under sprinkler irrigation with 225 mm, 150 mm, and 75 mm irrigation quotas. Future climate changes (increased precipitation, air temperatures, and CO 2 concentration) would bring effects of improving wheat yield by 6%− 7% and increasing groundwater vertical recharge by 58%− 122% in 2030–2049. The variations of predicted future shallow groundwater tables were − 0.37 m/yr, 0.01 m/yr, and 0.40 m/yr with the 225 mm, 150 mm, and 75 mm irrigation quotas, respectively, in the SSP2–4.5 scenario; and the corresponding results were − 0.45 m/yr, − 0.05 m/yr, and 0.34 m/yr in the SSP5–8.5 scenario. The optimization results showed that sprinkler irrigation could be implemented in approximately 50% of the study area to achieve shallow groundwater equilibrium and stable wheat production simultaneously in the near future. And the dominant sprinkler irrigation schedule after optimization was found to be scheme S3 (irrigation frequency for wheat was 5 times) with a limited irrigation quota of 150 mm. This study provides detailed data for planning sprinkler water-saving irrigation regimes and groundwater pumping strategies in China's breadbasket, as well as a reference for global irrigated agriculture with over-drafted aquifers to balance groundwater conservation and grain production. [Display omitted] • Percolation reduced by 3%− 37% for sprinkler compared to surface irrigation schemes. • Wheat yield increased under sprinkler irrigation with a 225 mm irrigation quota. • Future climate favored wheat yield, percolation, and groundwater recharge. • Climate-driven and sprinkler irrigation effects on water-food nexus were assessed. • Optimized sprinkler irrigation with a higher irrigation frequency was recommended. [ABSTRACT FROM AUTHOR]

[Display omitted] • Integrated dynamic CO 2 input and freeze–thaw cycle to establish a SWAT-BSR model. • Developed R code to screen extreme climate events and extract hydrologic variables. • Explored impacts of both single and integrated extreme climate events on hydrology. • Extreme cold and extreme drought had a strong negative impact on hydrology cycle. • Dryland corn zones were more vulnerable to integrated extreme events in the future. Extreme climate events may have a substantial impact on water conservation in the black soil region of China. This study integrated a dynamic CO 2 input method and an alternative freeze–thaw cycle function into Soil and Water Assessment Tool (SWAT) to create a SWAT-Black Soil Region (SWAT-BSR) model. The SWAT-BSR model was used to simulate extreme climate events in the Liao River Basin (LRB; 220,000 km2) and an R script was developed to extract days in which extreme climate events occurred along with corresponding hydrological variables. Extreme climate events included extreme heat, cold, precipitation, and drought events across seven zones in the LRB. Subsequently, we explored the changes in single and integrated extreme climate events during the historical period (1971–2000) and future periods (2041–2070 and 2071–2100) under the SSP2-4.5 and SSP5-8.5 scenarios. The results indicated that almost each hydrological variable was reduced under extreme cold and drought events compared to the normal conditions in the historical period. Canopy interception and percolation were decreased under extreme heat in the future periods while extreme cold was opposite with a pronounced negative impact on reference evapotranspiration and soil evaporation. In contrast, each hydrological variable under extreme precipitation increased relative to the normal conditions while hydrological variables were largely negatively affected under extreme drought. In addition, the integrated extreme climate index increased over time, with more pronounced changes under the SSP5-8.5 scenario than those of SSP2-4.5 scenario. Hydrological variables were most at risk in the dryland zones as compared to partially irrigated and irrigated zones. These findings indicated that more attention should be given to the impacts of extreme climate events on water losses from both historical and future perspectives. It is also necessary to develop mitigation strategies for water conservation in the LRB based on the extreme climate events projected to occur in different zones and periods in the future. [ABSTRACT FROM AUTHOR]

Facing the high possibility of future severe water resources scarcity, the Haihe River Basin in the North China Plain is an important region for climate change studies. This study employed an improved SWAT model with a Management Allowed Depletion (MAD) auto-irrigation method to assess the impacts of future climate change on water cycle and crop growth of irrigated winter wheat and summer maize in the Haihe River Basin, China. A total of 27 Global Climate Models (GCMs) from the latest Coupled Model Intercomparison Project 6 (CMIP6) and two Shared Socioeconomic Pathways (SSP2–4.5 and SSP5–8.5) were used to drive the SWAT-MAD model to simulate crop growth and water balance components for two future periods of 2041–2070 (2050s) and 2071–2100 (2080s). Data from almost all GCMs showed an increase in annual mean maximum air temperature, minimum air temperature, total precipitation, and solar radiation across the Haihe River Basin. Winter wheat yields increased by 15.6% (SSP245) and 16.3% (SSP585) by 2050s, and by 17.1% (SSP245) and 10.0% (SSP585) by 2080s, respectively. Summer maize yields increased by 8.9% (SSP245) and 9.6% (SSP585) by 2050s, and by 12.6% (SSP245) and 6.1% (SSP585) by 2080s, respectively. The change of leaf area index (LAI) indicated an advance in phenology of crops, which might be related to the increase in temperature. In the four future climate scenarios, precipitation, percolation, and surface runoff increased by 19.16–57.25 mm, 7.26–22.18 mm, and 1.45–4.87 mm during winter wheat growing season, 125.41–205.24 mm, 51.03–96.37 mm, and 33.97–68.69 mm during summer maize growing season. However, the irrigation amounts decreased by 18.82–83.74 mm and 35.71–50.92 mm during wheat and maize growing seasons, respectively. The annual Net Groundwater Use (NGU) decreased with time and the crop water productivity (CWP) increased in the future, which have positive effects on the sustainable development in this double-cropping system with intensive irrigation management. [Display omitted] • Bias-corrected CMIP6 of 27 GCMs for SSP-4.5 & 8.5 from 2041 to 2100 were downscaled. • A physically-based SWAT auto irrigation was used in the intensively irrigated basin. • Future CO 2 , temperatures, radiation, rainfall, percolation, & runoff were increased. • Decreases in irrigation and net groundwater use were found in most GCM projections. • Comprehensive future climate effects led to the increase of crop water productivity. [ABSTRACT FROM AUTHOR]

Over-exploitation of groundwater due to intensive irrigation and anticipated climate change pose severe threats to the water and food security worldwide, particularly in the North China Plain (NCP). Limited irrigation has been recognized as an effective way to improve crop water productivity and slow the rapid decline of groundwater levels. Whether optimized limited irrigation strategies could achieve a balance between groundwater pumping and grain production in the NCP under future climate change deserves further study. In this study, an improved Soil and Water Assessment Tool (SWAT) model was used to simulate climate change impacts on shallow groundwater levels and crop production under limited irrigation strategies to suggest optimal irrigation management practices under future climate conditions in the NCP. The simulations of eleven limited irrigation strategies for winter wheat with targeted irrigations at different growth stages and with irrigated or rainfed summer maize were compared with future business-as-usual management. Climate change impacts showed that mean wheat (maize) yield under adequate irrigation was expected to increase by 13.2% (4.9%) during the middle time period (2041-2070) and by 11.2% (4.6%) during the late time period (2071-2100) under three SSPs compared to the historical period (1971-2000). Mean decline rate of shallow groundwater level slowed by approximately 1 m a -1 during the entire future period (2041-2100) under three SSPs with a greater reduction for SSP5-8.5. The average contribution rate of future climate toward the balance of shallow groundwater pumping and replenishment was 62.9%. Based on the simulated crop yields and decline rate of shallow groundwater level under the future climate, the most appropriate limited irrigation was achieved by applying irrigation during the jointing stage of wheat with rainfed maize, which could achieve the groundwater recovery and sustainable food production., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)