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1. The greenhouse gas cost of agricultural intensification with groundwater irrigation in a Midwest U.S. row cropping system

   https://doi.org/10.1111/gcb.14472  

   McGill, Bonnie M.; Hamilton, Stephen K.; Millar, Neville; Robertson, G. Philip (October 2018, Global Change Biology)

   Abstract Groundwater irrigation of cropland is expanding worldwide with poorly known implications for climate change. This study compares experimental measurements of the net global warming impact of a rainfed versus a groundwater‐irrigated corn (maize)–soybean–wheat, no‐till cropping system in the Midwest US, the region that produces the majority of U.S. corn and soybean. Irrigation significantly increased soil organic carbon (C) storage in the upper 25 cm, but not by enough to make up for the CO₂‐equivalent (CO₂e) costs of fossil fuel power, soil emissions of nitrous oxide (N₂O), and degassing of supersaturated CO₂and N₂O from the groundwater. A rainfed reference system had a net mitigating effect of −13.9 (±31) g CO₂e m⁻² year⁻¹, but with irrigation at an average rate for the region, the irrigated system contributed to global warming with net greenhouse gas (GHG) emissions of 27.1 (±32) g CO₂e m⁻² year⁻¹. Compared to the rainfed system, the irrigated system had 45% more GHG emissions and 7% more C sequestration. The irrigation‐associated increase in soil N₂O and fossil fuel emissions contributed 18% and 9%, respectively, to the system's total emissions in an average irrigation year. Groundwater degassing of CO₂and N₂O are missing components of previous assessments of the GHG cost of groundwater irrigation; together they were 4% of the irrigated system's total emissions. The irrigated system's net impact normalized by crop yield (GHG intensity) was +0.04 (±0.006) kg CO₂e kg⁻¹yield, close to that of the rainfed system, which was −0.03 (±0.002) kg CO₂e kg⁻¹yield. Thus, the increased crop yield resulting from irrigation can ameliorate overall GHG emissions if intensification by irrigation prevents land conversion emissions elsewhere, although the expansion of irrigation risks depletion of local water resources. 

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2. Understanding the Food‐Energy‐Water Nexus in Mixed Irrigation Regimes Using a Regional Hydroeconomic Optimization Modeling Framework

   https://doi.org/10.1029/2022WR033691  

   Kumar, Hemant; Zhu, Tingju; Sankarasubramanian, A. (June 2023, Water Resources Research)

   Abstract Understanding the nexus between food, energy, and water systems (FEW) is critical for basins with intensive agricultural water use as they face significant challenges under changing climate and regional development. We investigate the food, energy, and water nexus through a regional hydroeconomic optimization (RHEO) modeling framework. The crop production in RHEO is estimated through a hierarchical regression model developed using a biophysical model, AquaCropOS, forced with daily climatic inputs. Incorporating the hierarchical model within the RHEO also reduces the computation time by enabling parallel programming within the AquaCropOS and facilitates mixed irrigation—rainfed, fully irrigated and deficit irrigation—strategies. To demonstrate the RHEO framework, we considered a groundwater‐dominated basin, South Flint River Basin, Georgia, for developing mixed irrigation strategies over 31 years. Our analyses show that optimal deficit irrigation is economically better than full irrigation, which increases the groundwater pumping cost. Thus, considering deficit irrigation in a groundwater‐dominated basin reduces the water, carbon, and energy footprints, thereby reducing FEW vulnerability. The RHEO also could be employed for analyzing FEW nexus under potential climate change and future regional development scenarios. 

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3. Irrigation benefits outweigh costs in more US croplands by mid-century

   https://doi.org/10.1038/s43247-023-00889-0  

   Partridge, Trevor; Winter, Jonathan; Kendall, Anthony; Basso, Bruno; Pei, Lisi; Hyndman, David (August 2023, Communications Earth & Environment)

   Abstract Irrigation can increase crop yields and could be a key climate adaptation strategy. However, future water availability is uncertain. Here we explore the economic costs and benefits of existing and expanded irrigation of maize and soybean throughout the United States. We examine both middle and end of the 21st-century conditions under future climates that span the range of projections. By mid-century we find an expansion in the area where the benefits of irrigation outweigh groundwater pumping and equipment ownership costs. Increased crop water demands limit the region where maize could be sustainably irrigated, but sustainably irrigated soybean is likely feasible throughout regions of the midwestern and southeastern United States. Shifting incentives for installing and maintaining irrigation equipment could place additional challenges on resource availability. It will be important for decision makers to understand and account for local water demand and availability when developing policies guiding irrigation installation and use. 

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4. Life cycle greenhouse gas emissions for irrigated corn production in the U.S. great plains

   https://doi.org/10.1016/j.envc.2023.100750  

   Koushki, Raana; Sharma, Sumit; Warren, Jason; Foltz, Mary E. (December 2023, Environmental Challenges)

   Agricultural management practices improve crop yields to satisfy food demand of the growing population. However, these activities can have negative consequences, including the release of greenhouse gas (GHG) emissions that contribute to global climate change. To mitigate this global environmental problem, the management practices that contribute the most to system GHG emissions should be identified and targeted to mitigate emissions. Accordingly, we estimated the cradle-to-product GHG emissions of irrigated corn production under various farmer-selected scenarios at an experimental testing field in the semi-arid U.S. Great Plains. We applied a carbon footprint approach to quantify life cycle GHG emissions associated with pre-field (e.g., energy production, fertilizer production) and in-field (e.g., groundwater pumping, fertilizer application) activities within fourteen scenarios in the 2020 Oklahoma Testing Ag Performance Solutions (TAPS) sprinkler corn competition. We determined that 63% of the total GHG emission from corn production was associated with in- field activities and that agricultural soil emissions were the overall driving factor. Soil biochemical processes within agricultural soils were expected to contribute an average of 89 ± 18 g CO2-eq kg− 1 corn of the total 271 ± 46 g CO2-eq kg− 1 corn estimated from these systems. On-site natural gas combustion for agricultural groundwater pumping, pre-field fertilizer production, and pre-field energy production for groundwater pumping were the next most influential parameters on total GHG emissions. Diesel fuel, seed, and herbicide production had insignificant contributions to total GHG emissions from corn production. The model was most sensitive to the modeled GHG emissions from agricultural soil, which had significant uncertainty in the emission factor. Therefore, future efforts should target field measurements to better predict the contribution of direct soil emissions to total GHG emissions, particularly under different managements. In addition, identifying the optimal application rate of irrigation water and fertilizer will help to decrease GHG emissions from groundwater irrigated crops. 

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5. Coupling a large-scale hydrological model (CWatM v1.1) with a high-resolution groundwater flow model (MODFLOW 6) to assess the impact of irrigation at regional scale

   https://doi.org/10.5194/gmd-15-7099-2022  

   Guillaumot, Luca; Smilovic, Mikhail; Burek, Peter; de Bruijn, Jens; Greve, Peter; Kahil, Taher; Wada, Yoshihide (January 2022, Geoscientific Model Development)

   Abstract. In the context of changing climate and increasing waterdemand, large-scale hydrological models are helpful for understanding andprojecting future water resources across scales. Groundwater is a criticalfreshwater resource and strongly controls river flow throughout the year. Itis also essential for ecosystems and contributes to evapotranspiration,resulting in climate feedback. However, groundwater systems worldwide arequite diverse, including thick multilayer aquifers and thin heterogeneousaquifers. Recently, efforts have been made to improve the representation ofgroundwater systems in large-scale hydrological models. The evaluation ofthe accuracy of these model outputs is challenging because (1) they areapplied at much coarser resolutions than hillslope scale, (2) they simplifygeological structures generally known at local scale, and (3) they do notadequately include local water management practices (mainly groundwaterpumping). Here, we apply a large-scale hydrological model (CWatM), coupledwith the groundwater flow model MODFLOW, in two different climatic,geological, and socioeconomic regions: the Seewinkel area (Austria) and theBhima basin (India). The coupled model enables simulation of the impact ofthe water table on groundwater–soil and groundwater–river exchanges,groundwater recharge through leaking canals, and groundwater pumping. Thisregional-scale analysis enables assessment of the model's ability tosimulate water tables at fine spatial resolutions (1 km for CWatM, 100–250 m for MODFLOW) and when groundwater pumping is well estimated. Evaluatinglarge-scale models remains challenging, but the results show that thereproduction of (1) average water table fluctuations and (2) water tabledepths without bias can be a benchmark objective of such models. We foundthat grid resolution is the main factor that affects water table depth biasbecause it smooths river incision, while pumping affects time fluctuations.Finally, we use the model to assess the impact of groundwater-basedirrigation pumping on evapotranspiration, groundwater recharge, and watertable observations from boreholes. 

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