Food systems contribute significantly to global greenhouse gas (GHG) emissions, demanding urgent emission reduction strategies. Simultaneously, enhancing food system resilience to climate change through adaptation measures is critical. Irrigation plays a vital role in boosting crop productivity and mitigating losses from drought and heat stress, making it a valuable climate adaptation strategy. However, irrigation also produces GHG emissions through energy consumption and other processes, potentially creating a conflict between climate adaptation and mitigation goals. The scale and distribution of these irrigation-related emissions are not fully understood, hindering effective policy development. This study addresses this knowledge gap by providing a comprehensive national-scale assessment of GHG emissions from US irrigation pumping, considering the interplay between irrigation expansion and GHG mitigation efforts. Understanding these emissions is crucial for developing effective climate-smart irrigation policies that balance the adaptive benefits of irrigation with its environmental impacts. Energy is needed for groundwater extraction, surface water transport, and pressurized irrigation system operation. Beyond on-farm energy use, off-farm infrastructure and energy are also involved in water diversion, transport, and storage. Additionally, irrigation influences soil-based N₂O emissions, reservoir CH₄ emissions, and CO₂ degassing from groundwater. Current national GHG accounting frameworks often omit irrigation energy use emissions, hindering the assessment of specific management practices' impacts. This study aims to bridge this gap by providing a spatially explicit, national-scale estimate of GHG emissions from irrigation pumping and analyzing the implications of recent climate policies for emission reduction.

While previous studies have investigated energy use in the US water sector, many focused on public water supply rather than irrigation, despite irrigation's significantly larger water withdrawals. Existing research on irrigation pumping energy use often features regional assessments, exclusively considers electrical use, lacks spatial resolution, or doesn't estimate associated GHG emissions. This study builds upon this existing literature by providing a national-scale, spatially explicit assessment of GHG emissions from irrigation energy use, thereby offering a more comprehensive understanding of the issue. The study fills a critical gap by providing a comprehensive national-scale analysis, resolving emissions to the level of specific management practices (irrigation) and offering a spatially explicit assessment.

This study quantified GHG emissions from US irrigation pumping energy use in 2018 using a three-step approach. First, it leveraged state-level data on fuel expenditures from the USDA Irrigation and Water Management Survey, fuel prices from the US Energy Information Administration, and emission factors from the US Environmental Protection Agency to calculate energy- and water source-specific emissions. These estimates were then downscaled to the county level using data on irrigation water withdrawals. Second, county-level emissions were allocated to 12 major irrigated crops using data on crop-specific irrigated area and crop water demand, adjusting for aridity. Third, the study projected the impacts of grid decarbonization and irrigation pump electrification on future irrigation emissions. The methodology included a Markov Chain Monte Carlo approach to capture uncertainty associated with input data. The researchers integrated data from various sources, including the USDA Irrigation and Water Management Survey, the US Energy Information Administration, the US Environmental Protection Agency, and the US Geological Survey. The county-level downscaling involved adjustments for groundwater depth and conveyance losses. Crop-specific emissions were estimated using crop-specific irrigation water application rates and irrigated area. Future emission projections under different policy scenarios incorporated state-level projections of changes in electrical grid emissions factors from the National Renewable Energy Laboratory. The study also modeled the impact of pump electrification on emission reductions. The key data sources included state-level data on fuel expenditures for on-farm irrigation pumps, fuel prices, emission factors, volume of water withdrawn for crop irrigation, crop-specific irrigated area and crop water demand, and projected changes in electrical grid emission factors. These data were integrated and analyzed using statistical methods and modelling techniques to generate comprehensive estimates of irrigation-related greenhouse gas emissions and their spatial variability. Uncertainty analysis was also incorporated to account for the uncertainties associated with input data.

The study found that in 2018, on-farm irrigation pumps in the US produced 12.64 million metric tonnes (MMT) CO₂e (90% CI: 10.44, 15.05 MMT CO₂e), representing 16% of the total energy use emissions attributed to the agriculture, forestry, and fisheries sector. Groundwater pumping contributed 85% of the total emissions. Electricity use dominated pumping emissions (69%), followed by natural gas (19%) and diesel (11%). Spatial variability in county-level emissions was significant, with Texas, Nebraska, and California accounting for 46% of national GHG emissions. Emissions were positively associated with irrigated area, irrigation water use, groundwater reliance, and groundwater depth, while negatively associated with pump fuel efficiency. High GHG emissions were concentrated in the High Plains Aquifer, Mississippi Delta, California's Central Valley, and the Gila and Imperial Valleys. Crop-specific analysis showed that corn for grain produced the most total emissions (2.82 MMT CO₂e), while sorghum and cotton exhibited high emissions intensities due to their geographic distributions and high groundwater reliance. Soybeans had the lowest emissions intensity. Projections under different policy scenarios indicated that reductions in the emissions intensity of the electrical grid would substantially reduce emissions from irrigation pumping. Under current policy, including the Inflation Reduction Act (IRA), annual pumping emissions are projected to decrease by 46% by 2050. Maintaining key IRA tax credits could further reduce emissions by 59%. Pump electrification, combined with current policy, could decrease annual emissions by 70%, reaching 86% reduction with the tax credit extension. The study also highlighted additional GHG emission sources associated with irrigation, such as increased N₂O emissions from irrigated soil and CO₂ degassing from groundwater depletion, emphasizing the need for a comprehensive approach to mitigate irrigation-related GHG emissions.

The findings highlight the significant contribution of irrigation pumping to agricultural GHG emissions and the substantial spatial heterogeneity of these emissions. This underlines the importance of considering energy use in irrigation when assessing the environmental impact of agricultural practices. The results show that irrigation expansion's emission intensity will vary significantly across regions, depending on factors like groundwater dependence, water table depth, and the cleanliness of the electrical grid. The study demonstrates the potential for significant emission reductions through both grid decarbonization and the adoption of electric pumps. The research also points towards the necessity of addressing infrastructural and economic barriers to pump electrification. The high spatial heterogeneity observed suggests that targeted policies focusing on specific regions and crops are needed for effective emission reductions. The study's findings have implications for the design of climate-smart irrigation policies, emphasizing the need to consider not just the adaptive benefits but also the carbon costs of irrigation expansion. This study contributes significantly to the understanding of the role of irrigation in agricultural GHG emissions and informs the development of policies aimed at enhancing sustainability in the agricultural sector.

This study provides a comprehensive, spatially explicit assessment of GHG emissions from US irrigation pumping, revealing its significant contribution to agricultural sector emissions and highlighting substantial spatial heterogeneity. The findings demonstrate substantial emission reduction potential through grid decarbonization and pump electrification. Future research should focus on quantifying additional irrigation-related GHG emission sources and analyzing the interactions between pump energy demand, electrification, and grid decarbonization under various climate change scenarios. Integrating these findings into climate-smart irrigation policies is crucial for achieving both agricultural productivity goals and ambitious climate targets.

The study's reliance on reported fuel expenditures and energy prices as proxies for energy use introduces potential uncertainties. Data limitations, such as incomplete response rates and potential misreporting in the USDA survey, might affect the accuracy of emission estimates. The study's allocation of emissions to individual crops is subject to data limitations and discrepancies between USDA and USGS data on water use. Future studies could improve the accuracy and spatial resolution of emission estimates by integrating more precise energy use data. The projection of future emissions relies on assumptions about grid decarbonization and pump electrification rates, and these projections are subject to uncertainties in future technological advancements and policy developments. The study's focus on on-farm pumping emissions does not capture all irrigation-related GHG emissions, including off-farm energy use, soil N₂O emissions, and reservoir CH₄ emissions. A more comprehensive study encompassing these emissions sources would improve the accuracy of the overall GHG footprint of irrigation.