Divergent effectiveness of irrigation in enhancing food security in droughts under future climates with various emission scenarios

Food security is a critical component of the UN Sustainable Development Goals (SDGs), particularly challenging for populous developing nations like China. Droughts, exacerbated by climate change, severely impact agricultural production, reducing cereal yields and increasing the inter-annual variability of agricultural output. Model simulations consistently project an increased risk of droughts under future climate change scenarios. While the impacts of climate extremes on agriculture are well-documented, the effectiveness of irrigation, the largest water consumption sector, in mitigating drought impacts under various climate scenarios remains poorly understood. This study uses China as a case study, focusing on wheat—a highly vulnerable crop—to quantify the impact of irrigation on wheat yields and its effectiveness in enhancing resilience to droughts under different climate change scenarios. The modified Palmer Drought Severity Index (MPDSI), incorporating irrigation into the water balance, allows a more nuanced assessment of irrigation's effectiveness.

Existing literature highlights the significant impact of droughts on crop yields, with studies employing various drought indices and methodologies. However, many evaluations based on meteorological drought indices neglect the role of irrigation and other anthropogenic factors in mitigating drought impacts. Other studies utilize agricultural drought indices based on soil moisture, which cannot definitively separate irrigation's effect from other factors. The limited understanding of irrigation's effectiveness under varying climate scenarios necessitates a deeper investigation to improve water resource allocation and management. The use of MPDSI offers a unique opportunity to address this knowledge gap by directly incorporating irrigation effects into drought characterization.

This study leverages outputs from four Global Climate Models (GCMs) from the ISIMIP program to project drought evolution under historical, RCP2.6, RCP6.0, and RCP8.5 scenarios. The modified Palmer Drought Severity Index (MPDSI), which incorporates irrigation into the water balance model, was employed to characterize droughts. Drought intensity, affected area, and duration were quantified using cumulative MPDSI values, drought-influenced area, and duration in months. Three methods—Multiple Linear Regression (MLR), Deep Learning (DL), and the Erosion-Productivity Impact Calculator (EPIC) model—were used to assess and project wheat yield changes under the different scenarios. MLR is a traditional statistical method, DL is an emerging machine learning algorithm, and EPIC simulates yield based on crop physiological processes. The three methods provide a more robust assessment of yield changes by accounting for different aspects of the problem. The analysis covered nine agricultural subregions of China. The Modified Mann-Kendall (MMK) test was used to assess trends in MPDSI. Wheat yield data were obtained from the National Bureau of Statistics of China, irrigation data from the National Agricultural Weather Station, and soil available water content data from the Chinese Ecosystem Research Network.

The analysis revealed that droughts are projected to become more intense in the future, with the RCP8.5 scenario showing the most severe drought events. Under RCP2.6 and RCP6.0 scenarios, irrigation significantly mitigated drought-induced wheat yield losses. However, under the RCP8.5 scenario, the effectiveness of irrigation in reducing yield losses was minimal. Historically, droughts caused slight decreases or even increases in wheat yield in some regions, with reductions generally remaining below 15%. Under RCP2.6, drought-induced wheat yield losses increased substantially across most regions. Under RCP6.0, while yield losses were less severe than under RCP2.6, they were still greater than during the historical period. Under RCP8.5, widespread and significant yield reductions were observed across all nine subregions, with some regions experiencing losses exceeding 80%. Comparing irrigated and non-irrigated areas, irrigation substantially reduced drought-induced yield losses under all scenarios except RCP8.5, where the difference was negligible. Mild to moderate droughts had minimal impact on irrigated areas, but significantly affected non-irrigated areas. Even under severe droughts, irrigated areas experienced considerably less yield loss compared to non-irrigated areas. The findings suggest that irrigation is a highly effective drought mitigation strategy for mild to severe droughts under lower emission scenarios but loses its effectiveness under extreme drought conditions and high emission scenarios.

The study's findings demonstrate that irrigation's effectiveness in mitigating drought-induced wheat yield losses is strongly dependent on the severity of droughts and the emission scenario. While irrigation is an effective drought mitigation strategy under lower emission scenarios and milder droughts, its capacity to enhance food security under high-emission scenarios and extreme droughts is limited. This highlights the importance of climate change mitigation efforts as a crucial complement to adaptation strategies like irrigation, particularly for regions heavily reliant on irrigation. The results align with previous research highlighting irrigation's ability to enhance drought resilience in specific regions but expands on this by analyzing its effectiveness across various drought intensities and climate scenarios. The use of three distinct modeling methods (MLR, DL, EPIC) enhances the robustness of the findings by accounting for different sources of uncertainty. The limitations of assuming constant future irrigation should be noted, as this parameter could change significantly based on evolving water management practices and resource availability.

This study underscores the complex interplay between climate change, irrigation, and food security. While irrigation remains a vital adaptation strategy for enhancing drought resilience in agriculture, its effectiveness is not unlimited, especially under high-emission scenarios and severe droughts. Climate change mitigation is paramount to reducing drought intensity and ensuring the long-term effectiveness of irrigation in maintaining food security. Future research could investigate the optimal combination of irrigation strategies and climate change mitigation policies to maximize food security in a changing climate.

The study assumes constant future irrigation practices, which might not reflect future reality due to changes in water management strategies and resource availability. The analysis also relies on GCM projections, which inherently have uncertainties. The focus on wheat and China limits the generalizability of the findings to other crops and regions. Further research could explore more detailed modeling of irrigation management strategies and investigate the economic implications of different adaptation and mitigation approaches.