Impact of irrigation on land-atmosphere coupling strength in main grain-producing regions of China

Irrigation was a critical measure for ensuring stable and increased grain production, and its influence on climate change was significant and could not be overlooked. Investigating the influence of irrigation on the strength of land-atmosphere coupling was essential for enhancing our knowledge of the impact of irrigation on climate change. This study leveraged the ERA5 reanalysis dataset alongside the global irrigation area dataset to perform a quantitative analysis of Coupling Strength Index (CSI) in both densely and reference irrigated areas within China's primary grain-producing regions. The analysis incorporated soil moisture and latent heat flux to explore the impact of irrigation on CSI. First, we applied the commonly used Pettitt test, Buishand test, and SNHT test in the field of agrometeorology to conduct a change-point analysis on the calculated CSI in China's main grain-producing areas. We found that CSI experienced a shift in 1996, changing from a decreasing trend before 1996 to an increasing trend afterward. Then, we explored how irrigation affected CSI by dividing the areas into intensive irrigation regions and reference irrigation regions. The results showed that large-scale irrigation expansion could have altered local climate conditions, significantly impacting CSI. Furthermore, the larger the proportion of irrigated area, the higher the CSI. The greater the difference in the proportion of irrigated area, the greater the difference in CSI. There was a significant difference in CSI values between northern and southern China. Historically, the annual CSI values in the north ranged between −0.008 MJ/m² and 0.009 MJ/m², while in the south, they ranged between −0.018 MJ/m² and −0.007 MJ/m². The annual CSI in northern China was generally stronger than in southern China, with the northern CSI values being distributed across both positive and negative ranges, whereas the southern CSI values were exclusively in the negative range. The impact of irrigation on CSI intensified with the increase in irrigation area disparity. Irrigation had a significant enhancing effect on the CSI in China's major grain-producing regions. This effect was more pronounced in northern China compared to southern China and was greater in densely irrigated areas than in reference irrigated areas. In recent years, this enhancing effect had been increasing at rates of 0.99×10⁻⁴ MJ/m² per year in the north and 2.53×10⁻⁴ MJ/m² per year in the south. This trend suggested that China was moving towards a more arid climate. After 1996, the trend in CSI changes for densely irrigated areas in both the north and south had become consistent. However, the trend in CSI changes for reference irrigated areas in the north was higher than in the south. This had resulted in a reduction in the CSI difference between densely and reference irrigated areas in the north, while in the south, this difference had significantly increased. A significant climatic shift occurred around 1996 in East Asia, leading to changes in precipitation and soil moisture trends. This shift caused a decrease in soil moisture in the main grain-producing regions of southern China, transitioning from a "radiation-limited" to a "soil moisture-limited" regime. Consequently, the enhancing effect of irrigation on CSI in southern China's main grain-producing regions increased. Although the overall decrease in soil moisture in China after the 1990s was an indisputable fact, there was still no clear conclusion on the relative contributions of human activities and climate shifts. This study revealed the impact of irrigation on CSI in China's main grain-producing regions, providing scientific references for national food security and climate change adaptation strategies.