Causative impact of air pollution on evapotranspiration in the North China Plain

Atmospheric dispersion conditions strongly impact air pollution under identical surface emissions. The degree of air pollution in the Jing-Jin-Ji region is so severe that it may impose feedback on local climate. Reference evapotranspiration (ET₀) plays a significant role in the estimation of crop water requirements, as well as in studies on climate variation and change. Since the traditional correlation analysis cannot capture the causality, we apply the convergent cross mapping method (CCM) in this study to observationally investigate whether the air pollution impacts ET₀. The results indicate that southwest regions of Jing-Jin-Ji always suffer higher PM_2.5 concentration than north regions through the whole year, and correlation analysis suggests that PM_2.5 concentration has a significant negative effect on ET₀ in most cities. The causality detection with CCM quantitatively demonstrates the significantly causative influence of PM_2.5 concentration on ET₀, higher PM_2.5 concentration decreasing ET₀. However, CCM analysis suggests that PM_2.5 concentration has a relatively weak causal influence on ET₀ while the correlation analysis gives the near zero correlation coefficient in Zhangjiakou city, indicating that the causative influence of PM_2.5 concentration on ET₀ is better revealed with CCM method than the correlation analysis. Considering that ET₀ is strongly associated with crop water requirement, the amount of water for agricultural irrigation could be reduced at high PM_2.5 concentrations. These findings can be utilized to improve the efficiency of water resources utilization, and reduce the exploiting amount of groundwater in the Jing-Jin-Ji region, although PM_2.5 is detrimental to human health.