Irrigation in the North China plain regulates the diurnal cycle of precipitation and regional water cycle

Irrigation, a significant human activity, impacts regional water cycles and climate. However, its effect on the diurnal cycle of precipitation (DCP) and the associated water cycle remains uncertain. In this study, we investigate the influence of irrigation in the North China Plain on precipitation and water cycle by utilizing a regional weather model with a water vapor tracing scheme. Our findings indicate that, during the afternoon, irrigation in the plain area generates a cooling effect, intensifying the thermal contrast between the mountains and adjacent plain and strengthening the mountain-plains solenoid. This leads to an increase in the amount and frequency of afternoon precipitation in the northwestern mountains, while the southeastern plains experience a decrease. This cooling and moistening effects of irrigation inhibit the development of the planetary boundary layer, posing a challenge for air to reach the lifting condensation level, which hinders cloud and precipitation formation and delays the peak of afternoon precipitation. Although irrigation's direct contribution to total precipitation does not exceed 6%, it is responsible for approximately 30% of the total precipitation changes induced by irrigation. The precipitation recycling ratio in the plain is elevated under irrigation, especially in the mountains, likely due to the enhanced moisture and altered local circulations. Irrigation's positive impact on precipitation is more pronounced under dry soil conditions, while under wet soil conditions, this positive impact diminishes and may even result in a negative impact.