Irrigation Cooling Effect on Local Temperatures in the North China Plain Based on an Improved Detection Method

Irrigation has excellent potential for altering surface characteristics and the local climate. Although studies using site observations or remote sensing data have demonstrated an irrigation cooling effect (ICE) on the air temperature (Tem) and land surface temperature (LST), it is difficult to eliminate other stress factors due to different backgrounds. We characterized the irrigation effect as the differences (Delta) of LST and DCT (DCT = LST Tem) between irrigated and adjacent non-irrigated areas. An improved method was proposed to detect it over the North China Plain (NCP) based on satellite observations. We also investigated the effects of irrigation on Tem, precipitation, NDVI, and ET, and explored the relationships between them. The results show that irrigation induced a decrease in the daytime/nighttime LST and DCT ( 0.13/ 0.09 and 0.14/ 0.07 degrees C yr(-1)), Tem (-0.023 degrees C in spring), and precipitation (-1.461 mm yr(-1)), and an increase in NDVI (0.03 in spring) and ET (0.289 mm yr(-1)) across the NCP. The effect on nighttime LST and NDVI increased by 0.04 degrees C 10 yr(-1) and 0.003 10 yr(-1), and that on ET weakened by 0.23 mm 10 yr(-1) during 2000-2015. The ICE on the LST had evident spatiotemporal heterogeneity, which was greater in the daytime, in the spring, and in the northern area of the NCP (dry-hot conditions). The daytime ICE in the NCP and northern NCP was 0.37 and 0.50 degrees C during spring, respectively, with the strongest ICE of 0.60 degrees C in Henan; however, the ICE was less evident (<0.1 inverted perpendicular C) in the southern NCP throughout the year. The DNDVI, DET, and DTem were the main factors driving ICE, explaining approximatively 22%, 45%, and 25% of the daytime ICE, respectively. For every unit of these measures that was increased, the daytime ICE increased by about 7.3, 4.6, and 1.5 degrees C, respectively. This study highlights the broad irrigation effect on LST, ET, NDVI, and the climate, and provides important information for predicting climate change in the future. The improved method is more suitable for regions with uneven terrain and a varying climate.