Characterizing spatial, diurnal, and seasonal patterns of agricultural irrigation expansion-induced cooling in Northwest China from 2000 to 2020

In the last two decades, agricultural irrigation areas expanded rapidly in arid Northwest China (NWC, including North and South Xinjiang and Hexi Corridor) and altered local and regional climates. However, the increasing irrigation's climatic effects with underlying biophysical mechanisms have rarely been investigated. Here we characterized the impact patterns of irrigation expansion on land surface temperature (LST) in NWC from spatial, diurnal, and seasonal dimensions from 2000 to 2020; we then decomposed the LST changes related to multiple biophysical components using the energy balance equation and quantified the contributions of albedo and evapotranspiration (ET) to LST changes based on partial correlation and dominance analyses. We found the space-and-time method can better represent the cooling effects of long-term irrigation expansion than the space-for-time method, despite that they derived similar cooling trends. The cooling effects were most evident in South Xinjiang, followed by North Xinjiang and Hexi Corridor, and varied in different irrigation expansion sources. Specifically, more intensive cooling occurred in new irrigated areas from unused lands (-0.69 ± 0.02 K) than that from grasslands (-0.47 ± 0.05 K) and forests (-0.28 ± 0.04 K). The cooling effects were dominated by marked daytime cooling compared to negligible nighttime warming. Seasonally, the cooling effects were concentrated in the growing season (May to September), especially in July and August. LST changes related to latent heat and sensible heat fluxes significantly surpassed other biophysical components. The changes in ET and albedo jointly modulated LST variations and increased ET dominated the daytime cooling effect (r = 0.52, p < 0.001) with a relative importance of over 55%, especially in North Xinjiang. But the ET-dominated cooling would be dampened by enhanced precipitation, implying the non-trivial impact of background climate. This study can provide important references for evaluating the climatic effects caused by irrigation expansion in arid regions and for improving the climate models.