Evapotranspiration increase is more sensitive to vegetation greening than to vegetation type conversion in arid and semi-arid regions of China

Evapotranspiration (ET) is a crucial component of both the water cycle and energy balance, with vegetation being a key factor influencing ET. Vegetation impacts ET primarily through two modes: vegetation growth change (VGC) and vegetation type conversion (VTC). Despite the different mechanisms by which VGC and VTC influence ET, previous studies have rarely differentiated their individual effects. This has left the relative contributions of VGC and VTC to ET changes unclear. To address this gap, this study focuses on the arid and semi-arid regions of China (ASAC), where substantial vegetation changes have been observed. The spatiotemporal patterns of ET in ASAC were analyzed using remote sensing ET data from 2001 to 2018. The leaf area index (LAI) and land use/land cover (LULC) data were incorporated to perform pairwise comparison and contribution analysis to investigate the specific effects of VGC and VTC on ET. The results revealed a significant increase in mean annual ET across ASAC, with a rate of 4.5 mm/yr (p < 0.05). This increase was more pronounced in forest land and cropland compared to grasslands. ET increased across all seasons, with the largest increase occurring in summer, accounting for approximately 50 % of the annual ET increment. The pairwise comparison and contribution analysis indicated that ET in ASAC is more sensitive to vegetation greening than to VTC. VTC played a moderating role in ET changes, with relative contribution ranging from 1.3 % to 57.8 % across different LULC change types. These findings enhance our understanding of how vegetation changes influence ET. They can also provide valuable insights for land management strategies in ASAC, aiming to optimize vegetation construction and promote ecosystems sustainability.