Large‐Scale Afforestation Enhances Precipitation by Intensifying the Atmospheric Water Cycle Over the Chinese Loess Plateau

Abstract

Afforestation is regarded as the most appropriate approach to control soil erosion (Jia et al., 2017;Porto et al., 2009), but expanding vegetation needs more water to maintain its growth (Huang et al., 2020;Teuling et al., 2019;Q. Wang et al., 2021). Precipitation thus plays a crucial role in the sustainability of afforestation, especially for water-limited areas. Atmospheric moisture is of great relevance to precipitation generation (Payne et al., 2020). There are two sources for atmospheric moisture overlying a land area: the first is external moisture flowing into this area by horizontal advection and the second is internal moisture provided by the local evapotranspiration (ET) (Y. Zhao & Zhou, 2021). The ratio of precipitation transformed from internal recycled moisture to the total precipitation received by this area is defined as the precipitation recycling ratio (PRR) (Holgate et al., 2020), which can serve as an indicator of the intensity of the water cycle (Gao et al., 2020;Trenberth, 1999). Both anthropogenic activities and climate variability affect the transport of atmospheric moisture and its transformation to precipitation (Baudena et al., 2021;Tuinenburg & Staal, 2020). To design science-informed and sustainable policies for afforestation, there is a growing need to explore the response of the atmospheric water cycle to afforestation and climate change.Afforestation alters moisture and energy fluxes interchanged between the atmosphere and land surface by affecting surface characteristics (Baudena et al., 2021;Hagos et al., 2014;. The Loess Plateau (LP) in China is an ideal platform to study the impact of large-scale afforestation on the atmospheric water cycle, since it launched the world's largest afforestation program (Cao et al., 2019), the Grain for Green Program (GFGP) in 1999. With an investment of US$54.57 billion (Bryan et al., 2018), the GFGP has led to a 57% decline

The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence

“…On the Loess Plateau, evapotranspiration has increased by about (Table 2). This reflects the limited change in the vegetation cover, which increased by only of the total area, from approximately in – to in – (Tian et al, 2022, figure 3c). An analysis of isotopic data, similar to the one performed by Wright et al (2017) for the Amazon, could help quantify the contribution of increased evapotranspiration to the added atmospheric moisture.…”

“…These values span abiotic and biotic regimes in Figure 9b. Increased greenness has been associated with a rise of atmospheric water content by about – mm/year across the plateau (Jiang & Liang, 2013; Tian et al, 2022). Our findings predict distinct responses across the region.…”

“…The analysis of the MERRA‐2 reanalyses data set by Tian et al (2022) revealed that the long‐term intense re‐greening on the Loess Plateau was accompanied by intensification of moisture convergence (Table 2). This suggest that the region is undergoing a transition towards the biotic regime.…”

“…As an example, the mean change of the water yield with precipitation established by Zhang et al (2022) for the Loess Plateau as a whole, see Figure 2b, is about two times the values characteristic of grassland ecosystems (if we use the dependencies in Figure 2 as a space‐for‐time substitution). This might be an indication that approximately half of the observed response reflects increased plant transpiration in the region with the rest due to external climatic conditions, for example, regional warming and, hence, increased carrying capacity of the atmosphere with respect to the water vapor (Tian et al, 2022). This is consistent with the findings of Cui et al (2022): the trend of moisture convergence attributed to re‐greening, mm/year 2 , is approximately half of the observed long‐term moisture convergence trend established by Hobeichi et al (2022; Table 3).…”

“…For the Loess Plateau, we used the data from figure 10 of Tian et al (2022), which include evapotranspiration, precipitation, atmospheric moisture content, and atmospheric moisture fluxes and from MERRA‐2 reanalysis (monthly data at resolution) for the years from to for the rainy season (from June to September).…”