Effects of vegetation restoration on evapotranspiration water consumption in mountainous areas and assessment of its remaining restoration space

In mountainous areas with inadequate precipitation, anthropogenic vegetation restoration would increase evapotranspiration water consumption and decrease regional water yield, affecting downstream water use. Assessing vegetation restoration-related water consumption and future vegetation restoration potential is important for vegetation management in mountainous areas. However, it is still difficult to evaluate the impact of vegetation restoration on evapotranspiration water consumption and determine the appropriate restoration level. This study focused on the Haihe River Basin as a typical afforestation area in China with growing competition for water use between the upstream afforested vegetation and the downstream urban development. This study adopted the Penman-Monteith-Leuning evapotranspiration model with a de-trending data series method to analyze actual evapotranspiration and vegetation restoration potential. The results show that 1) the average precipitation in the mountainous areas of the Haihe River Basin and the actual evapotranspiration (AET) between 2000 and 2019 were 508 and 317 mm, respectively, while the precipitation, leaf area index (LAI), and actual evapotranspiration all showed an increasing trend in the past two decades with growth rates of 3.8 mm/yr, 0.0015 (cm3/cm3)/yr, and 1.37 mm/yr, respectively; 2) in the past 20 years, vegetation restoration has cumulatively led to an actual evapotranspiration increase by 314, 269, 175, 429, 277, and 187 mm in the Luan-He Mountainous Basin (LHMB), Yong-Ding-He Mountainous Basin (YDHMB), Da-Qing-He Mountainous Basin (DQHMB), Bei-San-He Mountainous Basin (BSHMB), Zi-Ya -He Mountainous Basin (ZYHMB), and Zhang-Wei-He Mountainous Basin (ZWHMB), respectively; 3) the LAI maximum recovery level in the six sub-basins was deduced by using the linear fitting formula. When considering the replacement water of over-drafting groundwater in the downstream plain, the LHMB, YDHMB, ZYHMB, ZWHMB still have the potential for vegetation restoration, and the maximum LAI could be restored to 1.83, 1.30, 1.09, and 1.6 cm3/cm3, respectively. In summary, large-scale anthropogenic afforestation should consider the decrease in regional water yield to avoid the imbalance of ecological and human water use. This research method can be extended to other areas of water scarcity and provide a reference for mountain vegetation restoration to set a reasonable threshold.