全球树木恢复导致的区域水资源可用性变化

   树木恢复是储存大气碳和缓解气候变化的有效途径。然而，大规模的树木覆盖扩大会增加蒸发，导致当地的水资源可用性和流量减少。最近的研究表明，通过加强大气水分循环，增加降水量可以抵消这种影响。在这里，我们使用数据驱动的Budyko模型和UTrack水分循环数据集计算了9亿公顷全球树木恢复将如何影响蒸发和降水。我们的研究表明，直接增强的蒸发和间接增强的降水的综合效应创造了水资源可利用性变化的复杂模式。在一些地区，大规模的树木覆盖扩张可以将水资源利用率提高6%，而在其他地区，则可以将水资源利用率降低38%。对大型河流流域有着不同的影响：一些河流可能会因为蒸发增强而损失6%的流量，而对其他河流来说，更大的蒸发被更多的水分循环所抵消。几个所谓的森林恢复热点地区可能会失水，包括今天已经面临缺水的地区。树木恢复显著改变了陆地水通量，我们强调未来的树木恢复策略应考虑这些水文效应。

 

Fig. 1: Impacts of forest restoration on water fluxes and water availability. a, The tree-restoration potential: the percentage area of each pixel that is suitable for tree restoration. b , Model ensemble mean versus observed streamflow ( Q ) measurements for 19 validated river basins. The error bars for Q Budyko indicate the standard deviation over the six Budyko models. The error bars for Q validation indicate a 20% error. The river basins are Amazon (AM), Brahmaputra (BR), Colorado (COL), Congo (CON), Danube (DA), Ganges (GA), La Plata (LA), Mackenzie (MA), Mekong (ME), Mississippi (MI), Murray–Darling (MU), Niger (NIG), Nile (NIL), Orinoco (OR), Rhine (RH), Volga (VO), Yangtze (YA), Yenisei (YE) and Zambezi (ZA). c – f , The absolute annual change in water fluxes after tree restoration: change in evaporation ( c ), precipitation ( d ), water availability without evaporation recycling ( e ) and water availability with evaporation recycling ( f ). Note that e is the inverse of c : without the feedback of evaporation recycling, the local increase in evaporation equals the local decrease in water availability. g , The histogram shows the distribution of the global changes in water availability without and with evaporation recycling; 89% (without recycling) and 91% (with recycling) of the data fall within the displayed range of –20 ? mm ? yr ^–1 to +10 ? mm ? yr ^–1 . All maps display the 0.1° mean values, except for c , which displays the 0.5° mean value.

Fig. 2: Impacts of global tree restoration on hydrological fluxes in selected river basins. a , b , Mean wetness index (ratio of precipitation to potential evaporation; a ) and tree-restoration potential ( b ) for each river basin. c , The change in streamflow (Δ Q ) with evaporation recycling. The dots indicate the mean river-basin change in streamflow, and the bars indicate one standard deviation (the variation over the six Budyko models). d , The change in evaporation (Δ E , in green) and precipitation (Δ P , in blue). The green bar indicates the increase in evaporation without evaporation recycling, and the green line indicates the increase in evaporation with evaporation recycling (the increase in evaporation when taking into account the increased precipitation). The river basins are sorted from the lowest to the highest decrease in streamflow. The river-basin boundaries are taken from ref.

Fig. 3: Implications of the percentage restored area and the implications for water scarcity. a , Water availability could decrease in ‘hot spots for tree restoration’. The lower, middle and upper boundaries of the boxes display the 25th, 50th and 75th percentiles of the data (the interquartile range), respectively. The lines extend to a maximum of 1.5 times the interquartile range. b , Water availability will decrease in several regions that currently face water scarcity: the colour indicates whether local water availability will decrease (orange) or increase (blue) following the studied tree-restoration scenario. The cross-hatched regions currently face freshwater scarcity for at least three months per year, and the bold hatched regions face freshwater scarcity for at least nine months per year ³⁴ . Note that we quantified the change in water availability on an annual timescale. The sign of change could vary seasonally, and water availability could, for example, decrease on a yearly basis but remain equal, or even increase, in the dry season.