基于水量平衡原理的黄土高原林草植被覆盖度恢复潜力评估

为了科学评价黄土高原林草措施的可持续性，基于林草生态系统蒸散发（ET）与降雨补给之间的水量平衡动态关系，开展林草植被覆盖度恢复潜力研究具有一定的现实意义。利用实测水文气象要素数据和PML_V2 ET、GRACE和MOD13 A1 EVI等遥感产品，阐明了增强型植被指数（EVI）与ET的时空变化特征，构建了ET和EVI的定量响应关系，定量估算了不同降雨情景下黄土高原林草植被覆盖度恢复阈值与恢复潜力。研究结果表明：（1） PML产品基于通量站点观测值与水量平衡公式验证的误差分别为4.5 mm/8d、34.3 mm/a，产品精度优于MOD16 A2 GF。黄土高原2000-2018年多年平均ET为445.36 mm，多年平均EVI为0.17；黄土高原林草植被ET和EVI的增长速率分别为5.08 mm/a和0.0026/a。（2）利用多元逐步回归方法逐像元构建了ET与气象要素和EVI的最优响应关系模型，平均均方根误差（RMSE）为44.5 mm。（3）丰水年、平水年、枯水年不同降雨情景下黄土高原林草植被覆盖度平均恢复阈值为（71.5±37.3）%、（55.6±35.9）%和（22.4±26.0）%；平均恢复潜力为（9.4±30.4）%、（-6.4±23.8）%和（-39.7±26.2）%，平均RMSE为5.7%。（4）林草植被覆盖度恢复潜力存在空间异质性。不同历史降雨情景对干旱指数2.5-4区间的林草植被恢复潜力影响最为剧烈。若未来黄土高原经历长时期平水或枯水年时段，林草植被将会存在不同程度的退化风险。

Evaluation of restoration potential of shrubs-herbs-arbor vegetation coverage on the Loess Plateau based on the principle of water balance

In order to scientifically evaluate the sustainability of shrubs-herbs-arbor measures on the Loess Plateau, it is of practical significance to study the restoration potential of shrubs-herbs-arbor vegetation coverage based on the dynamic relationship between evapotranspiration (ET) and rainfall recharge of forest-grassland ecosystem. Based on the statistic of measured hydrometeorological elements and remote sensing products such as PML_V2 ET, GRACE and MOD13 A1 EVI, the temporal and spatial variation characteristics of the enhanced vegetation index (EVI) and ET were revealed. The quantitative response relationship between ET and EVI was established, and the restoration threshold and potential of shrubs-herbs-arbor vegetation coverage on the Loess Plateau were quantitatively estimated under different rainfall scenarios. The results show that:(1) the errors of PML products verified through the observed values of flux stations and water balance formula were 4.5 mm/8d and 34.3 mm/a, respectively, and the product accuracy was better than MOD16 A2 GF. The annual mean ET and EVI were 445.36 mm and 0.17 from 2000 to 2018 on the Loess Plateau. The growth rates of shrubs-herbs-arbor vegetation''s ET and EVI were 5.08 mm/a and 0.0026/a. (2) The optimal response model of ET and meteorological elements and EVI was constructed by using multiple stepwise regression method, and the average of root mean square error (RMSE) was 44.5 mm. (3) The average restoration thresholds of shrubs-herbs-arbor vegetation coverage in wet, normal and dry years were (71.5±37.3)%, (55.6±35.9)%, and (22.4±26.0)%, while the average recovery potential were (9.4±30.4)%, (-6.4±23.8)% and (-39.7±26.2)%. The average RMSE was 5.7%. (4) There was spatial heterogeneity in the restoration potential of shrubs-herbs-arbor vegetation coverage. The effects of different historical rainfall conditions on the vegetation recovery potential of shrubs-herbs-arbor were the most dramatic in the interval of drought index from 2.5 to 4. If the Loess Plateau experiences a long period of normal or dry years in the future, the shrubs-herbs-arbor vegetation will have different degrees of degradation risk.