极端降水、潜在植被和人类活动时空耦合对河龙区间水沙变化的影响评估

The changes of runoff and sediment load in the middle reaches of the Yellow River and their driving mechanism are the frontiers and hot topics in the Yellow River ecosystem research, but also the core problems in evaluating the ecological construction and ecosystem restoration of the Loess Plateau. At present, most of the studies mainly focus on the impacts of average climate change and human activity on watershed runoff and sediment load, but pay less attention to the impacts of spatiotemporal couplings of potential vegetation driven by climate change, human activity and extreme climate event on watershed runoff and sediment load. However, extreme climate event, potential vegetation and human activity are the main and key driving factors of runoff and sediment load changes in Hekouzhen-Longmen Region located in the Middle Yellow River, where soil erosion is serious and sensitive to extreme precipitation event but also ecological construction of soil and water conservation is strong. To overcome the above shortcoming, this study, taking the Hekouzhen-Longmen Region as the study area, distinguish and quantify the contributions of extreme precipitation event, potential vegetation and human activity to runoff and sediment load changes. The main objectives of this study are to (1) explore the dynamic change characteristics of runoff and sediment load in Hekouzhen-Longmen Region during 1960-2017; (2) select the key extreme precipitation indices which seriously affect the runoff and sediment load changes in Hekouzhen-Longmen Region, based on daily meteorological data of 12 national weather stations and 15 extreme precipitation indices; (3) simulate and analyze spatiotemporal change characteristics of potential vegetation based on the LPJ dynamic vegetation model; (4) distinguish and quantify the contributions of extreme precipitation event, potential vegetation and human activity to runoff and sediment load changes. The main methods used in this study are the moving average, Mann-Kendall nonparametric test, Sen slope estimate, Mann-Kendall falling sequence test, wavelet analysis, double cumulative curve, SWAT model and scenario setting.

黄河中游水沙变化及其驱动机制是黄河生态系统研究的前沿和热点领域，同时也是评价黄土高原生态建设和生态系统恢复的关键核心问题。现有对黄河中游水沙变化的归因分析研究，大多集中于平均气候变化和人类活动对流域水沙关系的影响，而忽视了因气候变化所致的潜在植被以及耦合人类活动、极端降水事件对流域水沙变化的影响。本课题以黄河中游对极端降水敏感的水土流失严重区和水土保持生态建设强烈区的河龙区间为研究对象，以水沙变化的关键影响因子分析为切入点，针对研究区水沙变化的主要驱动因素（极端降水、潜在植被和人类活动），在明晰区间1960-2017年水沙动态变化特征的基础上，基于国际通用的15个极端降水指标筛选出具有明显水沙响应的关键极端降水指标，基于动态植被模型LPJ模拟和分析潜在植被的时空演变特征，最后结合双累积曲线法、SWAT模型和情景设置手段，区分和量化极端降水事件、潜在植被和人类活动对河龙区间水沙变化的贡献。

针对黄河中游河龙区间水沙变化的主要驱动因素（极端降水、潜在植被和人类活动），在明晰1960-2017年河龙区间径流泥沙、极端降水事件变化特征的基础上，筛选引起水沙变化的关键极端降水指标；基于LPJ模型和综合顺序分类系统法模拟，阐明1960-2017年潜在植被的时空演变特征，揭示土地利用时空演变对潜在植被的影响；结合双累积曲线法、水文模型和情景设置手段，量化极端降水事件、植被和人类活动对水沙变化的贡献。结果表明，58 a间年径流量、输沙量呈极显著减少趋势，突变年份为1979年。15个极端降水指标中，I50、R1d、CWD、CDD、I20、R5d、R50p、R95p和P6-9呈不显著下降趋势，其余指标均呈不显著上升趋势。河龙区间稳定潜在植被类型为微温微干温带典型草原类、微温微润草甸草原类、暖温微干温带典型草原类和暖温微润森林草原类。因气温持续上升、2000年后降水显著增加，2000年后微润类植被面积大幅扩增，其中暖温微润森林草原类增加较多。植被覆盖度以45%以下占主导，82.52%的区域植被覆盖度得到了改善，12.55%区域植被覆盖度得到了显著改善。草地、林地、建设用地面积呈增长趋势，耕地是土地利用面积转化的主要贡献者。受人类活动影响，1980-2017年草原类潜在植被的31%～36%转变为耕地、11%～19%转变为林地，森林草原类潜在植被的19%～33%转变为耕地。R95p、年降水量、汛期降水量是引起水沙变化的关键降水指标。在1980-1989年、1999-2009年和2010-2017年，下垫面变化是水沙变化的主要因素，而1990-1998年极端降水是水沙变化的主要因素。下垫面因素中，植被变化是水沙变化的主导因素。综合而言，植被变化作用＞降水变化作用＞土地利用变化作用。但应关注极端降水增多增强导致的极端水文事件。研究结果将为黄河中游水土流失综合治理、生态环境建设提供科学依据和决策支持。

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