Long-term soil and water conservation in red soil region of southern China significantly reduced riverine sediment load: A case study of Pingjiang River in the upper reaches of Ganjiang River, Lake Poyang Basin

Abstract

长时间序列水沙数据分析有助于科学评价流域尺度水土保持减流减沙效应，为科学开展区域水土保持成效评估提供实现路径.本文选取南方红壤区水土保持工程最为集中、持续时间最长的典型流域——平江流域，基于1975—2014年的逐日降水量、流量、含沙量数据，综合应用预置白M-K检验法、Theil-Sen趋势度估计法、Pettitt检验法与累积量斜率变化率比较法，开展河流水沙变化特征及归因分析，揭示长期水土流失治理对河流水沙的影响.结果表明：（1）1975—2014年平江年输沙量、汛期输沙量和非汛期输沙量均存在显著减少趋势，年均减少率分别为2.38×10⁴、1.75×10⁴和0.44×10⁴ t/a，汛期输沙量在流域开始实施系统水土流失治理时即发生突变，而年输沙量和非汛期输沙量在系列国家水土保持重点工程实施一段时间后方开始突变；（2）流域长期水土保持显著减少平江输沙，但对径流无显著影响，重点治理期、治理后期以水土保持为主的生态保护工程对输沙量减少的贡献均在94.0%以上，且生态保护工程对汛期输沙量减少的贡献较非汛期的更为突出；（3）以水土保持为主的长期生态保护工程导致平江全年、汛期、非汛期输沙量分别减少59.3%、60.7%和55.7%；（4）在剔除大型水利工程（尤其是大中型水库）建设影响后，南方红壤区可以基于长时间序列水沙数据分析科学评价流域尺度水土保持减流减沙效应.研究结果可以为南方红壤区流域尺度水土保持减流减沙效应评价提供参考，并有望为区域系列国家生态保护与建设工程的布局和管理提供科学依据.;The analysis of water and sediment data with long time series is helpful for the scientific evaluation of flow and sediment reduction effect of watershed-scale soil and water conservation, and provides an effective method for the scientific evaluation of regional soil and water conservation effectiveness. The Pingjiang River Watershed in the upper reaches of Lake Poyang Basin, which is the typical watershed with the most concentrated and longest ecological protection engineering construction in the red soil region of southern China, was selected as the study area. Based on the daily rainfall, flow, and sediment concentration data of 1975-2014, pre-whitening Mann-Kendall test, Theil-Sen trend analysis, Pettitt's test, and comparative method of the slope changing ratio of cumulative quantity were conducted. Variation characteristics of flow discharge and sediment load were analysed, major influencing factors were identified, their contribution rates to flow discharge and sediment load variation were quantified, and effects of long-term soil and water conservation on flow discharge and sediment load were revealed. The results showed that: (1) From 1975 to 2014, sediment load of the whole year, flood season, and non-flood season all decreased significantly, with the yearly decreasing rate of 2.38×10⁴, 1.75×10⁴, and 0.44×10⁴ t/a, respectively, and the mutation of flood sediment load occurred once the soil and water conservation project began, while the mutation of the whole year and non-flood sediment load didn't happen until most of soil and water conservation projects accomplished. (2) Long-term soil and water conservation in the watershed significantly reduced the sediment load in Pingjiang River, yet had no significant effect on runoff discharge. During both the key control period and post-control period, the contributions of ecological protection projects, which mainly focused on soil and water conservation, contribution rate to the reduction of sediment load were more than 94.0%, while the contribution of ecological protection projects to the reduction of sediment transport in flood season was more than that in non-flood season.(3) Long-term ecological protection projects based on soil and water conservation resulted in a decrease of 59.3%, 60.7%, and 55.7% for sediment load of Pingjiang River during annual, flood, and non-flood period, respectively. (4) After excluding the impact of large-scale hydraulic engineering construction projects, especially large and medium-sized reservoirs construction, the effect on flow and sediment reduction in the watershed scale of soil and water conservation could be scientifically evaluated based on the analysis of long-term flow discharge and sediment load data in the red soil region of southern China. The results can be used as reference for the evaluation of the effect on soil and water conservation in the watershed scale of soil and water conservation in the red soil region of southern China, and are expected to provide scientific basis for the layout and management of national ecological protection and construction projects.