土壤侵蚀每年导致全球表土流失750亿吨以上，造成粮食减产等经济损失约合4000亿美元，并加剧土地退化与水体污染等生态环境问题，成为生态安全、河湖健康以及人类可持续发展的重要挑战（Montanarella，2007；Quinton et al.，2010；FAO，2015）。中国是世界上土壤侵蚀最严重的国家之一，经过70余年不懈治理，取得了显著成效，全国水土流失面积（土壤侵蚀强度超过1000 t/km ² ·a的面积）由20世纪80年代中后期的367.03万km ² ，减少为2011年的294.91万km ² 、2021年的 267.42万 km ² ，累计减幅达1/4以上，相应的中度及以上水土流失（土壤侵蚀强度超过2500 t/km ² ·a的面积）占比由49.32%降至35.58%，水土流失实现了面 积、强度“双下降”（水利部等，2010；水利部，2013；2021）。

Soil erosion causes topsoil loss of more than 75 billion tons per year globally. It results in reduced crop yields and other economic losses totaling approximately US$400 billion. The ecological and environmental issues, such as land degradation and water pollution, are aggravated, posing a serious threat to ecological security, river and lake health, and sustainable human development (Food Agriculture Organization of United Nations, 2015; Montgomery, 2007; Quinton et al., 2000). Soil erosion in China is particularly stern and the country has tackled this challenge unremittingly for over 70 years. The outcome speaks for itself: the national soil erosion area (with soil erosion modulus exceeding soil loss tolerance of corresponding region, e.g., 1000 t/km ² ·a in the Northwest Loess Plateau, 500 t/km ² ·a in the Southern Red Soil Area, and 200 t/km ² ·a in the Northeast Black Soil Area) (MWR, 2008) has dropped from 3,670,300?km ²  in the mid-to-late 1980s to 2,949,100?km ²  in 2011 and further to 2,674,200?km ²  in 2021, registering an accumulative reduction of over 1/4; and the proportion of soil erosion area at “moderate and above” level (with soil erosion modulus exceeding 2500?t/km ² ·a) has decreased from 49.32% to 35.58%, meaning both erosion area and the intensity have declined (MWR et al., 2010; MWR, 2013, 2021).

伴随经济社会快速发展，中国政府始终高度重视生态建设并不断深化认识、持续加大投入，2017年提出了“到2035年生态环境根本好转、美丽中国目标基本实现；到2050年生态文明全面提升”的发展愿景，2022年再次强调“推动绿色发展、促进人与自然和谐共生”的生态文明理念。水土流失防治作为流域生态保护修复、河湖生态环境复苏的根本措施，一直被视为中国生态文明建设的重要内容，并成为长期坚持的基本国策。为此，水土流失防治到什么程度才算行与好？才能满足国家2035年、2050年的生态文明与美丽中国建设总体目标？成为迫切需要回答的重大问题。与此同时，随着全球生态环境保护与治理持续深入，区域植被恢复适宜目标（Feng & Fu，2016）、生态系统服务功能权衡（Ouyang et al.，2016）、社会生态系统演变溢出效应（Wu et al.，2020）等诸多研究都表明，一定时空范围内的单一生态要素、过程及其效应，对于系统整体而言，都可能存在相对稳定或合理的阈值区间。水土流失作为水力、风力、重力及冻融等自然营力及人类活动作用下的地表过程，受到自然和社会因素综合影响，其面积、强度在一定区域与时期内处于变化消长的动态过程，既非通过治理就只减不增的静态存量，也不可能或不适宜彻底消除，势必存在科学治理的“天花 板”与综合权衡的“适宜度”。

The Chinese government has attached great importance to ecological improvement amid its rapid economic and social development throughout the years. Both the understanding of this issue and the investment for it are growing. In 2017, China called for “fundamentally improving the ecological environment and largely achieving the goal of a beautiful China by 2035; and comprehensively advancing ecological progress by 2050”; and the ecological civilization concept of “pursuing green development and promoting harmony between humanity and nature” was again highlighted in 2022. As a fundamental measure for the ecological protection and restoration of rivers and lakes systems, soil erosion and water loss control is taken as an important element of pursuing ecological progress and has become a fundamental state policy to be upheld in the long run. Therefore, to what extent is soil erosion control considered adequate to meet China's goal of achieving ecological progress and the vision of a beautiful China by 2035 and 2050 is a question that warrants speedy response. In the meantime, as global ecological protection and governance continue to progress, studies have been carried out on appropriate targets for regional vegetation restoration (Feng et al., 2016), trade-offs for ecosystem services (Ouyang et al., 2016), and evolution and spillover effects of the social-ecological system (Wu et al., 2020). These studies have shown that single ecological elements, processes, and their effects at a certain spatial and temporal scale may have relatively stable or reasonable threshold intervals for the system as a whole. Soil erosion is a surficial process that works under the action of natural forces such as water, wind, gravity, and freeze–thaw, as well as human activities. The erosion area and intensity are dynamically changing within a certain area and a period of time. It is neither a static stock that can only be reduced through control, nor is it possible or appropriate to eliminate it completely. There must be a “ceiling” for scientific control and an “appropriate degree” determined by comprehensive judgment.

因此 ，为科学确定全国及不同区域的水土流失防治目标， 2020 年水利部首次提出 “ 水土保持率 ” 指标，用于正向表征区域水土保持综合状况，将其阈值定义为符合自然规律并满足经济社会发展要求下的水土流失防治适宜程度，并被 列入 美丽中国建设评估的 22 项指标之一 （国家发展改革委， 2020 ） 。 2020-2022 年间，水利部专门组织中国水利水电科学研究院等 1 0 家科研机构，开展了全国及不同类型区水土保持率阈值研究（以下称 “ 《 专项 研究 》 ” ）。按照 东北黑土区、北方风沙区、北方土石山区、西北黄土高原区、南方红壤区、西南紫色土区、西南岩溶区、青藏高原区等 8 个全国水土保持一级分区， 综合分析各区自然地理条件、经济社会发展水平和趋势， 分别 提出 水土流失存量的远期防治情势研判原则 （曹文洪等， 2 021 ） 。在此基础上，以 30 米分辨率网格为空间单元，基于土壤侵蚀分类分级、土地利用、海拔地形、植被覆盖等地理空间数据叠加分析，逐片分析现存水土流失中哪些不需治理、哪些应当治理、哪些可以完全治理（治理后土壤侵蚀强度可降至轻度以下，不再计入水土流失面积）、哪些不可完全治理（治理后土壤侵蚀强度降低但仍在轻度及以上，依然计入水土流失面积），最终确定出全国及不同类型区的远期水土流失面积阈值（总体技术方案见图 1 所示 ）。

Therefore, to scientifically define soil erosion control targets nationwide and by regions, in 2020, the MWR came up with its first-ever indicator of “soil and water conservation rate,” which is used to proactively and comprehensively characterize the regional soil and water conservation status. The threshold is defined as the appropriate degree of soil erosion control determined in accordance with the laws of nature and deemed necessary to meet the requirements of economic and social development. The indicator of “soil and water conservation rate” has been listed as one of the 22 indicators for the assessment of building a Beautiful China (National Development and Reform Commission [NDRC],  2020 ). During the period 2020–2022, MWR commissioned 10 research institutions, including the China Institute of Water Resources and Hydropower Research, to conduct a study on soil and water conservation rate thresholds nationwide and by types of areas (hereinafter referred to as the “Special Study”). Based on the eight national primary subdivisions of soil and water conservation, namely the Northeast Black Soil Area, the Northern Sandy Area, the Northern Rocky Mountain Area, the Northwest Loess Plateau Area, the Southern Red Soil Area, the Southwest Purple Soil Area, the Southwest Karst Area, and the Qinghai-Tibet Plateau Area, a comprehensive analysis of the natural geographic conditions, economic and social development levels and trends of each area were conducted, and the principles of anticipating long-term soil erosion control situation were proposed (Cao et al.,  2021 ). The 30-m resolution grid was used as the spatial unit. Based on the superposition analysis of geospatial data such as soil erosion classification and grading, land use, elevation, topography, and vegetation cover, existing soil erosion areas were analyzed piece by piece as to which of them do not need to be controlled while others should be, and which can be fully controlled (i.e., the postcontrol soil erosion intensity can be reduced to below “mild” level, making them no longer counted as soil erosion area) and which cannot be fully controlled (i.e., the postcontrol soil erosion intensity can be reduced but remains at “mild or above” level, making them still counted as soil erosion area). Finally, the long-term soil erosion area thresholds nationwide and by types of areas were determined (see Figure  1  for the general technical process).

图1  水土保持率阈值确定技术流程

《专项研究》结果表明，全国2021年现存的水土流失面积中，约123万km ² 属于综合自然与社会因素而不需或不宜治理的水土流失面积，主要分布在西北地区沙漠、戈壁和流动沙地腹地的风力侵蚀，以及青藏高原、横断山地等高寒高海拔人口稀疏区的水力侵蚀；其余约144万km ² 水土流失则需因地制宜、分区施策的开展综合治理。综合分析与权衡水土流失规律、自然地理条件以及技术经济因素，到远期（2050年以后），通过治理约有52万km ² 水土流失可将土壤侵蚀强度降至轻度以下，按照土壤侵蚀分级标准可转变为非水土流失面积；其余约92万km ² 治理后的土壤侵蚀强度虽有不同程度降低，但因仍在轻度及以上，而将长期被计入水土流失面积，主要为西北地区固定沙地、绿洲与荒漠过渡带的风力侵蚀，华北、东南、西南等地山丘区和东北漫川漫岗区的部分坡耕地、园地和陡坡林草地的水力侵蚀，黄土高原沟壑陡坡的水力、重力混合侵蚀，以及必要生产建设活动所造成的阶段性人为水土流失（全国现存水土流失面积分布与远期防治分类示意见图2）。根据以上结果，全国水土流失面积远期应且能够减少至215万km ² ，水土保持率（水土保持状况良好面积占国土面积比例）阈值达77.5%，从而首次科学确定了为满足生态文明和美丽中国建设要求的国家尺度水土流失防治适宜目标。

The results of the Special Study show that of the existing soil erosion areas in China in 2021, about 1.23?million?km ²  either do not require control or are not inappropriate to control due to a combination of natural and social factors. These are mainly wind erosion area in the desert, the Gobi and the moving sandy hinterland in the northwest, and hydraulic erosion in the alpine and high-altitude sparsely populated areas such as the Qinghai-Tibet Plateau and the Hengduan Mountains; the remaining 1.44?million?km ²  requires site-appropriate comprehensive and targeted control. The comprehensive analysis and weighing of soil erosion laws, physical and geographical conditions, as well as technical and economic factors in the long term (after 2050) indicate that the soil erosion intensity of about 520,000?km ²  of postcontrol area can be reduced to below “mild” level, which can count as non-erosion area according to the soil erosion classification standards; that of the remaining 920,000?km ²  can be reduced to varying degrees but remains at “mild or above” level, making them counted as erosion area in the long term. These are mainly wind erosion in the fixed sandy areas and the transition zone between oasis and desert in northwest China, hydraulic erosion on sloping arable land, garden land and steep-slope forest and grassland in the hilly areas of north, southeast, and southwest China, and the rolling hilly region of Northeast China, mixed hydraulic and gravity erosion on the gullies and steep slopes of the Loess Plateau, and phased anthropogenic soil erosion caused by necessary production and construction activities (see Figure 2 for the distribution of existing soil erosion areas and the classification of long-term soil erosion control). Based on these studies, it is calculated that the national soil erosion area should and can be reduced to 2.15?million?km ²  in the long term, and the threshold of soil and water conservation rate (i.e., the proportion of the area under sound soil and water conservation conditions to the national land area) can reach 77.5%. For the first time, appropriate science-based targets for soil erosion control at national scale in the context of advancing ecological civilization and building a beautiful China are determined.

图 2 全国现存水土流失面积分布与远期防治分类示意图

依据以上结果， 《专项研究》还通过各省空间分布与统计，给出了各省的水土保持率阈值，明确了各省的水土流失防治适宜目标。为有序推进各地水土流失防治任务，有力落实各级地方政府的水土保持工作责任，水利部采取自上而下与自下而上相结合的方式，同步组织各省开展水土保持率阈值复核工作，合理规划确定了各省2025年、2030年和2035年水土保持率阶段目标，最终形成了未来15年各阶段的水土流失防治全国目标。2022年12月，中共中央办公厅、国务院办公厅印发《关于加强新时代水土保持工作的意见》（以下简称《意见》），明确提出：到2025年，人为水土流失得到有效管控，重点地区水土流失得到有效治理，水土流失状况持续改善，全国水土保持率达到73%；到2035年，人为水土流失得到全面控制，重点地区水土流失得到全面治理，全国水土保持率达到75%，生态系统水土保持功能显著增强（中共中央办公厅、国务院办公厅，2022）。《意见》是当前和今后一个时期中国加强水土保持工作的纲领性文件，在国家层面上明确了全国水土流失防治适宜目标，为科学、有序推进水土保持工作提供目标引领和重要遵循，对推动新时代水土保持高质量发展具有重大意义，也为促进世界各国共同应对全球土壤侵蚀的挑战提供了典型范例。

The Special Study took a further step to determine the respective soil and water conservation rate thresholds and erosion control targets for each province based on the provincial spatial distribution data and relevant statistics. To advance soil erosion control in a systemic manner and urge the local governments to act on the responsibilities for soil and water conservation, MWR adopted both top-down and bottom-up approaches: a simultaneous review of all provincial soil and water conservation rate thresholds was conducted, the soil and water conservation rate targets by 2025, 2030, and 2035 in each province were identified, and the national targets for soil erosion control in each stage for the next 15 years was determined. In December 2022, the General Office of the CPC Central Committee and the General Office of the State Council issued the Opinions on Strengthening Soil and Water Conservation in the New Era (hereinafter referred to as the “Opinions”). It is clearly put forward in the Opinions that by 2025, anthropogenic soil erosion shall be effectively managed, the erosion in key regions be effectively treated, the overall erosion situation be continuously eased, and the national soil and water conservation rate reach 73%; by 2035, anthropogenic soil erosion shall be fully managed, the erosion in key regions be brought under full control, the national soil and water conservation rate reach 75%, and the soil and water conservation functions of ecosystems be significantly enhanced (The General Office of the CPC Central Committee the General Office of the State Council, 2022). As a navigating document for strengthening soil and water conservation work in China both for the present and the long run, the Opinions defines the appropriate targets for soil erosion control at the national level and provides leading objectives and guidelines in a scientific and orderly way. It is conducive to promoting high-quality development of soil and water conservation in the new era and offers a template of addressing the common challenge of soil erosion globally.

确定远期阈值，水土保持率，土壤侵蚀，水土流失防治的适宜目标

determination of long‐term threshold, soil and water conservation rate, soil erosion, suitable target of soil and water loss prevention and control