三江源是中国重要的生态屏障，但在水力、风力和冻融等侵蚀营力的共同作用下，土壤侵蚀现象普遍。土壤侵蚀对草地、河流和湿地等生态环境造成了严重的破坏，制约三江源社会经济和生态环境的发展。由于该区域高寒、干旱、缺氧等环境条件，使得其土壤侵蚀综合观测较为困难，数据积累少，而¹³⁷Cs核素示踪法样品采集和计算方便，在计算土壤侵蚀模数、刻画土壤侵蚀强度空间分异方面优势明显。目前利用¹³⁷Cs核素示踪法在三江源地区开展的研究主要集中于小流域或者县级尺度，缺少基于全区域范围的¹³⁷Cs背景值和侵蚀强度评价。本文共选取79个土壤样地，利用¹³⁷Cs核素示踪法计算三江源背景值和土壤侵蚀速率，分析该区土壤侵蚀影响因素，得到三江源土壤侵蚀空间分布的规律，为水土保持规划和生态文明建设服务。主要研究结果如下：

（1）三江源中部和东部背景值在1182~1890 Bq/m²之间。其中，中部偏北的区域受核试验影响，背景值在1480~1890 Bq/m²之间。东部和中部偏南的区域受降雨量的影响，背景值大小与降雨量呈正相关，在1182~1849 Bq/m²之间。在未扰动的土壤垂直剖面上，¹³⁷Cs主要分布在土壤表层5 cm以内。

（2）三江源采集的非耕作土侵蚀样地面积活度在2~1779 Bq/m²之间，土壤侵蚀模数在0.02~41.17 t/(hm²?a)之间，微度、轻度和中度侵蚀样地数量占比分别为78%、21%和1%。三江源采集的耕作土侵蚀样地面积活度在332~1003 Bq/m²之间，土壤侵蚀模数在17.36~54.23 t/(hm²?a)之间，轻度、中度和强烈侵蚀样地数量占比分别为25%、25%和50%。

（3）三江源耕地土壤侵蚀模数>非耕地，非耕地植被覆盖度小于60%时，土壤侵蚀速率随覆盖度增加迅速降低，当植被覆盖度大于60%，土壤侵蚀速率随覆盖度的增加降低幅度较小。样地坡度在15°~25°时，平均土壤侵蚀模数最大，高降雨量（>400 mm）的样地平均土壤侵蚀模数约为低降雨量（<400 mm）样地的2倍。坡上、坡中、坡下土壤侵蚀强度大于坡顶和坡底。东南坡和南坡由于植被覆盖度低，因此土壤侵蚀模数大。

（4）三江源中部和东部土壤侵蚀速率在0.02~54.23 t/(hm²·a)之间，平均土壤流失速率为4.99 t/(hm²·a)，总侵蚀量为1.20×10⁸ t，。兴海县、玛多县、同德县和玛沁县侵蚀量范围在2.19×10⁷~1.32×10⁷ t之间，分别占总侵蚀量的18%、12%、12%和11%，其他县侵蚀量在1.34×10⁶~8.19×10⁶ t之间，占比均小于8%。黄河流域、长江流域和澜沧江流域侵蚀总量分别为7.54×10⁷、2.52×10⁷和1.95×10⁷ t，分别占总侵蚀量的63%、21%和16%，平均土壤侵蚀模数分别为7.07、2.59和5.32 t/(hm²·a)。

（5）微度、轻度、中度和强烈侵蚀面积分别占三江源东部和中部总面积的72.36%、26.43%、1.12%和0.09%，相应的侵蚀量分别占总侵蚀量的36%、55%、8%和1%。其中，微度侵蚀主要分布在三江源东南部和中部偏北区域，东南部主要受植被覆盖度影响，该区域植被覆盖度高，中部偏北区域受坡度影响，该区域坡度在7°以下；轻度侵蚀主要分布在三江源的东北部和南部，受坡度和降雨量影响，降雨量在400~600 mm范围内，坡度多在7~55°之间；中度和强烈侵蚀分布在三江源的东北角，该区域坡度在15°以上，也是三江源主要种植业分布区。

The Three Rivers’ Headstream Regions is an important ecological barrier in China, but under the combined action of erosive forces such as water, wind, and freeze-thaw action, soil erosion is common. Soil erosion has caused serious damage to the ecological environment such as grasslands, rivers and wetlands, restricting the development of the social economy and ecological environment of the Three Rivers’ Headstream Regions. Due to the high cold, drought, hypoxia and other environmental conditions in this area, it is difficult to comprehensively observe soil erosion, and the data accumulation is small. However, the ¹³⁷Cs measurement is convenient for sample collection and calculation. The advantages of differentiation and other aspects are obvious. However, the current researches using the ¹³⁷Cs measurement in the Three Rivers’-Headstream Regions mainly focus on small watersheds or county level scales, and lack the ¹³⁷Cs reference inventory and erosion intensity evaluation based on the whole region. In this paper, a total of 79 soil sample plots were selected, and the ¹³⁷Cs measurement was used to calculate the reference inventory and soil erosion rate of the Three Rivers’ Headstream Regions, analyze the influencing factors of soil erosion in this area, and obtain the spatial distribution law of soil erosion in the Three Rivers’ Headstream Regions, which is useful for soil and water conservation planning and ecological civilization construction services. The main findings are as follows:

(1) The reference inventory in the central and eastern parts of the Three Rivers’ Headstream Regions ranges from 1182 to 1890 Bq/m². Among them, the north-central area was affected by the nuclear test, and the reference inventory was between 1480 and 1890 Bq/m². The eastern and south-central regions are affected by rainfall, and the reference inventory is positively correlated with rainfall, ranging from 1182 to 1849 Bq/m². In the vertical profile of undisturbed soil, ¹³⁷Cs was mainly distributed within 5 cm of the soil surface.

(2) The area activity of the non-cultivated soil erosion samples collected from the Three Rivers’ Headstream Regions was between 2 and 1779 Bq/m², and the soil erosion modulus was between 0.02 and 41.17 t/(hm²·a). The proportions of moderately eroded plots were 78%, 21% and 1%. The area activity of the cultivated soil erosion plots collected from the Three Rivers’-Headstream Regions ranged from 332 to 1003 Bq/m², and the soil erosion modulus ranged from 17.36 to 54.23 t/(hm²·a). Quantity accounted for 25%, 25% and 50%.

(3) In the Three Rivers’ Headstream Regions, the soil erosion modulus of cultivated land > non-cultivated land. When the vegetation coverage of non-cultivated land is less than 60%, the soil erosion rate decreases rapidly with the increase of coverage. When the vegetation coverage is greater than 60%, the soil erosion rate decreases more rapidly with the increase of coverage. Small. When the slope of the plots was 15°-25°, the average soil erosion modulus was the largest, and the average soil erosion modulus of the plots with high rainfall (>400 mm) was about twice that of the plots with low rainfall (<400 mm). The soil erosion intensity on the slope, in the middle and under the slope is greater than that at the top and bottom of the slope. The southeast and south slopes have large soil erosion modulus due to low vegetation coverage

(4) The soil erosion rate in the central and eastern parts of the Three Rivers’ Headstream-Regions was between 0.02 and 54.23 t/(hm²·a), the average soil loss rate was 4.99 t/(hm²·a), and the total erosion amount was 1.20×10⁸ t. The erosion amount in Xinghai County, Maduo County, Tongde County and Maqin County ranges from 2.19×10⁷ to 1.32×10⁷ t, accounting for 18%, 12%, 12% and 11% of the total erosion amount, respectively. The erosion amount is between 1.34×10⁶ and 8.19×10⁶ t, accounting for less than 8%. The total amount of erosion in the Yellow River Basin, the Yangtze River Basin and the Lancang River Basin was 7.54×10⁷, 2.52×10⁷ and 1.95×10⁷ t, accounting for 63%, 21% and 16% of the total erosion, respectively. The average soil erosion modulus was 7.07, 2.59 and 5.32 t/(hm²·a).

(5) The areas of slight, mild, moderate and severe erosion account for 72.36%, 26.43%, 1.12% and 0.09% of the total area in the eastern and central parts of the Three Rivers’ Headstream Regions, respectively, and the corresponding erosion amounts account for 36%, 36% and 0.09% of the total erosion amount, respectively. 55%, 8% and 1%. Among them, the slight erosion is mainly distributed in the southeastern and centrally northerly regions of the Three Rivers’ Headstream-Regions, the southeastern part is mainly affected by the vegetation coverage, the vegetation coverage in this region is high, and the centrally northerly region is affected by the slope, which is below 7°; mild erosion is mainly distributed in the northeast and south of the Three Rivers’-Headstream Regions, affected by the slope and rainfall, the rainfall is within 400~600 mm, and the slope is mostly between 7~55°; Moderate and severe erosion are distributed in the northeast corner of the Three Rivers’ Headstream Regions, the slope of this area is above 15°, and it is also the main planting area of the Three Rivers’ Headstream Regions.