中国科学院水利部水土保持研究所机构知识库

黄土丘陵沟壑区是我国水土流失最为严重的地区之一，不合理的土地利用格局是引发和加剧该地区水土流失最主要的原因。自1999年大规模退耕还林（草）以来，该区域土地利用格局发生了显著地改变，但针对土地利用格局与土壤侵蚀关系的研究主要集中在小区尺度上，小流域尺度上不同降雨条件下侵蚀产沙对土地利用格局响应机制的研究还存在不足。因此，本研究在延安和子洲地区分别选取12个坝控小流域为研究对象，在明确现阶段土地利用现状的基础上，将土地利用类型与地形叠置构建土地利用类型格局指数及小流域格局综合指数，基于降雨资料和坝地泥沙淤积信息，反演现有生态环境下小流域侵蚀产沙特征，在分析土地利用结构特征对小流域侵蚀产沙影响的基础上，探索了小流域侵蚀产沙对土地利用类型格局的响应机制。主要研究结果如下：

1）退耕还林（草）工程实施20年后，丘一区、丘二区小流域林草广泛分布，但小流域侵蚀产沙依旧超过黄土高原地区容许水土流失量。延安、子洲地区各小流域内林草占比平均可达74%以上，但子洲地区坡耕地依旧大面积存在，平均占小流域面积的9.8%，最大占比26.5%。在无特大暴雨的2018-2020年期间，子洲与延安地区小流域年侵蚀产沙模数分别为4942.4 t/km²和1644.9 t/km²，子洲小流域年侵蚀产沙模数为延安地区3.0倍。

2）大暴雨及特大暴雨事件可使小流域年侵蚀产沙模数增大2.02-22.88倍。延安地区受2013年7月份持续强降雨过程影响，小流域年侵蚀产沙模数平均为14733 t/km²，可达同一流域2014-2020年年均侵蚀产沙模数的3.51-22.88倍，平均为8.7倍。子洲地区受2017年7月26日特大暴雨影响，小流域在2017年的年侵蚀产沙模数平均为24058 t/km²，为同一小流域2018-2020年年均侵蚀产沙模数的2.02-22.70倍，平均为6.30倍。

3）具有不同土地利用结构的小流域侵蚀产沙对降雨特征的响应存在差异，整体上小流域侵蚀产沙模数随着降雨量的增加呈现增加趋势，林草和梯田农地占比较大的流域对降雨的响应不敏感。延安地区，林地占比>60%的小流域侵蚀产沙对最大两小时降雨量、平均雨强的变化反映不敏感，梯田农地占比>25%的小流域对降雨各指标的变化敏感度较低；子洲地区，草地为主、坡耕地面积<8%且梯田农地面积>5%的小流域侵蚀产沙模数对次降雨降雨历时的变化反映极其不敏感，而草地为主、坡耕地面积>8%且梯田农地面积<5%的小流域侵蚀产沙模数随最大1小时降雨量、最大2小时降雨量、最大3小时降雨量、平均雨强的增大增幅最大。

4）土地利用结构特征对小流域侵蚀产沙影响显著。在2018-2020年，延安地区林地占比>60%小流域年平均侵蚀产沙模数（397.69 t/km²）在黄土高原土壤容许流失量范围内，林地占比<60%的两种土地利用结构小流域（梯田农地占比<25%与梯田农地占比>25%）年平均侵蚀产沙模数分别为2010.91 t/km²和2281.21 t/km²；子洲地区坡耕地占比>8%的小流域年侵蚀产沙模数显著高于坡耕地面积占比<8%的小流域，分别为6277.11 t/km²和3103.97 t/km²。

5）不同类型降雨下，导致小流域间侵蚀产沙状况存在差异的主要土地利用类型及其格局存在差异，且某一土地利用类型在同一格局下对流域侵蚀产沙“增强”与“减弱”的作用会发生转变。沟沿线以下坡耕地和沟沿线上方土质道路最易遭受降雨侵蚀，在雨量集中型的降雨侵蚀过程中，沟沿线以下坡耕地是泥沙的主要来源。2017年“7·26”特大暴雨过程中，沟沿线以下坡耕地以1.62%的流域面积贡献了32%的泥沙；雨量均匀型的降雨侵蚀过程中，沟沿线以上的土质道路是泥沙的主要来源。特大暴雨条件下，沟沿线以上的梯田农地（新修或老式梯田）作为小流域侵蚀产沙的非主要来源，与坡耕地和土质道路相比，其存在可以“减弱”小流域侵蚀产沙强度；在非特大暴雨条件下沟沿线以上新修的梯田农地是小流域泥沙主要来源，其存在“增强”了小流域侵蚀产沙强度。

The hill and gully region on the Loess Plateau is one of the most serious areas of soil and water loss in China. Unreasonable land use pattern is the main reason for triggering and aggravating sediment yield. Since the Grain for Green project launched in 1999, the land use pattern in this region has changed significantly. Relevant scholars have carried out some studies on the relationship between land use pattern and sediment yield. However, the research mainly focuses on the plot scale, whereas there is the insufficient research on the change of response mechanism of sediment yield to land use pattern under different rainfall conditions on the catchment scale. Therefore, this study selects 12 dam-controlled catchments in Yan’an and Zizhou areas as the research objects, to clarify the current situations of vegetation restoration and land use in these catchments. Based on that, the land use type and topography are superimposed to construct the Land Use Type Pattern Index of small catchments (W) and Land Use Pattern Integrated Index of Catchment (LUPIC). Based on rainfall data and sediment deposition information in the dam land, the sediment yield characteristics of catchments under the existing ecological environment are inverted. Furthermore, the impact of land use structure characteristics on sediment yield in catchments was analyzed. The response mechanism of sediment yield to land use type pattern in catchments was explored. The main research results are as follows:

(1) Two decades after the implementation of the Grain for Green project, forests and grasses are widely distributed in catchments in Yan'an and Zizhou areas, accounting for more than 74% of the total catchment areas. However, there is still a large area of slope cropland in Zizhou area, with an average area of 9.8% and the maximum proportion being 26.5%. Sediment yield in catchments still exceeds the permissible amount of soil and water loss on the Loess Plateau. During 2018-2020, the annual sediment yield modulus of catchments in Zizhou area is 3.0 times that of Yan'an area, with annual sediment yield modulus of 4942.4 t/km² and 1644.9 t/km².

(2) Downpour and heavy downpour events can increase the annual sediment yield modulus of catchments by 2.02-22.88 times. Affected by the continuous downpour events in July 2013, the average annual sediment yield modulus of catchments in Yan’an area is 14733 t/ km², which can reach 3.51-22.88 times of the average annual sediment yield modulus of the same catchment from 2014 to 2020, with an average of 8.7 times. The average annual sediment yield modulus of the catchments in Zizhou area in 2017 was 24058 t/km² due to the heavy downpour on 26 July 2017, which was 2.02-22.70 times that of the same catchments from 2018 to 2020 (4942 t/km²), with an average of 6.30 times.

(3) The responses of sediment yield to rainfall features vary among small catchments with different land use structure characteristics. Generally, the sediment yield modulus of small catchments increases with the increase of rainfall amount, and the catchments with large terraces are not sensitive to railfall features. In Yan’an area, sediment yield in the catchments with forest land accounting for more than 60% is not sensitive to the changes of maximum 2-hour rainfall and average rainfall intensity; sediment yield in the catchments with terrace accounting for more than 25% is less sensitive to the changes of maximum 1-hour rainfall and rainfall duration. In Zihzou area, the sediment yield modulus of the catchments with grassland as the dominated land use structure, sloping farmland area < 8% and terrace area > 5% was extremely insensitive to the change of rainfall duration; the sediment yield of the catchments with sloping farmland area > 8% and terrace area < 5% increases the most with the increase of maximum 1-hour rainfall, maximum 2-hour rainfall, maximum 3-hour rainfall and average rainfall intensity.

(4) The characteristics of land use structure in small catchments have a significant impact on sediment yield. In Yan’an area, the annual average sediment yield modulus of small catchments with forest land accounting for >60% (397.69 t/km²) is within the permissible amount of soil and water loss on the Loess Plateau; the annual average sediment yield modulus of the other two land use structures catchments with forest land < 60% (terrace<25% and terrace>25%) are 2010.91 t/km² and 2281.21 t/km² respectively. In Zizhou area, the annual sediment yield modulus of catchments with slope cropland accounting for more than 8% is significantly higher than that of the catchments with slope cropland accounting for less than 8%, which are 6277.11 t/km² and 3103.97 t/km², respectively.

(5) Under different rainfall conditions, there are differences in the most important land use type and their pattern that lead to the difference of sediment yield among catchments. In addition, under the same pattern, the role of a land use type in “enhancing” and “weakening” sediment yield will change. The slope cropland below the gully line and the unpaved road above the gully line are the most vulnerable to rainfall erosion. In the process of sediment yield caused by concentrated rainfall, the lower slope cropland is the main source of sediment, during the “7.26” heavy downpour event in 2017, the slope cropland below the gully line contributed 32% of the sediment with 1.62% of the total catchment area; in the process of sediment yield triggered by uniform rainfall, the unpaved road above the gully line is the main sediment source. As a non-major source of sediment yield, the terraces (newly built and old-type terraces) above the gully line can reduce the intensity of sediment yield in catchments under downpours, compared with slope-cropland and unpaved road; under non-downpour conditions, the newly built terraces above the gully line are one of the main sediment sources in catchments.