基于SWAT模型的北京平原区森林景观格局对雨洪减缓的影响研究

景观格局及其变化是影响地表径流及空间分布的重要因素，也是管理城市雨洪、治理生态环境的途径。以北京市平原区百万亩造林项目为背景，选取第一轮造林时间（2011、2013、2015、2017年）的遥感数据，以三环、五环、六环路为界，将平原区划分为核心区、中心区、近郊区、远郊区四个区域，分别分析整体及分区林地景观格局的变化。运用SWAT模型模拟不同格局对不同区域暴雨径流的影响，探究影响径流变化的主导林地景观指数。研究发现：1）平原区林地面积呈增长趋势，增幅为12.96％。除核心区和中心区的部分地区外，林地斑块更聚集、形状更复杂。2）百万亩造林工程对平原区雨洪风险的减缓有积极作用，但受城市建设及人类活动的影响，不同区域造林对径流的减缓强度与建设扩张的影响强度大小关系略有不同。核心区、中心区造林对径流的减缓强度大于建设扩张对径流的副作用，径流总量减小，减少比例为1.98％、4.29％。而近郊区、远郊区相反，径流总量变大，增大比例为0.09％与6.82％，其中远郊区建设扩张的强度是影响平原区径流风险的主导原因。3）不同区域雨洪减缓的关键性林地景观指数不同。核心区径流的主导林地因子为斑块数量与形状因子，中心区为面积因子，近郊区为聚合度因子和形状因子，远郊区为面积与聚合度因子。主导林地因子的大小调控将更高效的减缓雨洪风险。本研究通过SWAT模拟分析得到第一轮造林期间平原区森林格局与径流特征的空间变化及各区域径流减缓的关键性林地因子，为第二轮造林提高雨洪减缓功效提供参考。

Influence of forest landscape pattern on flood mitigation in Beijing Plain based on SWAT model

The landscape pattern and its change are important factors affecting surface runoff and spatial distribution and are also an approach of managing urban rainfall flood and governing ecological environment. Taking one million mu of afforestation project in the plain area, Beijing as the background, the remote sensing data during the first round of afforestation time (2011, 2013, 2015, and 2017) were selected in this paper. Defining the Third Ring Road, the Fifth Ring Road and the Sixth Ring Road in Beijing as the boundary, the plain area was divided into core area, central area, suburban area, and outer suburban area for analyzing the changes of landscape pattern of the whole forest land and the partitioned forest land, respectively. SWAT model was used to simulate the impacts of different patterns on storm runoff in different regions, and explore the dominant forestland landscape index affecting runoff change. The results show that 1) the forestland area in plain area presents an increasing trend, with an increase of 12.96%. Apart from some parts in the core area and the central area, the forestland patches are more concentrated and more complex in shape. 2) One million mu of afforestation project plays a positive role in reducing the risk of rainfall flood in the plain area. However, due to the influence of urban construction and human activities, the effecting intensity of afforestation and construction expansion on the runoff mitigation in different areas are slightly different. The effecting intensity of afforestation on runoff mitigation in the core area and central area is greater than the side effect of construction expansion on runoff, which is manifested as the case that the total runoff decreased, with the proportions of 1.98% and 4.29%, respectively. However, the situation is just on the contrary in the suburban area and outer suburban area:the total runoff increased by 0.09% and 6.82%. The intensity of construction expansion in the outer suburbs is the main factor affecting the runoff risk in plain areas. 3) The key forest landscape indices for different regions are different. The dominant forestland factors of runoff in core area are the number factors of patches and the shape factor; that for the central area is area factor; those for the suburban area are aggregation factor and shape factor; and those for the outer suburban area are area factor as well as aggregation factor. The control of dominant forestland factors will reduce the risk of rainfall flood more efficiently. In this study, through SWAT simulation analysis, the spatial changes of forest pattern and runoff characteristics in the plain area and the key forestland factors for runoff mitigation in different areas during the first round of afforestation were obtained which provides reference for improving the effect of mitigating rainfall flood during the second round of afforestation.