北运河流域雨洪“源-汇”景观时空演变

Spatial and temporal changes of ‘source-sink’ landscapes during stromwater processes in the North Canal Basin, China

近年来,海绵城市建设热潮涌现,这一强调雨洪弹性的领域却极度缺乏对于宏观水平过程的探究,未能系统理解雨洪过程的转变动态。由此,引入用以分析各类型生态过程及景观格局的重要理论--"源-汇"景观理论,以北运河流域洪涝过程的内在作用机制为例,着重强调与探求雨洪过程中"源"景观与"汇"景观的相对性与动态性。研究选取关键的景观因子指标,利用最小累积阻力模型测算雨洪径流动力值与地表景观阻力值,从而分析"源-汇"景观的动态演变特征,结果表明:北运河流域中,作为初始径流"源"的区域集中于蟒山山地以及百望山及阳台山山地,易成为山体径流的源头;在北运河流域范围内,自然排水条件下,产生径流的"源"景观类型比例大小排序为:林地>公建用地>道路>工业及设施用地>居住用地>荒地>绿地>农田;在5年一遇的重现期状态下,建设用地雨水消纳能力良好,10年以上重现期下其"源"作用偏强。非建设用地中的林用地在5年一遇重现期下"源"作用已很强,农用地一直有很强的"汇"作用。城市绿地在20年一遇的降雨下已有20%变成"源",其消纳雨水能力有待进一步的观察与实验;在北京城区内涝积水点体现出西多东少、北多南少的特征,径流过程随降雨强度的增大,以由西向东、由北向南的趋势扩散。北京城内洪涝灾害的成因多有缘于山体径流,需尽早探寻合理的手段与对策解决高山林地的径流问题。

‘Sponge city’ projects are booming all cross China over recent years. The term ‘sponge city’ emphasizes the elasticity of stormwater, but current ‘sponge city’ analyses have yet examined the large-scale horizontal processes and thus failed to systematically understand the dynamics of the stormwater ‘source-sink’. Taking the inner flooding mechanism of the North Canal Basin as an example, this paper introduces the ‘source-sink’ landscape theory in order to analyse the ‘source-sink’ dynamics during the stromwater runoff process. The study selected a series of key landscape factors, and applied the minimum cumulative resistance model to calculate the value of the stormwater runoff stimulus and the surface landscape resistance. The results demonstrate that in the North Canal Basin, the initial ‘source’ runoff locates in mountainous areas, including Mount Mang, Mount Baiwang, and Mount Yangtai. Under natural drainage, the sequences of the ‘source’ landscape types that generate runoff are: forest land > public construction land > road > industrial and facility land > residential area > wasteland > urban green space > farmland. Overall, the construction land has good rainwater absorption capacity with a 5-year rainfall return period, but becomes to be the ‘source’ after 10-year rainfall return period. Within the non-construction land categories, the forest land has the strongest ‘source’ effect even at a 5-year rainfall return period, and the agricultural land has the strongest ‘sink’ effect all the time. The urban green space gradually becomes source after 20-year rainfall return period, the rainwater absorption ability of green spaces warrants further exploration. Waterlogging points concentrate in the north and the west, but scatter in the south and the east of downtown Beijing. As the rainfall intensity increases, the runoff spreads out from the west to the east and from the north to the south. Since the floods in Beijing are probably started from mountain runoffs, practical approaches and strategies are urgently needed to solve the runoff problem caused by subalpine forests in the North Canal Basin.