自改革开放以来，我国社会经济迅猛发展，城市化进程不断加快，城市规模迅速扩张。然而，这种快速的城市化进程也带来了一系列环境问题，其中城市极端天气的频发尤为突出。由此引发的城市雨洪灾害给人类社会带来了巨大的威胁，对城市安全和社会稳定构成了严重威胁。为了提升城市抵御洪涝灾害的能力，减少人员伤亡和经济损失，探究在变化环境下城市降雨径流的变化规律成为了城市水文学领域的研究重点。北京市，作为快速城市化的典型代表，近年来极端降雨事件频发，使得防洪排涝工作成为城市建设中不可或缺的重要环节。因此，深入研究城市降雨径流的变化规律，对于提升城市防洪能力、保障城市安全具有重要意义。本研究选取北京市作为研究区域，旨在深入探究不同土地利用情况对降雨径流过程的影响。为此，我们采用了1985年、2000年、2015年的土地利用数据，并借助是中尺度天气预报模式（WRF），对北京市2012年“7·21”极端降雨过程进行了模拟。在此基础上，我们以WRF输出的降雨数据为驱动，通过暴雨雨水管理模型（SWMM），构建了通惠河、凉水河、清河和坝河四大流域的城市雨洪模型，用以模拟不同城市化程度下的“7·21”降雨径流过程。本研究不仅定量分析了城市下垫面对降雨径流过程的影响，还深入剖析了其主要影响机制。以下是本研究的主要结论：
（1）城市下垫面的扩张对北京市“7·21”极端降水事件产生了显著影响。具体而言，从1985年至2015年，随着城市化进程的加速，降雨量呈现出持续增长的趋势。其中，1985年至2000年间，降雨量增加了22.83%；而2000年至2015年间，降雨量更是增长了35.46%。此外，城市化还导致了城区雨带的提前出现，其覆盖范围更广，滞留时间更长。这些因素共同作用下，使得高降雨量地区不断扩大，从而加剧了城市极端降雨事件的频发。此外，城市下垫面对于影响降雨的气候要素同样具有不可忽视的影响，这一点在本次研究中也得到了充分的体现。城市化的推进使得下垫面不断扩张，显著提升了其辐射吸收能力。这一变化导致北京地区温度明显上升，城区升温幅度更是高达2℃，从而加强了城市下垫面与大气之间的湍流热交换。在此过程中，热量从地表向大气的传输现象变得更为强烈，感热通量也随之增加，特别是在城区，感热通量达到了180W/m²。此外，我们还观察到北京市内的不稳定能量CAPE呈现显著增长趋势，至2015年已达到375J/kg。尽管风向受城市化的影响相对较小，但城市下垫面的扩张却增大了下垫面的粗糙度，导致阻力系数上升，进而削弱了城区的风速。具体来说，城区风速的最大值从20m/s下降至14m/s，这在一定程度上阻碍了风向汇合带来的水汽，从而加剧了水汽在城市区域的聚集效应。此外，城市地表温度的升高促进了湍流活动的增强，同时建筑物的增多也阻碍了水分的蒸发，导致水汽混合比呈现下降趋势。然而，由于北京市“7·21”极端降雨事件的水汽条件异常充沛，城市化进程对这次降雨事件的水汽混合比影响相对较小。综合以上分析，城市化带来的气候要素变化促进了水汽的垂直输送，为更强烈对流的形成与发展提供了有利的热力条件。这些变化在极端降雨事件中尤为显著，对城市气候和水文循环产生了深远的影响。
（2）城市下垫面的扩张对径流过程具有显著影响。本研究成功构建了研究区域的SWMM模型，并通过了精度评估。我们将不同重现期（1年一遇、5年一遇、20年一遇和50年一遇）的设计降雨数据输入到不同下垫面情景的SWMM模型中，以探究其影响。结果表明，在相同重现期下，随着城市化程度的加深，径流过程线呈现出“窄尖”的特征。具体来说，下渗量逐渐减少，径流总量则相应增加，洪峰变化显著，且峰现时间明显提前。此外，由于城市化进程的“迅速-趋缓”特点，径流特征值在城市化前期的变化率高于后期。然而，值得注意的是，在城市化程度较高的地区，如通惠河流域，洪水特征值的变化率相较于其他流域要小得多
（3）随着城市化进程的推进和降雨强度的增强，地表径流显著增加。具体来看，2000年至2015年间，通惠河、凉水河、清河和坝河等流域的出流总量变化率呈现出不同的趋势。通惠河的出流总量变化率分别为0.27和0.11，显示出城市化进程中径流变化的“迅速-趋缓”特征；凉水河的变化率则为2.11和0.23，清河为2.78和0.58，坝河为2.78和1.38，均符合城市化发展的典型模式。研究还发现，位于上风向且降雨量较大的凉水河与通惠河流域，在城市化进程中，降雨量对径流的影响逐渐增强；而处于下风向的清河与坝河流域，下垫面对径流的影响始终占据主导地位。这一发现对于北京市不同地区的规划建设具有重要的指导意义。在上风向地区，应更加注重海绵城市和韧性城市的建设，以提高蓄水调控能力；而在下风向地区，则需加强低影响措施的建设，以减少不透水面对径流的影响。这样的规划策略将有助于实现城市水资源的可持续利用和生态环境的改善。

Since China’s reform and opening up, rapid socio-economic growth has accelerated the process of urbanization. However, this rapid urban expansion has led to frequent extreme weather events in cities, with urban rainfall and subsequent flooding posing a significant threat to society. In an effort to enhance urban resilience to flooding, minimize human casualties, and reduce economic losses, the exploration of the changing patterns of urban rainfall runoff under evolving environmental conditions has emerged as a key area of interest in the field of urban hydrology. As a prime example of rapid urbanization, Beijing has been subjected to an increasing frequency of extreme rainfall events in recent years. This has elevated the importance of flood prevention and drainage systems in the city’s infrastructure planning and construction. In this research, we focus on Beijing as a case study, employing land use data from 1985, 2000, and 2015. We choose the Weather Research and Forecasting Model to simulate the extreme rainfall event of July 21, 2012, in Beijing under various land use scenarios. The rainfall data, derived from the WRF, serves as a driving force in the development of urban stormwater models. The runoff simulation model is based on the Stormwater Management Model and is applied to the four primary drainage areas in Tonghuihe River, Liangshui River, Qing River, and Ba River. The aim is to simulate the runoff process of the “7.21” event under varying degrees of urbanization. This research conducts a quantitative analysis of the impact of urban subsurface structures on rainfall runoff processes, aiming to understand the primary influencing mechanisms. The main conclusions can be summarized as follows:
(1) The expansion of urban subsurface structures played a key role in influencing the “7.21” rainfall event in Beijing. Specifically, rainfall has significantly increased, with a rise of 22.83% observed from 1985 to 2000, and a further increase of 35.46% from 2000 to 2015. Conversely, urbanization has resulted in the progression of rainbands over urban areas, leading to a wider coverage and extended duration of rainbands. This has caused an expansion of areas experiencing high rainfall, thereby exacerbating the severity of extreme urban rainfall events. The urbanized subsurface has an important influence on the main climatic elements. The expansion of the urbanized subsurface has increased the radiation absorption capacity, resulting in higher temperatures in Beijing, up to 2°C in the urban areas, leading to stronger turbulent heat exchange between the urban subsurface and the atmosphere, stronger heat transfer from the surface to the atmosphere, and consequently an increase in sensible heat fluxes, especially in the urban areas, up to 180 W/m². The Convective Available Potential Energy (CAPE), a measure of atmospheric instability within Beijing, saw a significant increase, reaching 375 J/kg by 2015. However, the expansion of the urban subsurface led to an increase in surface roughness, resulting in a rise in the drag coefficient. This, weakened the wind speed within the urban area, with the maximum value decreasing from 20 m/s to 14 m/s. This reduction in wind speed hindered the convergence of winds carrying water vapor, thereby intensifying the effect of water vapor aggregation. Furthermore, the proliferation of buildings, increased turbulence, and elevated urban surface temperatures have collectively contributed to less favorable conditions for water evaporation, leading to a downward trend in the water-vapor mixing ratio. However, due to the severity of the “7.21” rainfall event in Beijing, the conditions for water vapor were more than adequate. It’s noteworthy that the impact of urban development on the water-vapor mixing ratio during the “7.21” rainfall event was minimal. In conclusion, the alterations in climatic variables induced by urbanization promote the vertical transportation of water vapor and establish thermal conditions conducive to the genesis and intensification of robust convection.
(2) As urbanization rapidly expands, intensified rainfall has resulted in a substantial rise in surface runoff. In this research, we have developed and validated a Stormwater Management Model (SWMM) specifically tailored to our study area. Design rainfall data for 1-year, 5-year, 20-year, and 50-year return periods were incorporated into the SWMM model under various subsurface scenarios. The findings revealed that, for the same return period, as urbanization intensified, the runoff process curve exhibited a “narrow tip” characteristic. There was a decrease in infiltration volume, an increase in total runoff, and significant changes in flood peaks. Furthermore, the time of peak occurrence was noticeably advanced. Additionally, due to the “rapid-slowing” characteristic of urbanization, the rate of change in runoff characteristic values is more pronounced during the initial stages of urbanization compared to the later stages. However, in areas with a higher degree of urbanization, such as the Tonghui River area, the rate of change in flood characteristic values is considerably less than that observed in other drainage areas.
(3) As urbanization expands and rainfall intensifies, there is a corresponding increase in surface runoff. Between 2000 and 2015, the rate of change in the total outflow from Tonghui River was 0.27 and 0.11, respectively. For Liangshui River, these rates were 2.11 and 0.23, respectively. The rates for the Qing River were 2.78 and 0.58, and for the Ba River, they were 2.78 and 1.38. These rates are consistent with the “rapid-slowing” characteristics of urban development, where changes during the early stages of urbanization are significantly more pronounced than those in the later stages. Utilizing a method of control variables, it has been observed that the Liangshui River and Tonghui River areas, which are situated in the upwind direction and receive higher rainfall, have seen a progressive increase in the impact of rainfall on runoff throughout the urbanization process. Conversely, the Qing River and Ba River areas, located in the downwind direction, have consistently exhibited a dominant influence from the lower bedding surface. These findings can serve as a valuable reference for the planning and development of various regions in Beijing. Upwind areas should prioritize the creation of sponge cities and resilient urban landscapes to enhance water storage and regulation. Conversely, downwind areas should emphasize the implementation of low-impact strategies to minimize the effects of impervious surfaces.