快速城市化对稻田流域蒸散发及生态水文过程的影响机制

Paddy wetlands are the main land use type across southern China. However, the ecosystem service functions of these artificial wetlands are being threatened by massive urbanization in the region. Paddy wetlands impact the regional environments by affecting evapotranspiration (ET) and other water and energy related processes. Converting water stress-free paddy fields to "dry" urban uses or impervious surfaces is bound to affect the watershed hydrological processes. However, the mechanisms and key processes of ET and other ecohydrological processes are still not fully understood, which limits the evaluation of watershed ecosystem services function of rice paddies. In this project, a multi-scaled (plot to watershed) water and energy balance, and multi-ecohydrological processes were investigated by integrating advanced techniques of field experiments, remote sensing, and simulation models. We conduct our case study in the Qinhuai river basin in the highly urbanized Yangtze River Delta region. The overall goal of this study is to understand the key driving processes of ET and its mechanisms underlining the hydrologic impacts of converting rice paddy fields to urban uses. The dynamic relationships between ET at the plot scale and the watershed scale will be identified. The changes of ET in different seasons for various land use and land covers will be compared, and their spatio-temporal differences, sensitivity and key thresholds will be quantitatively determined. The dominant hydrological disturbance processes controlling water balances and their variations with the converting of paddy fields to urban uses will be identified. Based on this study, the traditional watershed ecohydrological simulation model (SWAT) will be improved and adapted for the paddy field-dominated basin. Study results provide a reliable tool for ecosystem services assessment and watershed management in the highly urbanized regions in southern China.

稻田湿地是我国南方地区主要土地利用方式，但其生态系统服务功能受快速城市化的威胁日趋严重。稻田通过影响地表蒸散发等水热过程，对区域生态环境有着重要影响。大范围稻田湿地转为较“干”的城市用地，势必改变陆面水文过程。然而，由于对稻田影响流域蒸散发的深层次机理和关键过程不明，导致稻田生态系统服务功能被严重低估。拟选取长三角地区快速城市化的秦淮河流域，综合采用样地对比观测、遥感解译与模型模拟技术，开展多尺度、多要素、多过程的能量与水量平衡研究，阐明稻田蒸散发变化对流域水文过程的关键驱动与时空影响机制；建立蒸散发在样地与流域尺度间的动态转化关系；量化不同土地利用类型影响蒸散发变化的时空差异、敏感区间及关键阈值；厘清稻田转为城市用地影响流域水量平衡的主导干扰水文学过程；在此基础上，改善传统水文模型模拟稻田流域生态水文过程的能力。为合理评价南方快速城市化地区生态系统服务功能和提高流域管理水平提供科学基础。

采用野外调查、地面对比观测、遥感解译，模型模拟和情景分析等方法，从流域水热耦合、植被-大气耦合关系出发，统一认识了实际蒸散发与参考蒸散发间的关系；揭示了亚热带湿润地区城市化通过影响蒸散发，进而影响流域水文过程和水热平衡的关键驱动机制；确定了不同土地覆被类型影响流域蒸散发的时空差异和敏感区域；量化了不同地表覆被类型对流域水热平衡的影响及其时空差异；发展了流域尺度水文模型优化技术，改进了传统流域水文模型的稻田蒸散发模拟能力。研究发现，城市土地覆被变化的负贡献抵消并超过了气候变暖和城市热岛的正贡献，使得流域实际蒸散发在研究时段内下降，尤其在植被显著减少、地表温度显著上升的城乡交界区；稻田转为城市用地对流域蒸散发的负贡献率从53%（2000年）增至61%（2013年），气候的正贡献率从47%降到39%；只有考虑植被和土壤等下垫面条件的变化，才能统一认识实际蒸散发与参考蒸散发间的关系。发现植被叶面积指数的显著减少与不透水面的显著增加，使得蒸散发及其组分呈现相反的变化趋势, 植被蒸腾、冠层截留与土壤蒸发的减少量大约是积水蒸发增加量的2.5倍。稻田减少是流域总径流、基流、枯水径流和暴雨径流增加的主要原因，稻田每减少10%，流域蒸散发减少10%，导致年总径流增加30%。原下垫面是森林或稻田的城市化地区，不仅极端降水更易引发流域洪涝灾害，而且城市化降低了稻田湿地的“生物排水”功能，常规降水也可能导致极端水文效应，从而在流域尺度上出现“小雨大涝”现象。城乡交界区因地表蒸散发显著下降导致的潜热通量变化主导了整个流域地表能量再分配过程及地表热量平衡项的变化趋势，并加重了城市热岛等一系列城市环境效应。研究结果为从流域生态系统角度认识近年来新出现的不同尺度城市环境效应，提高流域水资源管理和决策水平提供了科学支撑。