典型喀斯特流域植被变化的水文响应及其空间差异

Positive trends in vegetation have been happened after the implementation of Ecological Restoration Projects in Southwest China. The strong heterogeneities of surface and subsurface structures in karst areas mean that the hydrological processes of karst systems can be very complex. The unclear hydrological behaviors on vegetation restoration of different geological catchments limit the quantitative assessment and further promotion of Ecological Restoration Projects. In this proposal, a typical karst watershed is selected as the study area, and hydrological, meteorological and remote sensing data are collected from 1980s~2015. Firstly, a discharge hydrograph separation method will be used to separate a karst watershed hydrograph into three outflow components: surface runoff, quick flow originating from focused recharge in conduits and slow flow in a porous annex system. The interaction between vegetation coverage and three outflow components will be analyzed. Secondly, the watershed will be partitioned to sub-basins. The statistical distribution functions of sub-basin characteristic parameters and cluster analysis will be used to group sub-basins into several landscapes. A stepwise model evaluation strategy will be used to select optimum parameters, and distinguish the domain processes of each kind of landscape. Then, the contributions of karst regions and non-karst regions to the watershed runoff will be estimated respectively. Lastly, a semi-distributed hydrological model will be applied and validated by taking the karstic characteristics into account. The spatial distribution of trends in runoff will be predicted by simulating different vegetation recovery scenarios at sub-basin and watershed scales. The results could make contributions to studies on hydrological behaviors on vegetation restoration. Additional, this study is useful for the optimal allocation of land use and water conservation function improvement at regional scale.

西南喀斯特地区生态恢复初见成效，植被总体呈增加趋势，但因水文过程复杂且时空异质性高，难以确定流域内不同地质地貌类型区植被变化对水文过程的影响差异，严重制约了石漠化综合治理生态水文效应的定量评估，阻碍了后续规划的制定与实施。申请项目拟以典型喀斯特流域为研究对象，基于水文气象监测数据和遥感影像，分析长时间序列植被格局与水文过程的时空变化特征及其相互联系；结合水文地质调查图划分子流域，基于流域特征分布曲线和聚类分析对子流域进行归类，利用迭代法建立子流域特征参数与水文过程的联系，明确喀斯特、非喀斯特类型区对流域径流量的贡献。在上述研究基础上，基于流域半分布式水文模型的构建与验证，分析不同地质地貌类型区水文过程变化的关键驱动因子，模拟与预测不同植被恢复情景下流域水文过程，阐明子流域-流域植被恢复的水文效应及其空间差异。研究结果能为西南喀斯特地区土地利用结构优化配置和流域水源涵养功能提升提供科学依据。

喀斯特地区土层浅薄，地表-地下二元结构发育；在全球气候变化和我国大规模实施生态治理的背景下，研究喀斯特地貌区等生态脆弱区植被恢复的水文效应对于实现区域社会和生态的可持续发展具有重要意义。本项目以典型喀斯特流域（龙江流域）为研究对象，分析了4个不同喀斯特地貌占比流域植被覆盖和降雨-径流监测数据的长时间序列变化；通过对流域内木连小流域坡地土壤水分、水位及流域出口流量、电导率和硝酸根离子的动态监测，明晰了喀斯特坡地土壤-表层岩溶带对降雨的响应规律，解析了小流域径流过程；结合植被-土壤-岩石的空间分布特征和喀斯特流域水文过程的特殊性，改进SWAT水文模型，模拟了龙江流域及其子流域的水文过程，阐释了子流域-流域植被恢复的水文效应及其空间差异。结果表明，退耕还林等生态工程增加了流域植被指数（NDVI），特别是前期受人类活动影响较大的喀斯特地貌区；喀斯特流域径流对植被变化响应敏感，植被恢复能有效提升区域水文调节功能，退耕还林等人类活动降低了汛期径流量，增加了非汛期径流量。坡地土壤和表层岩溶带平均厚度分别小于0.63和2.60m；岩溶发育强的坡地降雨产流阈值较大，岩溶发育弱的坡地水分和水位变化对降雨的响应迅速，存在优先流特征；表层岩溶带水是小流域径流的最重要补给来源，小流域出口径流量以基流为主。分别设置喀斯特地貌区和非喀斯特地貌区土壤层厚度，耦合双孔隙模型模拟喀斯特区表层岩溶带水文过程，提升了SWAT模型在喀斯特流域的适用性；模拟结果表明地表产流量占比和基流指数均与喀斯特地貌占比呈负相关关系。在植被破坏较为严重的环江流域（喀斯特地貌占比66%），生态工程实施后植被覆盖明显增加，NDVI存在突变点，非汛期径流增加主要受植被恢复影响；在植被覆盖较好的中洲小江流域（喀斯特地貌占比61%），NDVI不存在突变点，非汛期径流增加主要受气候变化影响。研究结果可以为该区水源涵养功能的提升提供科学依据，完善与补充石漠化综合治理生态水文效应的定量评估指标体系，从而推动后续政策与规划的制定和实施。