岩溶含水系统的森林退化/转变的水文效应

It is generally recognized that reforestation is tend to increase evapotranspiration (ET) and reduce water yield in the drainage area, while deforestation has reverse impact. Yet this may not be true in karst area. Underground drainage system and preferential flow are dominant patterns in karst area, which makes karst hydro-ecosystem different from non-karst area. So it is necessary to reconsider the hydrological effect due to rocky desertification and during its restoration. The objects of this project are to understand the impact of forest degradation and conversion on water yield and flow regime in mature karst area. Two karst catchments are selected for compared study. One is Banzhai karst spring in Guizhou province, which is covered by virgin forest. The other is Guancun karst spring in Guangxi, whose land cover is largely changed to cropland, shrubland, rocky desert, and only small forest residue. Nevertheless, the paired catchments are similar in hydrogeology, geomorphology and area. Water budget and some statistics methods are used for doing comparison through recharge coefficient, ratio of stormflow and baseflow, rainfall-runoff delayed time, and flow duration curve. The comparisons will show their difference in water yield and flow regime. Several subcatchment systems in epikarst zone in the catchments will be selected based on their land cover and will be monitored by epikarst springs. The ET will be calculated and runoff will be separated from hydrographs by the help of their chemograph. The ET and runoff ratio will help us to understand the impact of LULC. Combined multiscale analysis from epikarst and basins will finally help us to set up a hydrological model, and it will show the impact of forest degradation and conversion in Guancun on hydrological process.

一般认为森林恢复趋向于增加蒸散发减少流域产水量，而退化则相反。南方岩溶地区存在地下排水系统和大量优先流补给途径造成土地覆盖通过不同的方式影响流域的水文过程，并且导致石漠化及其治理成果的水文效应也存在不确定。本研究的目标是分析森林退化对岩溶流域水文过程的影响。选择贵州茂兰板寨地下河流域和柳州官村地下河流域进行对比。两个流域在面积大小、地貌和岩溶发育特征上相似，但是土地覆盖类型完全不同。板寨代表人类活动前的基本状态，以原始森林为主，官村有村庄、耕地等，森林退化为灌木林甚至石漠化。采用水均衡计算和流量统计分析的方法，通过补给系数、洪峰与基流的流量比、洪峰滞后时间等参数以及流量历时曲线的对比，揭示系统的产水量和径流特征的差异。选取两个流域内典型土地覆盖类型下的表层岩溶带水流子系统，以蒸散发和径流组成的对比，认识森林退化条件下山坡降雨补给方式的变化，建立流域水文模型，说明森林演变对水文过程的作用。

我国南方石漠化治理工程已经扩大到316个县，到2020年实现基本遏制石漠化土地扩展态势，为此治理结果如何衡量，未来石漠化治理需要达到什么程度等问题急需要答案。本项目以评价岩溶地区森林退化或石漠化土地的恢复治理所产生的水文效应为目标，从水文地质视角揭示岩溶生境植被与水循环的关系。项目选择了位于广西和贵州的四个岩溶流域，开展流域水文分析和对比，为森林水文效应的评判提供依据。为了揭示机理，水文对比的范围从流域尺度缩小至坡面尺度和钻孔剖面。流域和坡面水文对比的结果都显示，森林涵养水源的作用不明显，甚至于出现森林减小径流的效应。以茂兰自然保护区为例，在南方为数不多的岩溶原始森林区，地下河枯季径流模数在区域上处于较低水平，雨季洪水依旧猛烈。其水文特征与石漠化地区别无二致。利用钻孔分层观测装置一个水文年以上的数据提出岩溶地区植被生长存在“包气带障碍”。包气带即“厚”又“漏”的特性导致植被难以改变其表层岩溶带小生境。提出后期石漠化治理需要人工蓄水满足生态需水。项目开展了广泛的国内外合作，形成优势互补，促进了成果落地转化。在国际上，与加拿大圭尔夫大学G360地下水研究所建立了合作关系，学习并引进了地下水多层监测技术。该项技术成功应用在丫吉试验场，实现了表层岩溶带与下部包气带的分离，取得的结果是“包气带障碍”概念提出的重要依据，在项目研究中发挥了关键性作用。在国内通过与茂兰自然保护区管理局的合作，建立了茂兰自然保护区控制性的地下水观测断面，解决了“不完全产水量”问题，为今后保护区评价生态系统的服务功能打下基础。