水源涵养与水文调节:和而不同的陆地生态系统水文服务

水与生态系统的关系是重要的科学问题,并且受到社会广泛关注。水源涵养和水文调节都是陆地生态系统所能提供的水文服务,并从生态水文和水资源角度把生态系统的健康和完整性与人类社会的持续发展紧密联系起来,从而也要求动态和综合的视角加以深入研究。基于生态水文过程原理,对水源涵养和水文调节的概念进行了辨析,认为前者是后者的有机组成部分,相对具体、应用中务求精确明晰;后者则更具包容性、客观性和广泛适用性。文章进一步简要分析了当前国内外生态系统水源涵养和水文调节服务的主导评估方法,结果表明,水源涵养的评估以储水量法为主,而水文调节则以基于降水和蒸散的水量平衡法及综合模型法(如SWAT)为主。从生态系统服务相互作用的角度考量,实际上水源涵养和水文调节及其相关的其它服务类型(如固碳、土壤保持、生物生产、淡水供给等)存在着复杂的动态权衡或协同关系,在科学研究和生态系统管理实践中必须统筹考虑,以确保对科学问题的准确把握和促进"水-生态-社会系统"的高效、可持续发展。为此,必须加强对生态水文过程的长期观测和实验研究,并且关注空间异质性及尺度效应、时间动态性和利益相关者需求的多维性。

Water retention and hydrological regulation: harmony but not the same in terrestrial hydrological ecosystem services

The relationship between water and ecosystems, an important hotspot in the scientific fields, is highly concerned by the whole human society. Water retention and hydrological regulation are both key services produced by terrestrial ecosystems, which closely link the health and integrity of ecosystems and sustainable socioeconomic development that calls for dynamic and comprehensive perspectives for advancing the research paradigm. This paper firstly clarified the scientific concepts of water retention and hydrological regulation based on the principles of eco-hydrological processes. Water retention is the capacity of ecosystems to hold part of water input from precipitation at certain spatiotemporal scales, whereas, hydrological regulation is the capacity of ecosystems influencing the hydrological cycles across space and time. In fact, water retention is an integral component of hydrological regulation. The spatiotemporal variation of water retention can have important impacts on the roles of ecosystems in hydrological regulation services. The former is specific and should be used accurately, while the latter is more inclusive, objective, and widely usable. Then, a brief analysis on the assessment methods of the two hydrological ecosystem services was given in the paper. It was found that water storage estimation at different spatial scales was the staple method for the assessment of water retention service, whereas, water balance partition based on precipitation and evapotranspiration and complex models (e.g., SWAT) were most widely used for hydrological regulation service assessment. Anyway, spatial scaling and temporal dynamics should be incorporated in ecohydrological monitoring, assessment, and modeling, which is a weak point in contemporary research and applications. From the ecosystem services interaction point of view, there are complex trade-offs and dynamic synergies among water retention and hydrological regulation and other relevant ecosystem services (e.g., carbon sequestration, soil conservation, biomass production, and freshwater provision). More and more hydrological monitoring and experiments indicated that water retention and hydrological regulation by terrestrial ecosystems do not necessarily contribute to the fresh water provision to human society. Vegetation cover increment (e.g., from grass to forests) usually reduce the river runoff and thus the availability of water resources to human use from the surface water sources. This reduction is resulted at least partly from the increased water retention by changed ecosystem condition. Actually, the increase of water retained by ecosystems contributes to their services, such as carbon sequestration, soil conservation, biomass production, other than fresh water provision to river channel and human society. This is one of the important reasons for the trade-offs concerning ecosystem and water resources management. The tade-offs should be considered comprehensively in scientific research and ecosystem management applications, which guarantee accurate scientific understanding of the related issues and facilitating highly effective and sustainable development of the "water-ecological-social systems". As a prerequisite, long-term ecohydrological observations and experimental researches should be strengthened. Meanwhile, more attention should be paid to the spatial heterogeneity, scale effect, temporal dynamics, and the multi-dimensionality of the demands from various stakeholders pertinent to water resources and ecosystem management in integrative analyses. The advancement of the eco-hydrological understanding on hydrological services of ecosystems and related trade-offs and synergies are crucial for informing more rational and effective ecological restoration, conservation, and ecosystem management actions taken place on the ground.