Studies On The Driving Mechanism Of Hydrological Processes On The Landscape Pattern Evolution Of The Yellow River Estuary Wetland

Hydrological processes control the formation and evolution of wetlands. They are recognized as the significant driving forces of shaping structure, function and landscape pattern characteristics of wetland ecosystems. According to the interaction relationships between hydrological processes and wetland landscape pattern, a theoretical framework combining hydrological processes and the responses of wetland landscape pattern is proposed based on the drive-state-response model. The theoretical method is applied to the wetland ecosystem of the Yellow River Estuary. Firstly, the Indicators of Hydrological Alteration with great ecological significance are adopted to estimate the alteration of eco-hydrologic regime of river estuary. Secondly, the evolution characteristics of estuarine wetland landscape pattern are analysed using remote sensing and geographic information system technology. Thirdly, the quantitative relationships between eco-hydrologic factor and key landscape pattern index are established. Additionally, the response regularities of wetland landscape pattern characteristics are discussed under the circumstances of hydrological regime alteration. Finally, the environmental flow demands for estuarine wetland are summarized for ecological restoration.The primary contents and results of this research are described as follows.(1) The wetland ecosystem is a landscape mosaic consisting of various types of marsh plant communities and geomorphic units, where water serves as the controlling environmental factor. First, wetland hydrological processes influence the life history behaviors of plant communities and mosaic pattern directly. On the other hand, the hydrological processes act on environmental conditions, for example, biological, chemical and geomorphic characteristics, and induce dynamic change of structural and functional characteristics of wetland plant landscape indirectly. In view of this, conceptual model about wetland hydrological processes-chemical and geomorphic characteristics--landscape spatial-temporal heterogeneity is established based on the drive-state-response model.(2) The eco-hydrological index including five flow components with great ecological significance are adopted to estimate the effects of the integrated water regulation and water-sediment regulation on the eco-hydrology of river estuary. The results indicate that the distribution of estuarine monthly discharge within the year has changed obviously. The averaged discharge from May to November has significant change, and the averaged discharge of June has the greatest increasing extent. The annual minimum flows and base flow index have substantial increase simultaneously. The timing of annual1-day maximum flow is15days in advance, while the mean duration of low pulses has moderate increase. The rising rate and falling rates of two adjacent days are decreased, meanwhile, the flood pulse process disappears. After the integrated water regulation and water-sediment regulation, the estuarine hydrologic regime has improved effectively, while the hydrologic processes become flat due to the decrease of change rate and disappearance of flood pulse.(3) Environmental flows are effective management tools for river ecological conservation and restoration. Environmental flows are not only the simple ecological water requirements or suitable flow magnitude, but also the dynamic hydrograph with various hydrological elements and pulsing fluctuation.The current hydrological regime of estuary could meet the appropriate ecological flows during the dry period and that of the flood period could achieve the river channel scouring except for the lack of flood pulse process. While, the hydrological processes during April to June are incapable of meeting the ecological flows and short of the flow pulsing for fish reproduction.(4) The evolution characteristics of wetland landscape pattern after the water-sediment regulation are quantified using remote sensing technology and landscape pattern metrics. From2002to2010, the area of Wetland water, Reed swamp, Suaeda meadow and River increase, and the former two landscape types have increased by50.92km2and84.81km2respectively. It suggests that the degraded bare lands are replaced by wetland and swamp gradually. At the whole landscape level, landscape diversity and heterogeneity have improved. The landscape maintains natural features basically with rather feeble fragmentation. At the major landscape types level, aggregation of Wetland water patches increases accompanying by the distribution of patches close to the Yellow River; the aggregation, connectivity and structural complexity of Reed swamp also have an increasing trend; while, the aggregation and connectivity metrics of Suaeda meadow remain stable; moreover, the Shrub forest is subject to fragmentation and division processes. According to the ecological significance and sensitivity to environmental factors, the key landscape pattern metrics are selected at the level of whole landscape and major landscape types. The key pattern metrics include the diversity, connectivity, contagion, division and shape metric of the whole landscape, the connectivity, clumping, division, shape metric and total core area of the major landscape types.(5) The critical hydrological factors influencing landscape pattern characteristics are recognized based on the correlation method and stepwise regression analysis. Then the quantitative relationships between critical hydrological factors and pattern metrics are established. The results demonstrate that the pattern characteristics of whole landscape and Wetland water have significant correlations with flood peak processes, especially with the hydrological processes whose peak discharge is more than3500m3/s. The pattern characteristics of Reed swamp correlate with the flow discharge and change rates of April and May, the maximum discharge and flood peak processes. While, there is no obvious correlations between eco-hydrological factors and pattern characteristics of Suaeda meadow. The most critical eco-hydrological metric recognized by the stepwise regression models is the averaged peak discharge above3500m3/s.(6) The response regularities of landscape pattern characteristics to critical eco-hydrological factors are discussed based on the stepwise regression models. In addition, the hydrologic measures for estuarine wetland ecological restoration are proposed incorporating the environmental flow demands. When the flood peak discharge increases by500m3/s, the diversity and contagion metrics of whole landscape would improve obviously, and the total core areas of Wetland water and Reed swamp would increase by4.51km2and4.25km2respectively. If the falling rate of hydrological processes during April to May decrease by5%, the connectivity metric of Reed swamp would increase by15.6%. Combining the research results with environmental flow demands, the hydrological measures for improving the wetland ecological conditions are summarized as follows:flow pulsing processes taking place at least once during April to May with the pulsing discharge above800m3/s, the duration of pulsing process more than7days, the falling rate of pulsing process decreasing by5%; in addition, in virtue of the water-sediment regulation, the flood peak discharge increasing by500m3/s and the flood overbank process taking place as much as possible.The researches of relationships between hydrological processes and wetland landscape pattern extend the theoretical system of wetland eco-hydrology and river ecological restoration. The example of the Yellow River estuary wetland validates the reasonability and practicability of the theoretical framework combining hydrological processes and the responses of wetland landscape pattern. In addition, the results will provide theoretical foundations and references for estuarine wetland ecological restoration in the future.