| With ecological civilization construction on the agenda in China, how to guarantee the demand of ecological environment water via rational allocation of water in the premise of meeting the demand of human socio-economic development is an important and complex issue. Moreover, under the impact of climate change, drought which occurs more and more frequently has made the solution of this issue more difficult. Wetlands are perfect study objects on ecohydrology and environmental changes because their ecosystems have higher productivity and species diversity, and they are sensitive to environmental changes, particularly hydrological fluctuations. Thus, it is important to study on wetlands ecohydrological change, and to improve the ecohydrological theory against drought for ecological protection and allocation of ecological water during drought period.Baiyangdian Wetland is a typical northern lake wetland which is located in the North China Plain. In recent years, the wetland has been threatened constantly by drought. Although basin and inter-basin water transfer has alleviated this threat, the water transfer process should be implemented rationally based on ecohydrological changes during the wetland drought. In this paper, eco-hydrological change characteristics have been analyzed in Baiyangdian Wetland through laboratory experiments, field surveys, and numerical simulation. Based on drought vulnerability assessment in the wetland, maintenance and protection measures for the wetland have been proposed. The main works are as follows:(1) Connotation of wetlands drought has been discussed firstly. The results showed that the wetland has been suffering from comparatively serious drought since the1980s. The main reason causing this result is human activities (about75%), followed by climate change (about25%). With the development of socio-economy, human water consumption is bound to increase, which makes the wetland natural inflow is almost zero. Finally the wetland maintenance will rely on water transfer, particularly inter-basin water transfer, and this phenomenon will not change in a very long time.(2) Wetland hydrodynamic model has been established aiming at characteristics of complex topography and boundary, dense aquatic plants in wetland. In the model the topographic expression has second-order accuracy, and harmony approximate bottom slope items have been applied. Manning roughness employed equivalent Manning roughness in aquatic plants area and used normal Manning roughness in no plant area. Then the model has been verified by typical example, and presented well results. Finally flow field changes have been discussed using the model during the wetland drought.(3) Phragmites australis shows stronger tolerance and resilience of drought than Typha orientalis Presl in general. Their net photosynthetic rate, stomata conductance, transpiration rate and water use efficiency are on the rise with time in control, slightly drought and mild drought scenarios, while these indexes present declining trend in very drought and extremely drought. The threshold of enduring drought for Phragmites australis and Typha orientalis Presl are different, which are not less than20%,30%of soil water content respectively. Water supplement can enhance water absorption and increase fresh weight for them. If drought stress to them is not above the threshold, drought effect can be relieved by water supplement. On the other hand, when drought makes water volume decline and water quality worse in a wetland, the dominant community is transforming from submerged plants into emergent plants as a result.(4) Based on the wetland eco-hydrological characteristics during drought, drought vulnerability assessment index systems have been built. The wetland vulnerability is the lowest without outside influence when water level was not less than8.3m. Since2000, the wetland drought vulnerability has increased to level4. According to the results of wetland drought vulnerability assessment with specific external factor, considering the future socio-economic development will be more powerful, it is recommended that the lowest water level is over7.5m for the protection of wetland.(5) On the basis of ecological functions of Baiyangdian wetland, it is proposed that threshold for the wetland ecological protection is not less than7.5m combing the results of wetland drought vulnerability assessment. The corresponding lowest ecological flow was110million m3. Measures against the wetland drought are increasing wetland water and reducing wetland water consumption. Meanwhile it should be guaranteed that ecological land will not be occupied in the wetland and so on. |
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