Simulation Of Hydrological Processes For Wetlands In Naoli River Basin, NE China

As one of the three biggest ecosystems on this planet wetland provides human beings with not only great amount of resources, but also precious environmental functions and benefits. However, wetlands in China have been suffering constantly from the global change and anthropogenic disturbance over the last50years. The cumulative effects of increasing wetland loss, as well as intensified adjacent land use continue to threaten the structure and function of wetlands. Water regime in wetland has been changed significantly in many regions, which is the main reason for wetland degradation. Models, especially physically-based distributed hydrological models, can often accurately represent complex wetland hydrological situations. They can provide us detail information of the watershed hydrological processes, and improve our understanding of the physical, chemical and biological processes within a watershed and the way they interact. Distributed hydrological models such as MIKE SHE, can also be used to simulate hydrological impacts of future climate and landuse change on the functioning of wetlands, which can be expected to benefit wetland management, protection and restoration.The main purpose of this thesis was to establish a framework for wetland management and restoration based on an integrated surface/groundwater hydrological modelling system, i.e. MIKE SHE. To achieve this goal, a lumped conceptual NAM model was firstly built to help understand the basic hydrology characteristics of the study area-Naoli River Basin in northeast China. Then, the coupled MIKE SHE/MIKE11model was constructed using multisource data, based on which the main hydrological process of the watershed was able to be simulated. Furthermore, seven Global Circulation Models (GCMs) and their arithmetic ensemble mean (AEM) were used to predict the climate patterns in 2050s under three emission scenarios (A2, A1B, and B1). Hydrological impacts of climate change upon the wetland was investigated by running GCM output through validated the MIKE SHE/MIKE11hydrological model. Finally, the optimal wetland management and restoration scheme was proposed based on the outputs from modelling. The main conclusions are summarized as follows:(1) The MIKE11HD/NAM modelling system is capable of simulating the rainfall/runoff process of Naoli River Basin. Model performance during calibration period was generally better than that during validation period with modification on underlying surface characteristics caused by intensive human activities. The ongoing agricultural development since1950s has significantly changed the hydrological regime of the watershed, and there is no sign that the wetland degradation trend has stopped currently. The uncertainty of our conceptual model comes mainly from input data, model structure and the selection of optimal parameter set. Although NAM can effectively represent the major hydrologic processes based on limited data and a modest number of parameters, it deals with each catchment averagely. Therefore, it is not able to adequately address problems such as snowmelt, irrigation and impacts of land use change on water regime.(2) Quantitative statistical parameters and visual comparison of simulated and observed hydrograph show that the MIKE SHE/MIKE11modelling system can effectively simulate the major hydrological processes of Naoli River Basin. The Nash-Sutcliffe coefficient during both calibration and validation periods was above0.65, a criterion as satisfaction. According to water balance analysis, the total evapotranspiration during the simulation period exceeded the total precipitation, and baseflow contributed a large portion to the channel flow, which means that the watershed was of deficit in water balance. Water balance analysis for different landuse types demonstrated the wetland’s important functions in climate regulation and water conservation. The widely adopted well irrigation, a conventional practice, in the area has led to the constant decline of groundwater level. The uncertainty of the coupling model lies mainly in the input data, model structure and the selection scheme of optimal parameter set.(3) On the whole, the GCMs predicted that the precipitation, evapotranspiration and temperature in the watershed would rise considerably in2050s under all the three emission scenarios, compared with the baseline situation. However, there were differences in the amount of variation between GCMs. The three scenarios show consistently greater discharge for most of the year, with A1B scenario the highest. The AEM predicted that the annual discharge in the downstream would increase7.7%,10.1%and6.8%for A2, A1B, and B1scenarios, respectively. There was a delayed river flow response to the changing precipitation in the lowland catchment, owing to the functions of water retention and runoff regulation of wetlands. Water balance analysis showed that annual total precipitation within the area would increase significantly, but annual total evapotranspiration would increase much more. If the current landscape pattern stays unchanged and no other recharge measures are carried out, the water shortage situation would be more severe in the context of global climate change. Wetlands within the watershed are at greater risk of being shrunk and degraded.(4) Based on the conclusions of modelling practice, three wetland management and restoration-planning schemes (corresponding to the relevant regulation strategies and strength) have been presented. Scheme1concerns mainly on the continuity and integrity of the core wetland area, and it reconverts the fragmental croplands into an integrated wetland with the relatively virginal remnant. Scheme2reconverts all the paddy field while Scheme3all the dryland within the watershed into wetland. According to modelling results, Scheme1is optimal, as it is favorable to wetland protection in terms of most hydrological indicators and no harm on food security in the region would be expected. Considering the geohydrologic conditions of the Naoli River Basin, as well as the potential impact of climate change, the planning scheme should be carried out from the following aspects:reconversion of cropland into wetland at a rational level, effective water recharge, and wetland environmental protection.