| Wetland as well as forest and ocean are considered as the three greatest ecosystems in the world. A variety of resources for production, life and entertainment can be provided by wetland. Moreover, a great benefit of environmental regulation can also be obtained due to wetland. It can be concluded that wetland plays an important role in maintaining water source, regulating climate, preventing flood, controlling pollution and keeping biodiversity. However, the ecological service function of wetland is declining significantly because of unreasonable development and usage. Globally, there has been a broad understanding in the importance and urgency of strengthening wetland protection and restoration since the Ramsar Convention (Convention on Wetlands of International Importance Especially as Waterfowl Habitat) was established. Many works on wetland have been carried out such as planning and construction of wetland, evaluation of wetland ecological service function and calculation of eco-environmental water demand. Until now, however, there are no reports on evaluation of wetland water utilization effectiveness in the world. This study takes Baiyangdian wetland as an example to propose a method for assessing water use efficiency in wetland ecosystem through investigation of water cycle, calculation of eco-environmental water demand, evaluation of ecological service function and analysis of coupling relationship between eco-environmental water demand and ecological service function. Correspondingly, water use efficiency in Baiyangdian wetland is evaluated under current conditions. The main focus and conclusions of this study are as follows:(1) Investigation of water cycle in Baiyangdian wetlandWater cycle in wetland consists of evapotranspiration, precipitation and runoff, which is the main driving force for survival and development of wetland ecosystem. As the main way of water loss in wetland, evapotranspiration plays a key role in hydrological and ecological process. Evapotranspiration in Baiyangdian wetland was investigated at three different scales (station, sub-wetland and wetland). In station scale, the Multi-cylinder evapotranspirometer was developed for obtaining data of evapotranspiration from vegetation under different growth conditions and evaporation from open water surface. Then, the expansion effect of evapotranspiration between station and sub-wetland is studied through comparing analysis of evapotranspiration after measuring the water balance items of sub-wetland. In wetland scale, a combined model for remote sensing retrieval of evapotranspiration is developed. The model parameters can be calibrated by corresponding sub-wetland and pixel of remote sensing images. As a result of that, evaporation and transpiration in Baiyangdian wetland can be obtained separately.(2) Calculation of eco-environmental water demand in Baiyangdian wetlandAccording to the types of ecosystem service function in Baiyangdian wetland, the structure of water use can be divided into water requirements for evapotranspiration, evaporation, infiltration, soil storage, habitat, environmental purification, landscape protection and entertainment, respectively. The functional approach is used to calculate the eco-environmental water demand. Meanwhile, utilization of water resources in Baiyangdian wetland is investigated, which is a premise of the assessment of water use efficiency for the wetland.(3) Evaluation of ecological service function in Baiyangdian wetlandThe basic situation of geography, hydrology, climate, animals, history and culture in Baiyangdian wetland are investigated in this study. The ecological service function of Baiyangdian wetland is divided into eleven classes:biological resources supply, water resources supply, gas regulation, climate regulation, flood control, water purification, habitat, nutrient cycling, maintenance of ecosystem integrity, teaching and research and tourist. Correspondingly, the evaluation index system of ecological services of Baiyangdian is established. Moreover, the ecological service function of Baiyangdian wetland is evaluated quantitatively using value evaluation methods, which is the preparation for assessing the benefits generated by water utilization.(4) Assessment of water use efficiency in Baiyangdian wetlandThe coupling relationship between eco-environmental water demand and ecological service function is established according to the results obtained above. An approach on assessment of water use efficiency in wetland ecosystem is proposed. The water use efficiency of Baiyangdian wetland is divided into seven classes such as use efficiency of water demand in evapotranspiration from vegetation, evaporation from open water surface, infiltration, habitat, environmental purification, landscape protection and entertainment. Then, the water use efficiency of Baiyangdian wetland is evaluated.
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