| In the past few decades,the human have created unprecedented the wealth of society and promoted the development of the industrial civilization with the advanced technology and improved social productivity While developing economy and exploiting natural resources, the human have ignored the supporting capacity of the ecological system so that the environment has been affected in the unprecedented scale and intensity and the natural ecological system has been damaged and changed. Ignoring the relationship between the water resources and ecological environment system is the mismanagement of the water resources in the 20th century, which has directly caused the ecological environment to be worse and triggered many problems. The publication of such literature as Our Common Future, Care for the Earth, and 21st Century Agenda indicates that man's awareness of water and natural ecological system has changed. In the early 1980s, the prototype of the study on the distribution of water need of ecology and environment has been established. Up to the 1990s, the study on the pertinence of water resources and ecological environment, especially the study on the water requirements, has attracted the attention of the whole world. In China, the study on the water requirements of the ecological system is still in the initial stage. There is still no unified definition on the concept, intension and extension of the water need in the ecological system. The study on calculation methods are far from being in-depth and perfect and mainly use the method which combines the qualitative analysis with the microscopic quantification. Owing to the different climate, soil terrain and animal and plant sections, various ecological systems have formed in the surface of the earth. According to the force leading to the formation of the ecological system, they are divided into three categories: natural ecological system, semi-natural ecological system and platted ecological system. Different ecological systems have their own water requirement features. According to the interference that the man has done to the nature, the ecological water requirement consists of the water requirements for platted vegetation system and for the natural vegetation system. In the past few decades, the regularity of water requirements in the planted ecological system has been studied deeply home and abroad. Most of the estimating methods are applicable to study the water requirements of various crops in the planted vegetation system and a lot of achievements have been made in studying the water requirements of crops or planted woods and grass. However, the study on the water requirements of the natural vegetation system has developed slowly and fewer research reports about estimation of the evapotranspiration of natural vegetation and the quantity analysis of the water balance of the natural vegetation in the natural ecological system have been made. On account for the situation mentioned above, I have attend the key project, A Study of Scientific Allocation of Water Resources for Vegetation Construction in The Sandy Area Between Beijing and Inner Mongolia(50139040),supported by National Committee of Natural Science Foundation and have in charge of the study of evapotranspiration mechanisms of natural vegetation under guidance of professor Chaolunbagen. The evapotranspiration mechanisms and estimating approaches of water requirements for vegetation have been found by using interdisciplinary theories including hydrology and water resources, atmospheric physics and natural ecology and detail field experiments., The main contents of the study include the following five parts: Part one. minutely analyzes the features of the natural ecological system and also analyzes the latest methods and achievements made home and abroad in studying the evapotranspiration of various crops in the cropland ecological system. With reference to the theories and practice of the previous studies, the frame of embedding model for the study on the evapotranspiration of natural vegetation in the...
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