| Based on the theory of soil- plant- atmosphere continuum, studies on physiological plant ecology of Salix Psammophila, Elaeagnus angustifolia, Caragana microphylla Lam, Atripldex canescens ,Ulmus pumila and Populus alba L.var. pyramidalis, including the transpiration Characteristics of them, the rules of the transpiration water use and the soil evaporation water use of their wood stands, the actual evapotranspiration of the wood lands and the water requirement of them, as well as the soil water shortage of the wood lands, etc. were conducted in growing season of them, in the year 2006and 2007, for the purpose that the studies could provide the selection of tree species and the environmental water capacity of forests established in the sandy land during the process of local vegetation resumption and reconstruction with theoretical evidence.After the amount and distribution characteristic of water resource and the quantity and distribution characteristic of vegetation, as well as the status quo of social development in Yanchi County had been held macroscopically, the classified and the calculation system of its ecological water utilization were established. The quantities of ecological water uses in the light of the eco-environmental status quo were researched systematically. The quantities of ecological water requirements according with the aim of eco-environmental construction and protection were deeply studied based upon the sustainable development principle and the problems existing in water resource exploitation were also discussed for the intension that the evidences on decision-making were provided to the eco-environmental construction and harmonious development of the population, the economy, the resource and the environment in the county.Major conclusions are summarized as follows:Part I: The environmental water capacity1. Penman-Monteith equation was considered as a basic model in predicting stand transpiration. On basis of transpiration data measured by portable stomata instrument, the weather data measured by automatic weather station and establishing prediction models on all parameters such as net radiation on forest canopy, albedo, light reduction coefficient, forest canopy stomata resistance and boundary layer resistance, Penman-Monteith model is provided for accurately calculating stand transpiration in growing seasons. The results showed that there existed a good correlation between observed value and predicted value. And the equation can exactly predict the stand transpiration of the researched stand. The evaporation of the forest stand in experimental zones from 2006 to 2007 was calculated on the basis of Penman-Monteith model. The comparison between the values gained from calculation and measurement shows that the equation is feasible, which the relative errors are under 10% in.Salix Psammophila > Elaeagnus angustifolia > Caragana microphylla Lam >Atripldex canescens is the sequence of annual transpiration water use. The amount of annual transpiration water use of single Ulmus pumila is 0.5 time fewer than that of a Populus alba L.var. pyramidalis.2. The equation, that is Z=0.306-0.025X-5.387Y+9.851e-4 X2+1.423 X Y+22.335 Y 2 , is a model of the relationship of the soil evaporation of wood lands (Z) with the free water surface evaporation (X) and the soil moisture (Y) with a 0.713 correlative index (R2), which was established based on the investigated data.3. The relationship of transpiration rate (Tr: y) and water use efficiency (WUE: y) of tree to soil water content (SWC: x) answer for a kind of equation such as y=ax3+bx2+cx+d constructed upon the data observed with Li-6400. The critical value of SWC where the WUE of leaf is the highest serves as a lower limit of suitable soil moisture of tree growth and that where the Tr of leaf is the highest as a upper limit of that. The results from calculation with the equation are as follows: The range of Salix Psammophila is 10.72%~14.36%; Elaeagnus angustifolia 9.12%~11.40%; Caragana microphylla Lam 11.39%~13.20%; Atripldex canescens 10.07%~12.20%。4. Under suitable soil moisture, the lower limits of annual water requirements of a Salix Psammophila, a Elaeagnus angustifolia, a Caragana microphylla Lam, a Atripldex canescens, a Ulmus pumila and a Populus alba L.var. pyramidalis were 459mm, 445mm, 437mm, 387mm, 460mm and 602mm respectively in the sandy land, during the growth season (the months from 5to10). The soil water shortages were analyzed in every wood land by a means that the potential evapotranspiration was replaced with the actual evapotranspiration under suitable soil moisture. The conclusion indicated that water stress with different grades took place on trees in their growth season.5. In the light of the water consumption of tree transpiration (T), that of soil evapor- ation of forest plot and the precipitation quantity (P) under an assurance rate as 80%during the growth season, the least water nourishment area(S) for a tree growth in the lower limit was found from S=1000·T/(P-E).The results were as follows: 7m2 for a Elaeagnus angustifolia; 5m2 for a Salix Psammophila and a Caragana microphylla Lam; 4m2 for a Atripldex canescens; 15m2 for a Ulmus pumila and 20m2 for a Populus alba L.var. pyramidalis.Part II: The regionally ecological water use6. According to the theories of vegetation evapotranspiration and water balance, the quotas of ecological water use types were made certain based on the information gained from investigation and observation. The ecological water uses of types were calculated based on the year 2005 as a standard year in Yanchi County. The results are as follows: The ecological water use of forest cover was 6.05×108 m3/a; the ecological water use of soil and water conservation 0.19×108 m3/a; the ecological water use of meadow cultivation 8.18×108 m3/a; the ecological water use of lake and billabong 0.0627×108 m3/a; the ecological water use of plantation in town 0.0031×108 m3/a.7. Through an analysis of the characteristics of ecological water use types, ecological water use coefficient (Cu), which illuminates a proportion relationship between ecological water use amount and rainfall amount, and ecological water use modulus (Mu), which denotes the ecological water use amount in a square kilometer per year, were studied in the County. The results are as follows: Cu and Mu were 54.29% and 16.73% respectively in the year 2005,and will be 63.76% and 19.65% in the year 2010.8. 16.93×108m3/a will be the totally ecological water requirement of Yanchi county in 2010., which was calculated based on the ecological environment construction plan of the county. At the time, a water shortage of 1683.56×104m3/a will arise, and the water requirement of agricultural industry will amount to 1850×104m3/a as the largest portion in the ecological water uses.
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