| Process mechanisms and their simulations of substance (water, heat, solutes) circulation and energy transfer in Soil-Vegetation-Atmosphere Transfer (SVAT) are frontiers and key scientific issues of multi-disciplines across hydrology and water resource, eco-hydrology, soil physics, agro-ecology, physical geography, agricultural soil and water engineering, and so on. In China, with agriculture plantation structure adjusted, recently the plantation area of fruit trees expands increasingly and total water consumes volume of fruit trees augments continuously. In water shortage areas, it is especially important to manage water use of fruit trees effectively. Understanding transpiration process, root water uptake mechanism and soil water dynamics in root zone of fruit trees, not only benefits for establishing irrigation system in reason to ensure high yield and decrease water use, but also fulfills the research contents of water transport in SPAC and SVAT. In this dissertation, the present status and existing problems of water transport mechanisms and its simulation in SPAC were analyzed firstly. In 2002-2004 years, experiments were conducted in an apple orchard of the Irrigation Experiment Station of Northwest A & F University to investigate the canopy structure, hydraulic conductance, sap flow in trunks and roots, transpiration process, root distribution characteristics and water uptake mechanism. The main results as follow: (1) Canopy structure and the characteristics of spatial and temporal distribution of leaves physiological ecology index were investigated. On the basis of this, considered inhomogeneous canopy structure a three dimentional model including canopy radiation interception and transpiration was established. The excellent agreement between the modeled transpiration and measured sap flow indicates that the model can describe precisely the spatial and temporal variability of canopy radiation partition process and total leaves transpiration. There was a quadratic parabolic relationship between leaf area index and height of apple trees in vertical direction. In horizontal distribution, leaf area index decresed from the center to outer along the radius direction. Leaf area index in south, east and west which had 2~3 maximum, were greater that in north. Leaf area index gradually increased from 0.1 to 2.8 in the beginning of growth period, stabled in the middle of growth period and decreased to 0.1~0.2 in the late growth period , which correlated quadratic parabolically with the days after bud burst (DAB). In summer sunny day, photosynthesis active radiation (PAR) of the uplayer of canopy was higher 50% than that of downlayer, which resulted that leaf temperature, stomotal conductance, photosynthis rate, transpiration rate and water use efficiency of uplayer canopy were greater 0.5℃~3.5℃, 20%~40%, 30%~50%, 25%~50% and 30%~60% respectively than those of downlayer at same direction and same time. In the same layer, leaves in different direction intercept different amount photosynthesis active radiations (PAR), which result in that leaf temperature, stomotal conductance, photosynthis rate, and transpiration rate in east were greater than those of leaves in west before 12:00a.m; leaf temperature, stomotal conductance, photosynthis rate, and transpiration rate in east were smaller than those of leaves in west after 12:00a.m; leaf temperature, stomotal conductance, photosynthis rate, and transpiration rate in south were greater than those of leaves in north at same time. Assumed that apple canopy should be continuous, its surface should be ellipsoid, and solar radiation interception should abide with Beer law, considered radiation distribution characteristics, a three dimensional transpiration model was develoed and validated. The modeled results were compared with the sap flow in trunk, and the relative error in different weather ranked in order, sunny day < cloudy day < overcast sky, and the values were 8.6%, 23.6%, 30.8%, respectively. (2) Considered inhomogeneous spatial distribution of soil water potential in root zone and leaf water potential in canopy of apple trees, and the effects of canopy structure characteristics and root density distribution on water transport in the SPAC, an improved method for estimating whole tree hydraulic conductance of apple trees was developed and applied. Canopy conductances of apple tree were estimated using the measured sap flow in trunk and the inversion of Penman-Monteith transpiration model to analyse their diurnal variations. There was same diurnal variation trend among leaves water potential of top canopy in different direction, which is V type, and the minimum appeared at 14:00. At same time, there were exsited obvious difference between the underlayer and uplayer leaf water potential. The difference reaches the maximum 0.5 MPa at 14:00. Using the root density and leaf area index distribution as weighted factors, mean soil and leaf water potential were calculated. Sap flow in the trunks of apple trees was related linearly with the difference between mean soil and leafwater potential, the slope of the line is whole apple tree hydraulic conductance which is 18.821mmolMPa-1s-1. The canopy conductance of apple tree was estimtated from sap flow in the trunks using the inversion of Penman-Monteith transpiration model. The maximum of canopy conductance appear at 9:00 a.m. in sunny day, but the maximum of canopy conductance and solar radiation appear almostly at same time in cloudy days. (3) The diurnal and daily variations characteristics of sap flow in trunks and roots were analysed. Temporal sap flow was extremely correlated with the solar irradiation, and daily sap flow linearly correlates with reference crop evaportanspiration. There is often a lag between the sap flow in roots and in trunks because of soil water stress and atmosphere droughty. The diurnal variation trendecy of sap flow in roots and trunks of apple trees appear sigle apex curve respectively in sunny day; and multi-apex curve respectively in cloudy day. In growth period, daily sap flow presented marked season variation characteristics. There is often a lag about 1 hour between the sap flow in roots and in trunks because of soil water stress and atmosphere droughty. The correlation degree between the instantaneous sap flow and relative air humidity, air temperature, wind velocity and net solar radiation ranked in oder as follow, wind velocity < air temperature < air humidity < net solar radiation. The linear correlation coefficient between daily sap flow in trunks and reference crop evapotranspiration was 0.4-0.6 in the beginning and late of growth period, and greater than 0.6 in the middle of growth period. (4) Orchard evapotranspiration were estimated from sap flow of single trees and soil water changes in microlysmeters using diameters occurrence frequency as the weighted factors, which excellently agreed with the results estimated using soil water balance method. The basal crop coefficient and soil water stress modified factor were estimated for calculating evapotranspiration of apple orchard. On basis of this, evapotranspiration of apple orchard were estimated using the dual crop coeffient method, and the predicted evapotranspiration agreed excellently with the measured using sap flow and microlysimeter and the estimated using soil water balance method. Compared the evapotranspiration in apple orchard estimated from the sap flow and microlysimeter with that caculated using the water balance method, the relative error of them were low, indicated that it was feasible that sap flow in trunk of single trees and soil water changes in microlysmeters were converted into evapotranspiration in apple orchard using diameters occurrence frequency as the weighted factors when there were spatial variability of sap flow in the trunks and soil evaporation under the canopy of the single apple trees.In experimental period of 2002-2003 years, the daily average evapotranspiration in apple orchard estimated using sap flow-microlysimeter method was about 2.27 and 2.25mm/d respectively, and the maximum daily evapotranspiration were about 6.14 and 6.54mm/d respectively. In the middle of growth period the transpiration is about 80%-90% of evapotranspiration in apple orchard and in the beginning and late growth period 0-40%. Basal crop coefficient of apple trees gradually increased from 0.5 to 1.0 in the beginning growth period, stabled about 1.0 in the middle of growth stage period and decreased to 0.5 in the late growth period, which correlated linearly with leaf area index. As soil water content between 12.8% and 23.0% soil water stress modified factor is related linearly with the relative effective soil water content, as soil water content higher 23.0% soil water stress modified factor is 1.0, and soil water content below 12.8% soil water stress modified factor is 0.0 In 2004 apple orchard evapotranpiration estimated using dual crop coeffient method agreed excellently with those measured using sap flow-microlysimeter as the error about -4.9 % between of them, and also with those estimated using soil water balance method as the erro about -3.1%. (5) Spatial distribution characteristics of root density and water uptake rate in root zone of apple trees were analysed, considered root distribution and compensation mechanism allowing for increased root uptake under conditions of water stress, a two-dimensional model of root water uptake was established and validated, which includes root density distribution function, potential transpiration and soil water stress modified factor. The measured data were compared against the outputs soil water contents from the numerical simulation of the soil water dynamics that uses Richards'equation for 2-D water flow and the established root uptake model. The results showed an excellent agreement between the measured data and the simulated outputs. The roots of apple trees centralized in the root zone with depth between 0 and 60 cm and radial space between 0 and 105 cm, and the roots amount in this main area accounted for 70% of total root in the root zone. Vertical and horizontal distributions of root density of apple trees are minus exponent. Two dimensional distribution of root density of apple trees was fitted using two dimensional minus exponent, and the coeffient is 0.639. Ratio of one dimensional root wate uptake rates to the maximum root wate uptake rates presented minus exponent distribution, which was like ratios of two dimensional root wate uptake rates to the maximum root wate uptake rates. Considered root distribution and a compensation mechanism allowing for increased root uptake under conditions of water stress, a two-dimensional model of root water uptake was established and validated, based on the relationship root uptake rates among root lengthdensity, potential transpiration and soil water distribution. The soil water dynamics in apple orchard under the different water status were simulated for 10 days using the two dimentional model, the results showed that, the relative errors between the predicted soil water content and the measured in the surface and center area of root zone were greater that in the other area. Under the full waterd condition the relative error of surface soil layer was higher than that in water stressed condition. The relative error in surface layer under full watered condition was higher 0.81% than that underwater stressed condition, and the relative error in root density centeralizing area under full watered condition was lower 1.03% than that under watere stressed condition. The model is adapted to be applied in the simulation of soil water movement in apple orchard watered after the water stressed for short time. The radial variation of soil evaporation should be consided in simulation of soil water movement in root zone of apple trees...
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