Example of SWP measurements (tree 2, 1 – 7 March 2011) 

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Context 1

... calculated from TVET and actual transpiration (T) based on sap fl ow measurements for the whole experimental period are shown in Figure 3. The transpiration rates of both trees are similar. For most time, except for the raining days, they are far below the PT rates, suggesting that both trees are under water stress for root water-uptake process. Figure 4 provides an example of SWP measurements. Each day, SWP reaches its maximum value around 06:00 h (local standard time). Similar phenomenon is reported by Ritchie and Hinckley (1975). After sunrise, SWP decreases quickly with an increase of transpiration rate and comes to its minimal value at around 14:00 h. Then, it goes up quickly until sunset. After sunset, SWP increases gradually until the water potential within soil – plant system becomes equilibrium. In the following analysis, daily predawn SWP is selected from the continuous output of stem psychrometers around 06:00 h every day, and daily root zone soil water potential is calculated by averaging predawn SWP of a day and the next day to account for the slightly change in root zone soil water potential within 1 day. For the whole week, there was not a rainfall event. The predawn SWP shows a decreased trend (Figure 4), indicating that the root zone became drier and drier mainly because of evapotranspiration. This supports that the predawn SWP re fl ects the changes of root zone soil water condition. Figure 5 shows the changes of predawn SWP during the experimental period. Both trees show a good consistence, and SWP responses sensitively to the rainfall events. In the dry intervals between rainfall events, predawn SWP decreases primarily because of root zone water loss through evapotranspiration. This trend agrees well with the experimental and modelling studies of root zone soil water dynamics (e.g. Shang et al . (2004)). In the following analysis, all data, except for those of rainy days, are used to examine and parameterize the water stress function (50 days in total). This is because a relative humidity of approximately 100% occurs during rainy days, which precludes transpiration from the canopy (Granier, 1987). Figure 6 shows the relationship between predawn SWP and T/PT based on the fi eld measurements. The data clouds of both trees are similar. On the basis of visual observation, they are different from the Feddes model piecewise linear shape but more close to the S-shape model form. Thus, the S-shape function is chosen for further analysis. The S-shape model fi ts the observation data fairly well (Figure 7), with a coef fi cient of determination ( R 2 ) of 0.74 ( n = 100, signi fi cance level < 0.01). Two parameters are À 0.52 MPa for h 50 and 1.04 for p , respectively. However, some data points (within the oval) greatly deviate from the fi tted curve. This suggests that some factors, other than the soil water condition, also in fl uence the root water uptake. In a coupled soil – plant – atmosphere continuum model, these factors, such as vapour pressure de fi cit, solar radiation and air temperature, are considered to calculate their stress effect (e.g. a Jarvis-type function). For a decoupled model discussed in this article, these factors are often not considered. Nevertheless, it is possible to lump ...

Context 2

... calculated from TVET and actual transpiration (T) based on sap fl ow measurements for the whole experimental period are shown in Figure 3. The transpiration rates of both trees are similar. For most time, except for the raining days, they are far below the PT rates, suggesting that both trees are under water stress for root water-uptake process. Figure 4 provides an example of SWP measurements. Each day, SWP reaches its maximum value around 06:00 h (local standard time). Similar phenomenon is reported by Ritchie and Hinckley (1975). After sunrise, SWP decreases quickly with an increase of transpiration rate and comes to its minimal value at around 14:00 h. Then, it goes up quickly until sunset. After sunset, SWP increases gradually until the water potential within soil – plant system becomes equilibrium. In the following analysis, daily predawn SWP is selected from the continuous output of stem psychrometers around 06:00 h every day, and daily root zone soil water potential is calculated by averaging predawn SWP of a day and the next day to account for the slightly change in root zone soil water potential within 1 day. For the whole week, there was not a rainfall event. The predawn SWP shows a decreased trend (Figure 4), indicating that the root zone became drier and drier mainly because of evapotranspiration. This supports that the predawn SWP re fl ects the changes of root zone soil water condition. Figure 5 shows the changes of predawn SWP during the experimental period. Both trees show a good consistence, and SWP responses sensitively to the rainfall events. In the dry intervals between rainfall events, predawn SWP decreases primarily because of root zone water loss through evapotranspiration. This trend agrees well with the experimental and modelling studies of root zone soil water dynamics (e.g. Shang et al . (2004)). In the following analysis, all data, except for those of rainy days, are used to examine and parameterize the water stress function (50 days in total). This is because a relative humidity of approximately 100% occurs during rainy days, which precludes transpiration from the canopy (Granier, 1987). Figure 6 shows the relationship between predawn SWP and T/PT based on the fi eld measurements. The data clouds of both trees are similar. On the basis of visual observation, they are different from the Feddes model piecewise linear shape but more close to the S-shape model form. Thus, the S-shape function is chosen for further analysis. The S-shape model fi ts the observation data fairly well (Figure 7), with a coef fi cient of determination ( R 2 ) of 0.74 ( n = 100, signi fi cance level < 0.01). Two parameters are À 0.52 MPa for h 50 and 1.04 for p , respectively. However, some data points (within the oval) greatly deviate from the fi tted curve. This suggests that some factors, other than the soil water condition, also in fl uence the root water uptake. In a coupled soil – plant – atmosphere continuum model, these factors, such as vapour pressure de fi cit, solar radiation and air temperature, are considered to calculate their stress effect (e.g. a Jarvis-type function). For a decoupled model discussed in this article, these factors are often not considered. Nevertheless, it is possible to lump ...