ment through soils. The rate of water flow through the soil () depends on the driving force (the gradient in water potential) and the resis- tance to water movement. This resistance, in turn, depends on the hydraulic conductivity (L) of the soil, and the path length (l) of the column through which the water travels. (4.5)
Fluid Pressure
The term fluid pressure is coined as, the measurement of the force per unit area of a given surface of a closed container. It is a branch of physics that helps to study the properties of fluid under various conditions of force.
Gauge Pressure
Pressure is the physical force acting per unit area on a body; the applied force is perpendicular to the surface of the object per unit area. The air around us at sea level exerts a pressure (atmospheric pressure) of about 14.7 psi but this doesn’t seem to bother anyone as the bodily fluids are constantly pushing outwards with the same force but if one swims down into the ocean a few feet below the surface one can notice the difference, there is increased pressure on the eardrum, this is due to an increase in hydrostatic pressure.
- A spruce tree is 20 m tall with a trunk diameter of 0.4 m. Its needles have a measured water potential of –2.5 MPa, while the roots have a measured water potential of –1.2 MPa. Hydraulic conductivity through the stems is 7×10–8m3s/kg. What is the transpiration rate of the tree (in g H2O/s), assuming that 1.5% of the stem cross sectional area contains water-
conducting vessels?
Hints:
- 1 MPa = 106kg/(m·s2); water density = 1 g/cm3
- Start by using the flow equation (p. 104 CMV textbook) to calculate the rate of water movement in m s-1
- Multiply the rate by the cross-sectional area of water-conducting vessels
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