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Soil Physics October 19, 2009 Homework 4 Due October 26 Water Potential and Retention 1. Assume that a soil has a water table at 0.32 m below the surface. Calculate the vertical distributions of the pressure, gravitational and total potential (J/kg) in the interval (0-0.5 m) assuming that the reference level is at the surface. Present the results in a table using steps of 0.1 m (see Table below). Repeat the calculation for the reference level placed at the water table. In each case, convert the values of potential to units of energy/volume and energy/weight. z (m) ψg ψp ψh 0 0.1 0.2 0.3 0.4 0.5 2. A curious student has constructed a manometer tensiometer that uses alloy X instead of mercury. Alloy X is insoluble in water, is colored liquid at room temperature, and has a density of 8.0 Mg m-3. A tensiometer is buried 0.5 m deep in the soil where the pressure potential of the soil water is –1.5 m. The surface of the reservoir of alloy X is 0.1 m above the surface. How high above the soil surface will alloy X rise in the manometer tube when equilibrium is reached in the system? 3. A core sample of 100 cm3 is on a tension table (“hanging column”) apparatus. At negative pressure potentials –hi (cm) the equilibrium outflow Vi (cm3) is read on the burette: -hi 0 10 20 30 40 50 60 80 100 120 Vi 31 31 30.3 29.2 27.5 25.5 23.3 21.5 19.1 17.8 Compute the soil water contents θi and plot the soil water retention curve when the bulk density of the soil is 1.38 Mg m-3. Soil Physics October 19, 2009 Homework 4 Due October 26 4. Make a graph showing the relative vapor pressure (or relative humidity, RH) at equilibrium with the matric potentials showed in the table below. What is the RH range that can be found in field soils with water contents between saturation and wilting point? 5. Columns of Soil A and Soil B are sampled from the field. The columns are 1-m long. In the laboratory, the columns are placed in the upright position with the bottom in contact with the interface of unlimited pure water (at the interface ψp = 0). After equilibrium, both columns are placed in the horizontal position with their ends sealed. Using the water retention properties of Soils A and B, predict the approximate value of matric potential in each column after the second equilibrium is achieved. In which of the two soils is that prediction more uncertain and why? hm, kPa -1 -10 -30 -70 -100 -300 -500 -1,000 -1,500 -3,000 -7,000 -10,000 -100,000 Soil A Soil B hm, cm θ , m3 m-3 hm, cm θ , m3 m-3 0 0.571 -3 0.518 -25 0.521 -14 0.516 -50 0.485 -21 0.516 -100 0.407 -29 0.515 -150 0.377 -40 0.442 -200 0.353 -46 0.38 -300 0.338 -51 0.34 -400 0.328 -59 0.291 -500 0.319 -72 0.249 -600 0.31 -100 0.186 ...





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