1 / 35

Chapter 3: Energy State of Water in Soil

Chapter 3: Energy State of Water in Soil. 100 cm 3 soil Sand 20 Vol.-% H 2 O. ?. 100 cm 3 soil Clay 20 Vol.-% H 2 O. Ch. 3 Learning Objectives. Why can soils store water? What are the driving forces for water movement in soils? How can we measure these driving forces?.

benita
Download Presentation

Chapter 3: Energy State of Water in Soil

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 3:Energy State of Water in Soil 100 cm3 soil Sand 20 Vol.-% H2O ? 100 cm3 soil Clay 20 Vol.-% H2O Petzold & Schwärzel : HSE

  2. Ch. 3 Learning Objectives • Why can soils store water? • What are the driving forces for water movement in soils? • How can we measure these driving forces? Petzold & Schwärzel : HSE

  3. Forces acting on Soil Water Petzold & Schwärzel : HSE

  4. Electrostatic forces Water Soil • Water is bonded to • other water molecules by • cohesive (water-water) • forces • the soil surface by • adhesive (soil-water) forces Petzold & Schwärzel : HSE

  5. Bond Strength of adsorped waterdependends on water film thickness From: Hartge & Horn Petzold & Schwärzel : HSE

  6. Capillary Rise - Capillarity Source: Flühler et al. 2005 Petzold & Schwärzel : HSE

  7. Surface Tension Petzold & Schwärzel : HSE Source: Jury & Horton 2004

  8. Contact Angle γ Liquid moistened the surface = hydrophilic the surface is water repellent = hydrophobic Petzold & Schwärzel : HSE Source: Jury & Horton 2004

  9. Why does water rise? Petzold & Schwärzel : HSE Source: Brady & Weil 1996

  10. Calculate the height of rise of water in glass tube of radius R at equilibrium. 2 H in [m] r in [m] Petzold & Schwärzel : HSE

  11. Capillary rise in a soil Source: Brady & Weil 1996 Petzold & Schwärzel : HSE

  12. Take home messages • Every pore diameter is related to a suction. • The water filled pores and the corres-ponding diameter can be calculated at given suction. Petzold & Schwärzel : HSE

  13. Soil Water & Energy • Kinetic energy: associated with motion • of minor importance (can be neglected in soils) • Potential energy: associated with position • of primary importance in determining the state and movement of water in soil Petzold & Schwärzel : HSE

  14. The bow is able to store energy by virtue of its position . Potential Energy No energy is stored. Petzold & Schwärzel : HSE

  15. Potential Energy m F s1 s0 Petzold & Schwärzel : HSE

  16. Total Soil Water Potential • = the work that is required for moving a quantity of water from a reference state into the desired state within the soil. • The performance is reversible and isotherm. • = describes the energy density of soil water. Petzold & Schwärzel : HSE

  17. Reference or Standard State • = pure water, • at atmosphere pressure, • at the same temperature as that of soil and, • constant elevation. • Soil water potential at the reference state is 0 (per definition). Petzold & Schwärzel : HSE

  18. Units of Total Soil Water Potential Petzold & Schwärzel : HSE

  19. Components of Soil Water PotentialΨ Petzold & Schwärzel : HSE

  20. Osmotic potential Petzold & Schwärzel : HSE

  21. Hydraulic Potential - Hydraulic Head Often, a simpler notation is used: H = z + h Petzold & Schwärzel : HSE

  22. Soil Water Potential versus Soil Water Content Petzold & Schwärzel : HSE

  23. Driving force for water movementwithin the soil Water moves from High to Low energy Flow path A = Area Distance = x Petzold & Schwärzel : HSE

  24. Capillary tube with an internal solute membrane Petzold & Schwärzel : HSE

  25. Direct Measurements of Components of Water Potential • Gravitational potential • Measure the vertical distance between the reference elevation and the point of interest. • Osmotic potential • Extract soil solution, measure its concentration. • Pneumatic potential • Measure the soil air pressure with a barometer. • Hydrostatic potential • Measure the vertical height of saturated water above the point of interest. Petzold & Schwärzel : HSE

  26. Gravimetric Potential & Reference Elevation Petzold & Schwärzel : HSE

  27. Measurement of the Matric Potential Petzold & Schwärzel : HSE

  28. Tensiometer Petzold & Schwärzel : HSE

  29. VariousTensiometers are available Petzold & Schwärzel : HSE

  30. What is the matric potential at the cup? The pressure in the cup is: Related to the weight: a = porous cup b = vacuum gauge c = connecting tube d = rubber stopper Petzold & Schwärzel : HSE

  31. Calculate the height of rise of water in glass capillary of Radius R placed in an open vessel of pure water. Assume the contact angle of the water on glass is zero. Assume that the evaporation of water is negligible. Analysis of Systems at Equilibrium Petzold & Schwärzel : HSE

  32. The equilibrium principle to estimate components of the water potential 1. Verify that the system is at equilibrium. 2. Define a reference elevation and a reference air pressure. 3. Find a location in the sytem where YW may be estimated. Call this YWo value . 4. Set YW = YWo at the point of interest. 5. Divide YW into appropriate components. 6. Evaluate directly those components for which appropriate information at the point of interest is available. 7. Determine the remaining component or sum of components using the equation in step 4. Petzold & Schwärzel : HSE From Jury & Horton, 2004

  33. From Jury & Horton, 2004 Height of rise of water in a clean glass capillary 1. The system will reach equilibrium as soon as the water in the capillary stops rising. 2. Define z = 0 at point A and po = Patm 3. At point A all components of YW may be evaluated. Yg = 0 (since z=0) Yo = 0 (pure water) Ya = 0 (P=po=patm) Ym = 0 (no soil) Yp = 0 (no overlying hydrostatic pressure) Thus, relative to the reference state, YW = 0 at A. 4. At point B, YW = 0 since B is at equilibrium with point A where YT = 0. 5. At point B, we have YW = Yg + Yo + Ya + Ym + Yp 6. By definition, at point B the components are Yg = rwgH Yo = 0 (pure water) Ya = 0 (P=po=patm) Ym = DP = -2s/R Yp = 0 (no overlying hydrostatic pressure) 7. YW = 0 = rwgH -2s/R  H = 2s/R rwg. Petzold & Schwärzel : HSE

  34. No equilibrium  Hydraulic gradient Petzold & Schwärzel : HSE

  35. Take home messages • Water is stored in soil because of electrostatic forces. • Soil water potential characterizes these forces and the energy density of soil water. • Water in soil moves spontaneously because of differences in the energy density of soil water. Petzold & Schwärzel : HSE

More Related