SOIL222: Module 5. - Lecture 6: Infiltration and Water Movement

Describe the processes of infiltration and redistribution of soil water, and the soil properties that influence them.

• Infiltration: Process of water entering the soil surface.

• Percolation: Movement of water through the soil profile.

• Water levels change continuously due to: drainage, evaporation, and plant uptake.

• Water moves through soil in liquid or vapor phases.

• Soil Properties Influencing Movement (Hydraulic Conductivity based):

  • Affected by: water content, pore size distribution, and continuity of pores.

  • Conductivity categorized by soil texture:

    • Poor (fine texture): < 10^{-7} ext{ m/sec}

    • Medium (aggregated): $10^{-7} ext{ to } 10^{-4} ext{ m/sec}$

    • High (coarse texture): > 10^{-4} ext{ m/sec}

Explain Darcy's law and the concept of hydraulic conductivity

• Darcy's Law: Describes water movement from high potential to low potential: $q = rac{Q}{A} = k \times rac{F<em>{2} - F</em>{1}}{l_{12}}$

  • Flux (q): Volume per unit time per unit area.

• Hydraulic Conductivity (k):

  • Represents the ease with which water moves through soil.

  • Affected by: water content, pore size distribution, and continuity of pores.

  • Categorized by soil texture:

    • Poor (fine texture): < 10^{-7} ext{ m/sec}

    • Medium (aggregated): $10^{-7} ext{ to } 10^{-4} ext{ m/sec}$

    • High (coarse texture): > 10^{-4} ext{ m/sec}

Calculate the direction of water flow in soil from soil water potentials.

• Water flows from areas of high potential to low potential (less negative).

• Water movement is primarily influenced by matric and gravitational potentials, determining the hydraulic potential.

• Types of Potential Energies:

  • Matric Potential (Φ m): Energy required to overcome absorption and capillarity forces in soil.

  • Osmotic Potential (Φ o): Energy needed to balance solute concentrations.

  • Gravitational Potential (Φ g): Energy stored due to water's elevation above a reference point.

  • Total Soil Water Potential (Φ T): $ext{Φ}<em>{T} = ext{Φ}</em>{m} + ext{Φ}<em>{o} + ext{Φ}</em>{g}$

  • Hydraulic Potential (Φ H): $ext{Φ}<em>{H} = ext{Φ}</em>{m} + ext{Φ}_{g}$

  • Water Potential (Φ w): $ext{Φ}<em>{w} = ext{Φ}</em>{o} + ext{Φ}_{g}$

Calculate soil water flow rate from Darcy's Law.

• Darcy's Law Formula: The flow rate (flux, $q$) can be calculated using: $q = rac{Q}{A} = k \times rac{F<em>{2} - F</em>{1}}{l_{12}}$

  • Where $Q$ is the volume of water, $A$ is the cross-sectional area, $k$ is the hydraulic conductivity, $(F<em>{2} - F</em>{1})$ is the potential difference, and $l_{12}$ is the distance over which the potential difference occurs.

  • Flux (q): Represents the volume of water passing through a unit area per unit time.

• Application: Calculations involve using given matric potential data to determine water flow rates between soil depths, such as between soil depths A and B.