4.2 Infiltration processes and models

Infiltration is a crucial process in the water cycle, determining how much rainfall enters the soil. This topic covers the stages of infiltration, from initial abstraction to steady-state, and explores various models used to predict infiltration rates.

Understanding infiltration is key to managing water resources and predicting runoff. We'll examine popular models like Green-Ampt and Philip's equation, comparing their strengths and limitations in different soil conditions and rainfall scenarios.

Infiltration Process Stages

Stages of infiltration process

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• Initial abstraction
  • Occurs before ponding begins
  • Includes interception by vegetation (grass, leaves), surface depression storage (puddles), and evaporation
  • Reduces the amount of water available for infiltration
• Ponding
  • Begins when the rainfall intensity exceeds the infiltration capacity of the soil
  • Water accumulates on the soil surface forming ponds or puddles
  • Marks the start of the actual infiltration process into the soil
• Steady-state infiltration
  • Occurs when the infiltration rate reaches a constant value over time
  • Depends on soil properties (texture, structure) and initial moisture content
  • Governed by the soil's saturated hydraulic conductivity (permeability)
  • Reached when the soil becomes fully saturated and cannot absorb more water

Infiltration Models

Application of infiltration models

• Green-Ampt model
  • Assumes a sharp wetting front and a constant soil moisture content behind the front
  • Infiltration rate equation: $f(t) = K_s \left(1 + \frac{\psi \Delta \theta}{F(t)}\right)$
    • $K_s$: saturated hydraulic conductivity (mm/hr)
    • $\psi$: wetting front suction head (mm)
    • $\Delta \theta$: change in moisture content (dimensionless)
    • $F(t)$: cumulative infiltration (mm)
  • Useful for estimating infiltration in homogeneous soils (sand, loam)
• Philip's equation
  • Based on the solution of the Richards equation for vertical infiltration
  • Infiltration rate equation: $f(t) = \frac{1}{2} S t^{-1/2} + A$
    • $S$: sorptivity (mm/hr^1/2)
    • $A$: steady-state infiltration rate (mm/hr)
    • $t$: time since the start of infiltration (hr)
  • Accounts for the decrease in infiltration rate over time
  • Applicable to a wider range of soil conditions compared to Green-Ampt

Empirical vs physically-based models

• Empirical models
  • Based on observed data and statistical relationships between infiltration and time
  • Examples: Kostiakov, Horton, and Holtan equations
  • Advantages: simple to use, require fewer input parameters (soil type, initial moisture)
  • Disadvantages: limited transferability to different soil types and conditions, do not consider physical processes
• Physically-based models
  • Derived from fundamental physical principles (Darcy's law, conservation of mass) and soil properties
  • Examples: Green-Ampt, Philip's equation, and Richards equation
  • Advantages: more accurate, applicable to a wider range of soil conditions, consider physical processes (capillarity, gravity)
  • Disadvantages: require more detailed soil data (hydraulic conductivity, porosity), computationally intensive

Limitations of infiltration models

• Green-Ampt model
  • Assumes a homogeneous soil profile and a sharp wetting front, which may not be realistic in many cases
  • Does not account for soil layering (clay over sand) or preferential flow paths (macropores, cracks)
  • May overestimate infiltration rates in heterogeneous soils with variable properties
• Philip's equation
  • Assumes a semi-infinite, homogeneous soil profile, which is rarely found in nature
  • Does not consider the effect of soil layering or initial moisture content on infiltration
  • May not accurately represent infiltration under ponded conditions or high rainfall intensities
• Empirical models (Kostiakov, Horton, Holtan)
  • Based on limited data sets and may not be applicable to all soil types and conditions
  • Do not explicitly account for soil physical properties (texture, structure) or initial moisture content
  • May not accurately predict infiltration rates for long durations (days) or extreme events (floods, droughts)