The vadose zone, spanning from the land surface to the water table, plays a crucial role in the hydrologic cycle. It regulates water infiltration, stores moisture for plants, and facilitates groundwater recharge. Understanding this zone is key to managing water resources effectively.

Unsaturated flow in the vadose zone is driven by capillary forces and gravity. The Richards equation models this complex process, combining Darcy's law with mass conservation. This knowledge helps predict water movement and contaminant transport, impacting groundwater management and environmental protection.

The Vadose Zone and Unsaturated Flow

Vadose zone in hydrologic cycle

• Vadose zone (unsaturated zone) extends from land surface to water table
  • Contains both air and water in pore spaces (soil, rock)
• Regulates infiltration of water from land surface into subsurface
  • Controls amount and timing of water entering soil
• Stores water in soil matrix
  • Available for plant uptake (transpiration) or evaporation back to atmosphere
• Allows water to percolate downward and recharge groundwater aquifers
  • Replenishes water in saturated zone below water table

Processes of unsaturated flow

• Unsaturated flow driven by capillary forces and gravity
  • Capillary forces attract water molecules to soil particles causing upward movement against gravity (capillary rise)
  • Gravity pulls water downward through soil profile (percolation)
• Capillary rise moves water upward from water table into unsaturated zone
  • Height of rise depends on soil pore size distribution
    • Finer-grained soils (clay) have smaller pores and higher capillary rise
    • Coarser-grained soils (sand) have larger pores and lower capillary rise
• Preferential flow paths are channels or macropores allowing rapid water movement through vadose zone
  • Caused by root channels, animal burrows, or soil cracks
  • Bypass soil matrix leading to fast transport of water and contaminants to water table

Modeling Unsaturated Flow and Its Implications

Richards equation for soil water

• Richards equation describes water movement in unsaturated soils
  • Combines Darcy's law for fluid flow with conservation of mass
• Equation: $\frac{\partial \theta}{\partial t} = \frac{\partial}{\partial z} \left[K(\theta) \left(\frac{\partial \psi}{\partial z} + 1\right)\right]$
  • $\theta$ = volumetric water content
  • $t$ = time
  • $z$ = vertical coordinate (positive upward)
  • $K(\theta)$ = unsaturated hydraulic conductivity (depends on water content)
  • $\psi$ = matric potential (represents capillary forces)
• Solving requires soil hydraulic properties
  • Water retention curve and unsaturated hydraulic conductivity function
  • Measured in lab or estimated from soil texture and structure

Vadose zone's environmental impact

• Controls rate and timing of groundwater recharge
  • Recharge occurs when water percolates through vadose zone to water table
  • Amount and timing depend on precipitation, evapotranspiration, soil properties
• Acts as buffer for contaminants slowing transport to water table
  • Contaminants can be adsorbed to soil particles, degraded by microbes, or diluted by dispersion
  • But preferential flow paths can bypass soil matrix allowing faster contaminant transport
• Understanding vadose zone crucial for:
  1. Sustainably managing groundwater resources
  2. Assessing aquifer vulnerability to contamination
  3. Designing remediation strategies for contaminated sites