10.1 Soil water retention and hydraulic conductivity

Soil water retention and hydraulic conductivity are crucial concepts in unsaturated zone flow. They determine how water moves and is stored in soil, affecting plant growth and groundwater recharge.

Understanding these properties helps predict water availability, drainage, and contaminant transport. Soil texture, structure, and organic matter content influence water retention and conductivity, with clay soils holding more water but having lower conductivity than sandy soils.

Soil Water Content vs Potential

Relationship between Soil Water Content and Potential

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• Soil water content refers to the amount of water stored in the soil pores, while soil water potential represents the energy state of water in the soil
• Soil water potential decreases as soil water content decreases indicating water is held more tightly by the soil matrix at lower water contents (e.g., at wilting point)
• The relationship between soil water content and soil water potential is non-linear and is influenced by soil texture, structure, and organic matter content
  • Coarse-textured soils (sandy soils) have lower water retention compared to fine-textured soils (clay soils) at the same matric potential
  • Well-structured soils with stable aggregates tend to have higher water retention than poorly structured soils
  • Soils with higher organic matter content generally have higher water retention due to increased porosity and surface area

Components of Soil Water Potential

• Soil water potential components include matric potential, osmotic potential, and gravitational potential, with matric potential being the most important in unsaturated soils
  • Matric potential arises from the capillary and adsorptive forces that attract and bind water to the soil matrix
  • Osmotic potential is due to the presence of dissolved solutes in the soil water, which reduce the water potential
  • Gravitational potential is the potential energy of water due to its elevation relative to a reference level
• The relationship between soil water content and soil water potential is crucial for understanding water movement, storage, and availability to plants in unsaturated soils
  • Water moves from regions of high water potential to regions of low water potential
  • Plant roots can extract water from the soil until the soil water potential becomes too low (around -1500 kPa, permanent wilting point)

Soil Water Retention Curve

Characteristics of the Soil Water Retention Curve

• The soil water retention curve, also known as the soil moisture characteristic curve, describes the relationship between soil water content and soil water potential (matric potential) for a specific soil
• The soil water retention curve is typically plotted with soil water content on the x-axis and soil water potential (matric potential) on the y-axis, with the potential expressed as the logarithm of the negative pressure head (pF)
• The shape of the soil water retention curve is influenced by soil texture, structure, and organic matter content, with coarser soils having lower water retention compared to finer soils at the same matric potential
  • Sandy soils have a steeper soil water retention curve, indicating a rapid decrease in water content with decreasing matric potential
  • Clay soils have a more gradual slope in the soil water retention curve, indicating a slower decrease in water content with decreasing matric potential

Key Points and Applications of the Soil Water Retention Curve

• Key points on the soil water retention curve include the saturation point (zero matric potential), field capacity (matric potential around -33 kPa), and permanent wilting point (matric potential around -1500 kPa)
  • Saturation point represents the maximum water content when all soil pores are filled with water
  • Field capacity is the water content held by the soil after excess water has drained due to gravity
  • Permanent wilting point is the water content at which plants can no longer extract water from the soil
• The soil water retention curve is essential for estimating water storage, water availability to plants, and water flow in unsaturated soils, as well as for modeling soil water dynamics and solute transport
• Hysteresis can occur in soil water retention curves, meaning that the relationship between soil water content and matric potential differs depending on whether the soil is wetting or drying
  • Wetting curves generally have higher water content at a given matric potential compared to drying curves
  • Hysteresis is caused by factors such as pore size distribution, entrapped air, and contact angle effects

Factors Influencing Hydraulic Conductivity

Soil Properties Affecting Hydraulic Conductivity

• Soil hydraulic conductivity is a measure of the soil's ability to transmit water and is a function of soil water content (or matric potential) in unsaturated soils
• Soil texture significantly influences hydraulic conductivity, with coarser soils generally having higher saturated and unsaturated hydraulic conductivities compared to finer soils
  • Sandy soils have larger pores and less resistance to water flow, resulting in higher hydraulic conductivity
  • Clay soils have smaller pores and more resistance to water flow, resulting in lower hydraulic conductivity
• Soil structure, particularly the presence of macropores and preferential flow paths, can greatly enhance soil hydraulic conductivity, especially near saturation
  • Macropores are large pores (> 75 μm) that allow rapid water flow and bypass of the soil matrix
  • Preferential flow paths, such as root channels or cracks, can create high-conductivity zones in the soil
• Soil organic matter content can affect soil hydraulic conductivity by influencing soil structure, porosity, and water retention properties
  • Organic matter can improve soil aggregation and create more stable pores, enhancing hydraulic conductivity
  • Organic matter can also increase soil water retention, which can affect unsaturated hydraulic conductivity

Other Factors Influencing Hydraulic Conductivity

• Soil hydraulic conductivity decreases as soil water content decreases (or matric potential becomes more negative) due to the reduction in water-filled pore space and increased tortuosity of the flow path
  • As the soil dries, larger pores drain first, and water flow is restricted to smaller pores with more tortuous pathways
  • The relationship between hydraulic conductivity and water content (or matric potential) is highly non-linear, with hydraulic conductivity decreasing by several orders of magnitude from saturation to dry conditions
• The presence of layering, compaction, or other heterogeneities in soil can result in anisotropic hydraulic conductivity, meaning that the conductivity varies with direction
  • Layered soils may have higher horizontal hydraulic conductivity compared to vertical conductivity due to the presence of low-permeability layers
  • Compacted soils may have reduced hydraulic conductivity, especially in the vertical direction, due to the destruction of macropores and increased bulk density
• Temperature affects soil hydraulic conductivity by influencing water viscosity and density, with higher temperatures generally increasing hydraulic conductivity
  • Water viscosity decreases with increasing temperature, allowing for easier flow through soil pores
  • Temperature effects on hydraulic conductivity are more pronounced in fine-textured soils and at lower water contents

Measuring Soil Water Properties

Methods for Measuring Soil Water Retention

• Direct methods for measuring soil water retention include the hanging water column method, pressure plate apparatus, and dew point potentiometer, which apply different matric potentials to soil samples and measure the corresponding water content
  • Hanging water column method applies matric potentials up to -10 kPa by suspending a soil sample on a porous plate above a water reservoir
  • Pressure plate apparatus applies matric potentials up to -1500 kPa by placing soil samples on a porous ceramic plate in a pressurized chamber
  • Dew point potentiometer measures the relative humidity of the air in equilibrium with a soil sample, which is related to the soil water potential
• Indirect methods for estimating soil water retention include pedotransfer functions, which use easily measurable soil properties (e.g., texture, bulk density, organic matter content) to predict water retention curves
  • Pedotransfer functions are developed using regression analysis or machine learning techniques on large datasets of soil properties and water retention data
  • Examples of pedotransfer functions include the Saxton-Rawls model and the Rosetta model

Methods for Measuring Soil Hydraulic Conductivity

• In-situ methods for measuring soil hydraulic conductivity include the instantaneous profile method, which involves measuring soil water content and matric potential at multiple depths over time, and the tension infiltrometer method, which measures water infiltration rates at different applied tensions
  • Instantaneous profile method allows for the estimation of unsaturated hydraulic conductivity by solving the Richards equation using measured soil water content and matric potential data
  • Tension infiltrometer method measures the steady-state infiltration rate at different applied tensions, which can be used to estimate unsaturated hydraulic conductivity using the Wooding equation
• Laboratory methods for measuring saturated hydraulic conductivity include the constant head and falling head permeameter methods, which involve measuring water flow through a soil sample under a known hydraulic gradient
  • Constant head permeameter method maintains a constant water level above the soil sample and measures the steady-state flow rate
  • Falling head permeameter method measures the time taken for the water level to fall a certain distance through the soil sample
• Unsaturated hydraulic conductivity can be estimated from saturated hydraulic conductivity using mathematical models, such as the van Genuchten-Mualem model or the Brooks-Corey model, which relate hydraulic conductivity to soil water retention parameters
  • These models use fitted parameters from the soil water retention curve to predict the unsaturated hydraulic conductivity function
  • The van Genuchten-Mualem model is widely used and relates hydraulic conductivity to the effective saturation using the following equation: $K(S_e) = K_s S_e^{1/2} [1 - (1 - S_e^{1/m})^m]^2$ where $K(S_e)$ is the unsaturated hydraulic conductivity, $K_s$ is the saturated hydraulic conductivity, $S_e$ is the effective saturation, and $m$ is a fitting parameter related to the pore size distribution
• Inverse modeling techniques can be used to estimate soil hydraulic properties by fitting simulated data to observed data of soil water content, matric potential, or water fluxes
  • Inverse modeling involves adjusting soil hydraulic parameters in a numerical model until the simulated data closely matches the observed data
  • Commonly used inverse modeling tools include HYDRUS, RETC, and PEST