Soil 9/24 Soil Water Characteristic Curve and Measurement Methods

Water Characteristic Curve: Concepts and Measurement

Understanding the Water Characteristic Curve

• Porosity and Density Relationship

  • The highest point on the water characteristic curve (at zero pressure) represents the saturated volumetric water content ($\theta_s$), which is equivalent to the porosity ($\Phi$) of the soil.

  • Particle density ($\rho_p$) is consistently approximately $2.65 ext{ g cm}^{-3}$.

  • From the porosity ($\Phi$), the bulk density ($\rho<em>b$) can be calculated using the relationship: $\frac{<br>ho</em>b}{<br>ho_p} = 1 - rac{ ext{Pore Volume}}{ ext{Total Volume}} = 1 - ext{Porosity}$.

  • Therefore, $\rho<em>b = (1 - heta</em>s) <br>ho_p$. This allows determination of bulk density solely from the saturation point on the curve.

• Water Evacuation with Pressure

  • As pressure increases, water is evacuated from larger pores first.

  • The air entry point is the pressure at which air first enters the largest continuous pores, replacing water.

  • With continued pressure application, pores progressively empty, filled by air.

  • The residual water point represents the minimum water content held in the soil even under very high pressures, typically in very small, disconnected pores.

• Key Water Content Points

  • Permanent Wilting Point (PWP): Represents the soil water content at which plants can no longer extract sufficient water and permanently wilt. This is a higher pressure (less water) point.

    • Specific values provided: $15 ext{ bar}$, or $100 ext{ cm}$, or $1,500 ext{ kPa}$.

  • Field Capacity (FC): Occurs at a lower pressure ($0.33 ext{ bar}$ or $33 ext{ kPa}$) and represents the amount of water remaining in the soil after gravitational drainage has ceased following saturation.

  • Available Water Holding Capacity (AWHC): The volume of water available for plant uptake, calculated as the difference in volumetric water content between Field Capacity and Permanent Wilting Point ($AWHC = heta<em>{FC} - heta</em>{PWP}$).

Hysteresis in Soil Water Characteristic

• Concept: The drying curve (starting wet and allowing soil to dry) differs from the wetting curve (allowing dry soil to get wet).

• Observation: The wetting curve typically lies below the drying curve on a water characteristic graph.

• Reason: This phenomenon, known as hysteresis, occurs due to factors like varying pore geometries (e.g., inkbottle pores) and entrapped air, meaning the water content at a given matric potential is different during wetting versus drying.

Sponge Analogy for Soil Water Dynamics

• Dry Sponge in Water: When a dry sponge is dipped in water, water rises due to capillarity. The height of water rise depends directly on the pore size (smaller pores allow higher rise). This mimics the initial wetting of a dry soil.

• Saturated Sponge: If the sponge is fully submerged, all pores fill with water, representing the saturation point ($\theta_s$) or porosity. Measuring its mass would allow calculation of its porosity, similar to saturated soil.

  • Note on Real-World Soils: Some soils (e.g., Vertisols) exhibit shrinking when dry and expanding when wet, changing their porosity and density.

• Draining Sponge (Gravimetric vs. Matric Potential)

  • When a saturated sponge is lifted out of the water, water drains from the largest pores first due to gravity.

  • Water continues to drain until the matric potential (negative pressure due to surface tension in smaller pores) balances the gravimetric potential (downward pull of gravity).

  • The remaining water is held by matric forces in smaller pores. This illustrates how soil retains water against gravity.

• Flipping the Sponge: Flipping the sponge demonstrates the influence of gravimetric potential on water distribution. Water drains more from the side that was previously at the bottom (higher gravimetric potential), showing that water content is not uniform throughout a soil column but varies with height, with lower points being wetter (requiring lower matric potential to hold water up).

• Pore Size Distribution (Sponge): The sponge shows a water characteristic curve; smaller pores at the top remain filled, while all pores at the bottom are filled, illustrating a pore size distribution within the material.

Causes of Dry Spots and Soil Management

• Dry Spots in Lawns: Can occur in well-irrigated lawns due to variations in the soil profile.

  • A common cause is an underlying sandy layer that is closer to the surface in certain areas.

  • Sandy layers have larger pores, leading to lower water holding capacity and faster drainage compared to finer-textured (e.g., loamy) layers.

  • This results in grass roots in sandy areas experiencing less available water.

• Soil Amendment: For sandy soils, adding organic matter is beneficial as it significantly increases the soil's water-holding capacity.

  • However, if organic matter becomes very dry, it can become hydrophobic, reducing its ability to absorb water.

Alternative Water Characteristic Curve Representation (Critique of Textbook Graph)

• Axis Orientation: Some textbook graphs present water potential on the x-axis and volumetric water content on the y-axis (inverted from the preferred representation).

• Organic Matter (Peat): Peat (organic material) exhibits a much higher water characteristic curve, indicating it holds significantly more water than clay or sand.

  • Peat also has a very narrow pore distribution range; most of its water drains within a small range of potential (e.g., by $100 ext{ kPa}$), which is conveniently close to field capacity.

• Inaccurate Sand Porosity Example

  • A depicted sand graph shows a saturated volumetric water content ($\theta_s$ or porosity $\Phi$) of $0.25$.

  • Using the formula $\rho<em>b = (1 - heta</em>s) <br>ho<em>p$: $\rho</em>b = (1 - 0.25) imes 2.65 ext{ g cm}^{-3} = 0.75 imes 2.65 ext{ g cm}^{-3} = 1.9875 ext{ g cm}^{-3}$.

  • A bulk density of $1.9875 ext{ g cm}^{-3}$ is considered very high for sand, suggesting an unrealistically low porosity for saturated sand. This highlights a potential inaccuracy in such graphs, as typically, sand would have higher porosity and lower bulk density.

  • Such high bulk densities are rare, potentially found in very dense, cemented layers like a fragipan.

• Impact of Compaction: Dash lines on the graph depict compacted soil compared to native (solid line) soil.

  • Reduced Porosity: Compaction generally leads to a slightly smaller saturated water content ($\theta_s$), indicating a loss of overall porosity.

  • Pore Size Distribution Shift: At higher pressures, the compacted soil curve crosses the native soil curve, meaning compacted soil has relatively more smaller pores and fewer beneficial macropores (which hold available water). This makes it harder for plants to extract water from compacted soil.

• Summary of Factors Affecting Water Holding Capacity

  • Sandy particles: Lead to lower porosity and lower overall plant available water holding capacity (PAW).

  • Aggregation: Improves macropore formation, increasing the availability of water.

  • Organic matter: Essential for sandy soils to increase water retention, as it has good water-holding properties.

Calculating Available Water Holding Capacity (Example)

• Example from a Hypothetical Graph (Clay Soil):

  • Assume a clay soil graph with a logarithmic kilopascal (kPa) scale.

  • Identify Field Capacity (FC) at $33 ext{ kPa}$. From the graph, its volumetric water content is $\theta_{FC} hickapprox 0.45$.

  • Identify Permanent Wilting Point (PWP) at $1,500 ext{ kPa}$. From the graph, its volumetric water content is $\theta_{PWP} hickapprox 0.36$.

  • Available Water Holding Capacity (AWHC) = $\theta<em>{FC} - heta</em>{PWP} = 0.45 - 0.36 = 0.09$. This represents the fraction of soil volume holding plant-available water.

Methods for Measuring Soil Moisture in the Field

1. Gravimetric Water Content

• Principle: Involves taking a moist soil sample, drying it in an oven, and calculating water content from the difference in mass.

• Application: Useful for any relevant water content range.

• Limitation: Inefficient for continuous, real-time monitoring of rapid changes in the field.

2. Neutron Scattering (Neutron Probe)

• Principle: A neutron probe emits high-energy (fast) gamma particles. These particles slow down significantly upon collision with hydrogen atoms (primarily in water molecules).

• Measurement: A detector measures the ratio of fast to slow neutrons returning to the probe. This ratio is inversely proportional to the soil's water content.

• Field Use: A tube is inserted into the soil (e.g., $1 ext{ m}$ depth), and the probe is lowered to different depths to measure moisture profiles rapidly (e.g., $5-10$ minutes).

• Calibration: Requires calibration based on soil texture, organic matter, salinity, and other parameters.

• Limitations: It's a radioactive source, requiring specialized training and adhering to regulations, even with very low radioactivity. It's a manual method and cannot be left for continuous, autonomous measurement at a single point.

3. Electric Conductivity-Based Methods (TDR/FDR)

• Principle: Water, especially with dissolved ions (salts), conducts electricity. Measuring the soil's electrical conductivity (EC) can be related to its moisture content.

  • Time Domain Reflectometry (TDR) and Frequency Domain Reflectometry (FDR) are common types.

• TDR/FDR Mechanism: An electrical pulse is sent through stainless steel rods embedded in the soil. The way the pulse travels and is reflected back (differences in signal travel time or frequency response) provides information about the soil's dielectric constant, which is strongly influenced by water content.

• Influence Factors: Soil texture, mineral composition, and particularly salinity (dissolved salts) can affect EC measurements and thus require careful calibration or compensation.

• Advantages: Relatively small, arguably cheap, can be permanently installed (stick in the soil) and connected to data loggers for continuous, automated measurements.

• Limitations: More limited range of effectiveness; requires some water to measure accurately.

4. Tensiometers

• Principle: A tensiometer is essentially a vacuum gauge connected to the soil through a porous ceramic or silicate tip that is entirely saturated with water.

• Measurement: As soil water potential changes, water moves in or out of the porous tip, creating a vacuum or pressure that is directly measured by the gauge. This directly measures the matric potential of the soil water.

• Pore Size Importance: The pores in the ceramic tip must be sufficiently small to maintain saturation and prevent air entry, even under relatively high tensions (negative pressures). This ensures it can measure within the desired range, e.g., typically up to $0.8$ to $1 ext{ bar}$.

• Limitations:

  • Not usable once the soil becomes too dry (i.e., beyond its air entry point), as air can enter the tip, breaking the water column.

  • Requires the porous tip to remain saturated.

• Modern Versions: Newer, smaller versions can be digitalized for continuous monitoring.

5. Pressure Plate/Membrane Apparatus

• Purpose: A lab method used to generate the entire water characteristic curve.

• Mechanism: A moist soil sample (in a cylinder or ring) is placed on a porous plate (or membrane) inside a sealed chamber. Pressure is then applied to the chamber.

• Measurement: At different applied pressures, the water that drains from the soil sample through the porous plate is collected and measured. The remaining water content in the soil is then determined.

• Porous Plate Requirements: The pores of the plate must be smaller than the desired air entry point for the pressure range being measured (e.g., a pressure plate for lower pressures, a pressure membrane for very high pressures up to $50 ext{ bar}$ or more).

6. Tension Table

• Purpose: A lab method used for measuring water characteristic curves at very low pressure ranges.

• Mechanism: A moist soil sample is placed on a porous table connected to a water reservoir. The height of the water reservoir can be adjusted, which in turn changes the hydrostatic (volumetric) potential applied to the soil.

• Measurement: Water drains from the soil until equilibrium is reached at a given height (potential). The water removed is then measured, providing the matric potential at that water content.

• Limitations: The range of measurable pressure is limited by the practical height difference of the water reservoir (e.g., cannot exceed the height of the room, typically only for very low negative pressures/tensions).

Importance of the Water Characteristic Curve for Field Management

• The water characteristic curve is a fundamental tool for field management and studying soil properties.

• Instead of constantly measuring matric potential, one can:

  1. Determine the water characteristic curve for the specific soil in the lab.

  2. Use relatively inexpensive, continuously monitoring probes (e.g., TDR/FDR) to measure volumetric water content ($\theta_v$) in the field.

  3. Refer to the established water characteristic curve to estimate the soil water potential from the measured $\theta_v$.

• Plant Relevance: The soil water potential, rather than just the total water content, is the most critical factor for plant water uptake. Plants can struggle to extract water if the potential is too low, even if a significant amount of water is still present (e.g., in clay soils).