Scaling Infiltration & Soil Moisture Concepts

• Infiltration is the movement of water from the soil surface into the soil.
• Percolation is the downward flow of water through the unsaturated zone.
• Infiltration and percolation affects water availability, runoff, groundwater recharge, and flooding.

What Happens After Water Infiltrates?

• After water infiltrates, there are three main pathways: subsurface flow, soil storage & evapotranspiration, and groundwater recharge.
• Subsurface flow moves underground toward rivers and lakes.
• Soil storage & evapotranspiration involves water being retained in soil and released back into the atmosphere.
• Groundwater recharge contributes to aquifers, supplying wells and springs.

Introduction to Field Capacity

• When soil is fully saturated, with all pores filled with water, water begins to drain because of gravity.
• After water is fully saturated drainage slows down and eventually becomes negligible or close to zero.
• Field Capacity (θfc) is the point at which gravity drainage becomes insignificant, meaning that it is negligible.

Why Defining Field Capacity is Challenging?

• In real soils, gravity drainage continues indefinitely, field capacity is difficult to define when water flow ceases entirely.
• Defining field capacity at qz' = 0 is impractical. Instead, operational definitions are used in practice.

Operational Definitions of Field Capacity

• Time-Based Definition measures water content after a specific drainage period, sandy soils is 3 days and finer-grained soils is 6+ days.
• Tension-Based Definition measures water content at a specific tension where ψ = −33 kPa (or -340 cm).
• Drainage Rate Definition measures water content at a specific small drainage rate.

Field Capacity Across Different Soil Types

• Sandy soil has large pores, drains fast, and has a lower field capacity.
• Clayey soil has small pores, holds more water, and has a higher field capacity.
• Silty Soil has intermediate behavior.

Field Capacity & Water Resource Management

• Field capacity helps determine irrigation schedules for crops.
• Field capacity is important for flood control because soils at field capacity can't absorb more water.
• Used to model groundwater recharge and soil water storage.

Permanent Wilting Point (PWP)

• The Permanent Wilting Point (θpwp) is the water content at which plants can no longer extract water from the soil, leading to irreversible wilting.
• Transpiration ceases, and plants wilt permanently.
• Occurs when soil moisture tension reaches -15 bar (-1,470 kPa).

Estimating Permanent Wilting Point (θpwp)

• Mathematical Definition: θpwp = 0(−15 bar)
• Van Genuchten Equations can estimate θpwp for different soils.
• Different soils have different θpwp.
• Sandy soil dries quickly and has a lower PWP of ≈ 0.06.
• Clayey soil holds more water, but not always available to plants and has a higher PWP of ≈ 0.27.

Real-World Example – Drought & Wilting in Management

• Drought conditions cause soil moisture to drop below PWP, preventing plants from extracting water.
• Sandy soils dry out faster than clayey soils, making crops in sandy areas more vulnerable.
• Knowing Ofc, Opwp, and θa is important in irrigation strategies to ensure enough water is available.

Soil-Water Status Terms

• Field Capacity (0fc) is the maximum water held after drainage slows (~−33 kPa).
• Permanent Wilting Point (0pwp) is the moisture level where plants wilt permanently (~−1,470 kPa).
• Available Water Content (θa) is the difference between Ofc and Opwp, where: θα = 0 fc - θρωρ
• The difference between the field capacity and permanent wilting point is considered to be the available water content.

How to Determine Soil-Water Status?

• Sources of Soil-Water Information:
• Moisture Characteristic Curve (Ofc, θρwρ, θα from lab data).
• Soil-Hydraulic Class (Using standard soil texture data).
• Pedotransfer Functions (PTFs) (Empirical models from soil properties).

Soil-Water Status – How Much Water is Available to Plants?

• Field Capacity (0fc) is the maximum water soil can hold after drainage stops.
• Permanent Wilting Point (0pwp): Water content at which plants wilt permanently.
• Available Water Content (0a) = 0fc – өрωρ, this is the water actually usable by plants.
• Extreme Dryness (Hygroscopic Water): Water so tightly bound to soil that only air can remove it

Hydrologic Soil Horizons

• Defined based on water content and soil-water pressures.
• Thickness and depth vary in time and space.
• Some horizons may be absent depending on the environment.

Ground-Water Zone (Phreatic Zone)

• Fully saturated zone with positive pressure.
• Pressure increases with depth.
• Water table boundary is at atmospheric pressure.
• Flows are based on recharge and discharge zones.

Tension-Saturated Zone (Capillary Fringe)

• Tension-Saturated Zone occurs directly above the water table.
• It is almost saturated due to capillary forces.
• Water is under negative pressure (tension).
• Pressure distribution remains hydrostatic, even though pressure is negative.
• Soil type affects thickness: gravel has a very thin fringe (~10 mm), while clay can have several meters of capillary rise.

Why Does Capillary Action Occur?

• Capillary action happens because of two main forces: Adhesion and Cohesion.
• Adhesion (Water-Sticking-to-Surface Force): Water molecules are attracted to solid surfaces, and this pulls water upward.
• Cohesion (Water-Sticking-to-Water Force): Water molecules are attracted to each other (due to hydrogen bonding), and this drag pulls water molecules along.
• Surface Tension: The thin film of water at the interface between air and liquid creates tension, helping water resist gravity and move upwards in tiny spaces.

Intermediate Zone

• Located between the capillary fringe and root zone.
• Water drains downward via gravity.
• Water content fluctuates due to precipitation.
• The Intermediate Zone does not go below field capacity.

Root Zone

• Layer where plant roots extract water.
• Water enters via infiltration and capillary rise.
• Water exits via evapotranspiration and drainage.
• Water content fluctuates between field capacity and wilting point.

Hydrologic Horizons and Water Movement

• Water enters and moves through zones in different ways:
• Infiltration → Root Zone → Intermediate Zone
• Percolation → Tension-Saturated Zone → Ground-Water Zone
• Capillary Rise → Moves from the Ground-Water Zone upward
• Evapotranspiration → Removes water from the Root Zone

Introduction to Infiltration

• Infiltration is the process by which water from rainfall or snowmelt enters the soil.
• Infiltration replenishes groundwater, reduces surface runoff and erosion, and supports plant growth.

Key Terms & Concepts

• Infiltration rate is the speed at which water enters the soil (mm/hr or cm/hr).
• Infiltration capacity is the maximum rate at which soil can absorb water.
• Percolation is the downward movement of water within the soil profile.

The Infiltration Process at a Point

• At a single location (few m²), infiltration involves water arriving at the surface, the initial rapid absorption, and decreasing rate over time.
• Infiltration process is controlled by soil properties, land cover, rainfall characteristics, and surface conditions.

Darcy's Law and Infiltration

• Darcy's Law describes the flow of water through porous media: q = -K (dh/dz).
• In uniform soil matrix there are no abrupt texture changes and no macropores (cracks, root channels, or burrows).
• Macropore Flow is a rapid water flow that bypasses smaller pores
• Matrix Flow is the slow and uniform movement of water flow

Field Measurement of Infiltration

• Common methods for measuring infiltration include double-ring infiltrometer, tension infiltrometer, and rainfall simulation.
• Measuring infiltration helps to assess soil health, model water movement, and improve irrigation efficiency.

What is a Ring Infiltrometer?

• A ring infiltrometer is a device for measuring infiltration in a small area (0.02–1 m²).
• Consists of a cylindrical ring that extends above and into the soil.
• Creates a ponding condition to simulate natural infiltration. Water is added to the ring to create ponding and the infiltration rate is measured using change of water level, amount of water added, or using a water balance equation.

The Issue of Lateral Water Movement

• Lateral water movement is due to capillary forces.
• This can lead to an overestimation of infiltration rates.
• Solution: Use a double-ring infiltrometer or apply a correction factor.
• Double-Ring Infiltrometer uses two concentric rings filled with water. The outer ring acts as a buffer to reduce lateral flow from the inner ring, ultimately providing more accurate infiltration rate measurements.

Single-Ring Correction Method

• A correction factor can adjust for lateral flow effects.
• Tricker (1978) proposed correction factors based on measurement duration and infiltration rate.
• A simpler alternative to a double-ring infiltrometer.
• Infiltration rates vary across locations due to soil heterogeneity, and multiple measurements are required to get an accurate average
• Studies suggest at least six measurements for reliable results.

Tension (Disc) Infiltrometers

• A device that allows water to infiltrate through a porous disk under controlled pressure.
• Measures infiltration without creating ponding
• It is portable and easy to install.
• A tension (Disc) Infiltrometer is supplied through porous disk at controlled measure. and the time-domain reflectometry and constant-flow reservoirs can improve accuracy.

Advantages and Limitations of Tension Infiltrometers

Advantages:

• It is Portable and easy to set up
• Measures unsaturated infiltration rates accurately
• Suitable for field studies Limitations
• Sensitive to soil surface conditions and Requires careful setup for accurate results.

Sprinkler-Plot Studies

• Uses artificial rainfall to measure infiltration and runoff.
• Controlled rate is applied until ponding occurs.
• Infiltration is calculated as the difference between applied rainfall and runoff.
• Constant rainfall rate is applied using a simulator, runoff is collected and measured, and then the infiltration rate is calculated.

Advantages and Limitations of Sprinkler-Plot Studies

Advantages:

• Simulates real rainfall conditions, measures infiltration over a larger area, and is useful for erosion studies. Limitations:
• Equipment can be complex and it is difficult to maintain uniform rainfall distribution.

Modeling Infiltration in Hydrology

• Quantitative Models: Green-Ampt Model, Horton's Equation, and Philip's Equation.
• Models matter because they predict runoff and groundwater recharge and aid in planning for flood and irrigation.

Tension Infiltrometer vs. Sprinkler-Plot Studies

• Tension Infiltrometer has a simple setup, portable, small Area, does not simulate Natural Rainfall?, and suitable for Dry & Unsaturated Soils.
• Sprinkler-Plot Study has a more complex setup, large area, simulates natural rainfall, and suitable for surface Runoff Studies.

Philip Equation

• The Philip equation is a widely used infiltration model that describes how water infiltrates into soil over time where:
• f(t) = (Sp/√t) + Kp
• f(t) is Infiltration rate at time t
• Sp is Sorptivity (cm/hr1/2) – represents the capillary forces that control early-stage infiltration.
• Kp is Hydraulic conductivity (steady infiltration rate) (cm/hr) – represents the long-term infiltration controlled by gravity.
• t is Time (hours)

Introduction to Infiltration Variability

• Infiltration rates are highly variable across different areas.
• Saturated hydraulic conductivity significantly affects infiltration.
• Saturated hydraulic conductivity (Ks) controls how fast water infiltrates into gravity and capillary forces. Rapid infiltration is a result of high Ks while low KsK causes ponding, and erosion.
• Influencing factors include soil type, land use, vegetation, and climate.

Case Study - Burgy and Luthin (1956)

• Study measured infiltration in a 335-m² basin.
• Used a basin flooding method and ring infiltrometers.
• Infiltrometer values ranged from near zero to 110 cm/hr, the average values of different methods were similar.

Case Study - Sharma et al. (1980)

• Conducted in Oklahoma over a 0.096-km² watershed.
• Displayed an 11-fold variation in Sp and 29-fold variation in Kp (Philip equation parameters).
• Demonstrated no consistent correlation between infiltration and soil type.

Case Study - Loague and Gander (1990)

• Watershed in Sharma et al study was used to conduct this study.
• Found spatial correlation of infiltration rates to be less than 20m.
• Vegetation and animal activity affected infiltration more than soil texture.

Case Study - Tricker (1981) and Wilcock & Essery (1984)

• Tricker found nearly zero infiltration to 256 cm/hr in a 3.6-km² watershed in England.
• Results showed infiltration correlated with organic matter.
• Wilcock & Essery found seasonal variability in Northern Ireland, and that higher summer infiltration was up to (0.9 cm/hr) than in winter (0.06 cm/hr)
• High organic matter meant a correlation with winter.

Scaling point infiltration models to larger areas presents challenges. Geometric & dynamic scaling aid in generalizing infiltration across soils. The Permanent Wilting Point (θpwp) is critical for plant-available water.