Soil Water Movement: Infiltration, Percolation, and Management
Chapter 1: Introduction to Infiltration Measurement
Infiltration Measurement (Double Ring Infiltrometer):
Method: Large, concentric cylinders are used. An inner cylinder is filled with water, and the rate at which water drains from it is measured. An outer cylinder, also filled with water, helps create a more homogeneous (even) measurement within the inner cylinder.
Procedure: When adding water, a cover must be placed over the soil. This cover allows water to move freely but prevents disturbance to the soil surface upon water addition. This is a simple, easy field method for measuring infiltration.
Infiltration Rate Dynamics:
Initial High Rate: When dry soil is first exposed to water, the infiltration rate is initially very high (observed in gardens, for example) because large water pores fill rapidly.
Decrease and Plateau: As the soil wets, the infiltration rate decreases.
Sandy Soil: Shows less initial decrease and reaches a plateau relatively quickly since it has mostly large pores.
Silt Loam and Clay: Exhibit a more significant decrease in infiltration rate.
Plateau (Saturated Hydraulic Conductivity): The plateau infiltration rate represents the a saturated hydraulic conductivity () under saturated conditions. This is because water is now moving through the soil when it's fully saturated.
Soil Texture and Infiltration:
Finer Textured Soils (e.g., Silt Loam, Clay): The infiltration rate decreases more rapidly.
Expansive Clays (Vertisols): Show an initially very high infiltration rate that sharply decreases after about an hour or less. This is due to:
Initial Dry State: Vertisols shrink when dry, forming large cracks that allow rapid water entry initially (high infiltration).
Wetting and Expansion: Once wet, they expand, eliminating large pores and leaving only finer pores that do not permit high water flow. The saturated hydraulic conductivity of clay is significantly smaller than that of sand or other coarser textures.
Infiltration vs. Percolation:
Infiltration: The process of water entering the soil surface from the atmosphere or irrigation.
Percolation: The downward movement of water through the soil profile below the surface after infiltration has occurred.
Wetting Front Propagation:
Definition: As water infiltrates and percolates, a 'wetting front' advances into the dry soil.
Characteristics: The top surface is typically wet (close to saturation). The wetting front often shows a sharp transition between the wetted zone and the drier, unwetted soil below. An example from Alabama shows moist subsoil, a dry middle area, and a topsoil where the wetting front is propagating.
Real-world Example: After heavy rain, water infiltrates and percolates deeper, but a drier subsurface may still exist compared to deeper, moist subsoil, especially where roots don't take up as much water.
Zones: Typically consists of a completely saturated area at the top, followed by a wetting zone (close to saturation but not fully), and then the sharp wetting front boundary leading to moist and finally dry soil.
Importance of Infiltration Rate (Infiltrability):
Knowing the infiltration rate is crucial because if precipitation or irrigation rates are higher than the soil's infiltration capacity, optimal water penetration will not occur.
Instead, water will accumulate on the surface.
Chapter 2: Surface of Soil and Water Movement
Runoff and Erosion:
When precipitation or irrigation rates exceed infiltrability, water that cannot penetrate the soil surface flows as 'runoff'.
Runoff is often associated with soil erosion, a process where initial runoff creates erosion, which in turn can stimulate more runoff (a positive feedback loop).
Crusting/Hydrophobic Crusts: If a soil crust forms (e.g., in deserts during flash floods) or if the surface is hydrophobic, water cannot penetrate effectively. Even if the surface appears wet, the soil a few centimeters below can be quite dry, leading to significant runoff.
Water Percolation Through Soil Layers (Discontinuities):
Counterintuitive Movement: When water percolates and encounters a sharp boundary between two soil textures, for example, a finer textured silt loam overlying a coarser sandy layer, water does not immediately move into the sand.
Water Potential Principle: Water moves from higher water potential to lower water potential. At the boundary, the dry silt loam (finer pores) will have a lower (more negative) matrix potential due to smaller pore sizes and higher adhesion forces. Therefore, water will first saturate the silt loam layer above the sand before percolating into the coarser, sandy layer.
Significance: This phenomenon is critical for understanding natural soil processes (e.g., pathogenic conditions) and in designing irrigation systems or landscape features.
Endogenic Examples (Perched Water Table):
A 'perched water table' can develop when water percolating through a finer-textured layer (e.g., silt loam) encounters a sharp boundary with a coarser, more porous underlying material (e.g., sandy loam).
The delay in water movement across the boundary due to water potential differences causes the finer layer to become saturated temporarily, creating a perched water table.
Gleyed Layers: This saturation leads to 'gleyed' layers (grey color) and sometimes 'orange' mottles, indicating redox conditions where iron is reduced due to waterlogging.
Sand Layers for Compaction Prevention:
A sand layer in compacted areas (e.g., high-traffic bays, golf greens) can prevent further compaction from traffic.
Sand, being already a compact material with predominantly large grains and fewer small pores, is less susceptible to significant compaction.
Water Movement in Sand: While it prevents compaction, water movement (wetting front propagation) is much faster and narrower in sand, leading to rapid drainage. This can be undesirable if moisture retention is needed for plants.
Chapter 3: Layered Soils and Water Flow Types
Gravel Layer for Water Retention in Sand:
To overcome rapid drainage in sand, a gravel layer can be placed below the sand.
This seemingly counterintuitive approach doesn't increase drainage but creates a temporary 'perched water table' effect, holding water in the root zone (above the gravel) until the water potential above and below the boundary equilibrates.
This provides enough time for grassroots to take up water, not acting entirely like a heavy soil but offering sufficient retention.
Nuclear Waste Storage Example:
A similar concept was proposed for nuclear waste protection in the Nevada desert.
Weather-resistant containers holding nuclear waste were to be covered with a layer of gravel, then desert soil. The gravel layer acts as a barrier, preventing moisture from reaching the waste barrels by delaying water movement due to water potential differences, similar to the plant root zone example.
Vapor Flow and Hygroscopic Coefficient:
Definition: Vapor flow occurs when water is held as thin films (about water molecules thick) around soil particles.
Hygroscopic Coefficient: This condition corresponds to a very low soil water potential, approximately . This potential is almost double the wilting point potential.
Relative Humidity: At this pressure, the relative humidity around particles is about , close to saturation vapor pressure but not quite liquid water.
Mechanism: Water vapor moves from areas of high vapor pressure (higher potential, higher humidity) to areas of low vapor pressure (lower potential, lower humidity). For instance, from to (drier conditions).
Vapor Flow in Desert Environments:
Significance: Vapor movement is crucial in desert areas with dry surfaces but relatively moist subsurface conditions, especially due to day-night temperature gradients.
Nighttime Effect: During the night, the surface cools more rapidly than the subsurface. The subsurface, being warmer and moister, has higher water vapor potential. This creates an upward gradient of water vapor towards the cooler surface.
Plant Survival: This upward movement can be sufficient for desert plant roots to accumulate some water from the atmosphere near the surface.
General Movement: Water vapor moves from warm to cool and from moist to dry areas. The net effect (upward or downward) depends on the specific temperature and moisture gradients, potentially leading to upward movement even if surface soil moisture is higher.
Osmotic Potential and Fertilization:
Definition: Osmotic potential, often ignored, becomes significant when solutes (like fertilizers) are present in soil water.
Effect of Fertilizers: When salt-based fertilizers dissolve, they decrease (make more negative) the osmotic potential in the soil water.
Water Vapor Gradient: This creates a steep water potential gradient that draws water vapor towards the fertilized spots near the soil surface.
Three Types of Water Flow (Summary):
Saturated Flow: Occurs when the soil is completely saturated, and the matrix potential is zero.
Unsaturated Flow: Occurs in partially saturated or dry soil, typically between the hygroscopic coefficient and saturated conditions (e.g., to ).
Vapor Flow: Dominant when water is present only in thin films at very low potentials (below ). Although a relatively small amount of total water, it's vital under very dry conditions.
Chapter 4: Hydrologic Cycle and Water Distribution
Hydrologic Cycle Overview:
Key concepts include evapotranspiration (and potential evapotranspiration), saturated and unsaturated zones, aquifers (confined, unconfined, perched), and water management.
Global Water Distribution:
The vast majority is saline ocean water.
Significant amounts are in permanent frozen areas (Arctic) and deep groundwater.
Only a very small fraction of global water is available freshwater (for terrestrial life or human use).
Freshwater Breakdown (Surface/Terrestrial Water): Includes saline lakes (unusable without treatment), fresh lakes, swamps, rivers, atmospheric water, and soil water.
Water Residence Times (Key Figures):
Arctic Ice: Approx. years (reflecting ice ages).
Deep Groundwater: Approx. years (very slow recharge).
Oceans: Approx. years.
Shallow Groundwater: Approx. years.
Lakes: Varies widely based on size and precipitation, often days to years or longer.
Rivers: Much faster, less than a year (e.g., half a year, a few months).
Soil Water: Relatively short, from several months to a few days.
Atmosphere: Shortest residence time on average (not active in a room but globally).
Watersheds (Drainage Basins):
Definition: A terrestrial area (also called a drainage basin) where water drains from various locations into a larger surface water body (stream, river, lake, ocean).
Control Factors: Primarily determined by topographic differences, but hydraulic differences between points are also significant.
Example (Alabama): The larger Mobile watershed (including Alacusa, Augusta, Alabama, Black Warrior rivers) drains of Alabama's water into Mobile Bay. Other watersheds include the Tennessee River watershed to the north and smaller ones draining directly to the Gulf of Mexico (e.g., the Choctawhatchee, Escatawpa, Perdido, Yellow, and Conecuh/Sepulga watersheds).
Water Balance Equation:
Precipitation/Irrigation: Inputs of water.
Soil Storage: Water held within the soil profile.
Evapotranspiration (ET): Combined effect of evaporation from the soil surface and transpiration from plant leaves (water returning to the atmosphere).
Discharge: Water leaving the system, either as surface runoff or infiltration into groundwater.
In essence, water that falls on the surface is either stored in the soil, returned to the atmosphere, or drained via surface runoff or groundwater.
Chapter 5: Environmental and Management Impacts on Water Balance
Timing of Cold Temperatures and Snowfall:
Frozen Topsoil Before Snow: If the topsoil freezes solid before snowfall, the snow falls onto an impermeable ice layer. When this snow melts, it will predominantly become surface runoff because it cannot infiltrate the frozen ground.
Unfrozen Topsoil Before Snow: If the soil is warm enough to remain unfrozen before the first snowfall, the snow acts as an insulating blanket, keeping the soil surface warmer. This can allow some microbial activity to continue. When this snow melts, it will infiltrate and percolate through the soil rather than creating large amounts of runoff.
Vegetation Effects:
Canopy Interception: Precipitation or irrigation doesn't always directly hit the ground; it's intercepted by vegetation canopies.
Example (Corn Plants): Corn plants can direct water closer to the base of the stalk, creating higher water potential near the plant and lower potential between plants. This has implications for the movement of fertilizers and agrochemicals in the soil.
Turf Grass and Runoff (Hydrographs):
Hydrograph: A graph representing water flux or flow in a stream over time.
Bentgrass: Has a more dense thatch and a root zone that facilitates better infiltration and percolation, resulting in lower and slower runoff (lower peaks in the hydrograph).
Perennial Ryegrass: Does not enhance water infiltration as effectively, leading to faster and higher runoff peaks in the hydrograph, meaning more water leaves the system as surface drainage.
Forest Management and Water Loss:
Humid/Sub-humid Forests: Before harvesting, these forests typically have low hydrographs and low water loss, as most precipitation is held in the soil and evapotranspired.
Immediately After Harvest: With no soil cover and less transpiration, water becomes surface runoff, leading to rapidly increased hydrographs (high peaks).
Forest Re-establishment: Once a new forest establishes, the low flow surface water conditions return.
Soil Management Practices to Retain Water:
Ponds/Spray Furrows: Creating ponds or spray furrows in fields allows water more time to infiltrate and percolate into the soil, rather than funneling off the field as runoff.
Cover Crops: Useful for preventing soil surface disturbance and creating 'biopores' (channels formed by roots and organisms), which improve water infiltration and percolation.
Chapter 6: Conclusion - Soil Restrictions and Urbanization
Urban Soil Compaction:
Urban soils (both silt loam and silt clay) often exhibit much lower saturated hydraulic conductivity in both topsoil and subsoil compared to natural or less disturbed pasture soils.
This is typically due to compaction and disturbance from human activity.
Fragipan (Fragic Horizon):
Definition: A dense, restrictive layer in the subsoil (denoted as 'x' in soil horizon designation, e.g., 'Bx'). It's a naturally occurring, brittle, and compacted layer.
Impact: A fragipan restricts water infiltration and percolation. If saturated, it leads to shallow subsurface flow or can cause surface runoff if infiltration approaches zero above it.
Hydrograph Effect: Areas with fragipan soils show distinct patterns in hydrographs, often with delayed or reduced water flux in streams compared to soils without such a restrictive layer.
Soil Biology and Agrochemicals:
Beneficial Organisms: Soil organisms like nematodes and termites create 'macropores' (large channels) that significantly improve infiltration and percolation, leading to better soil water storage and reduced runoff.
Agrochemical Impact: Pesticides and other agrochemicals, while targeting pests, can also harm beneficial organisms. This reduction in soil biology (e.g., lower numbers of nematodes and termites) leads to a decline in macropores, reducing infiltration and percolation.
Hydrograph/Water Content Effect: This results in lower soil moisture content and higher exclusion (more runoff) for the same amount of applied water, as reflected in increased hydrograph peaks.
Urbanization and Stream Erosion:
Impact of Lower Percolation: Urbanized areas often have lower soil percolation due to impervious surfaces (buildings, roads) and compacted soils. Even if the surface appears wet, there's limited infiltration below, leading to dry subsurfaces.
Increased Runoff: Water from urban areas rapidly flows into watersheds and streams, creating very high water fluxes (elevated hydrograph peaks).
Severe Erosion: This intense water flux causes severe damage downstream, eroding stream banks and even exposing tree roots previously deep within the soil.
Urban Management Practices:
Green Parking Lots: Incorporating permeable surfaces and vegetation to allow water infiltration.
Drainage Basins: Directing surface water into groundwater or engineered soil water infiltration systems to reduce runoff.
Soil-Plant-Atmosphere Continuum (SPAC):
Process: Describes the continuous pathway of water movement from the soil, through the plant, and into the atmosphere.
Resistance Points: Two main resistance points are associated with the plant:
Soil to Plant Roots: Water moving from the soil into the plant roots.
Plant Leaves to Atmosphere: Water moving from the leaf surface (transpiration) into the atmosphere.
Water Potential Gradient: For water to move efficiently along the SPAC, a strong gradient of water potential is required across these resistance points, driving water from higher (less negative) potential in the soil to lower (more negative) potential in the atmosphere.