Ch. 5-6: Soil Water Dynamics and Hydrologic Cycle in Soil Science

Soil Science Exam 2 Ch. 5-6

Polarity

Has positive & negative sides.

Hydrogen bonding

+ attracts to − between water molecules.

Hydration

Other molecules attracted to + or − sides of water.

Cohesion

Water attracted to water.

Adhesion

Water attracted to solids.

Surface tension

Attraction between water molecules (cohesion) is greater than attraction to air.

Capillarity

Phenomenon by which water moves in soil due to adhesion and cohesion.

Gravimetric soil water content

The mass of water in a given mass of soil.

Volumetric soil water content

The volume of water in a given volume of soil.

Saturation

The state when all soil pores are filled with water.

Field capacity

The amount of soil moisture or water held in the soil after excess water has drained away.

Wilting coefficient

The soil moisture level at which plants can no longer extract water.

Hygroscopic coefficient

The amount of water held tightly to soil particles.

Gravitational water

Water that drains through soil under the influence of gravity.

Plant available water

Water that can be absorbed by plant roots.

Unavailable water

Water that is not accessible to plants.

Capillary water

Water held in the soil pores against the force of gravity.

Hygroscopic water

Water that is tightly bound to soil particles and unavailable to plants.

Available water holding capacity

The amount of water that can be stored in soil and made available to plants.

Soil water potential

The potential energy of water in the soil, affecting its movement.

Matric potential

The potential energy associated with the attractive forces between water and soil particles.

Osmotic potential

The potential energy of water due to solute concentration.

Soil-plant-atmosphere continuum

The system describing the movement of water from soil to plants to the atmosphere.

Potential energy

Most important energy type in soils.

Soil water characteristic curves

Relationship between soil water potential (Ψ) and the amount of water in the soil, influenced by soil texture, structure, and pore size distribution.

Gravimetric water content

θg = mass of water associated with a given mass of dry soil, calculated using oven-dried soil compared to sampled soil.

Volumetric water content

θv = volume of water associated with a given volume of dry soil; θv = Gravimetric water content (θg) × bulk density.

Soil water content

Amount of water in soil considered by mass (g/g) and by volume (cm3/cm3).

Energy state of water

Water moves to where its energy state will be lower.

Pore size distribution

Influences how soil water behaves and drains.

Water film thickness

Causes most of the pore space to be filled with water.

Soil texture

Influences the relationship between soil water potential and water content.

Soil structure

Influences the relationship between soil water potential and water content.

Water potential

Determines how soil water behaves and its supply to plants.

High energy areas

Areas where soil water is wetter.

Low energy areas

Areas where soil water is drier.

Gravitational Potential

Water moves from high potential energy to low potential energy.

Soil water dynamics

Depends more on water potential (energy status) than on water content (amount).

Soil water supply

Longer-term supply depends on understanding both water potential and water content.

Negative pressure in soil

Caused by matric potential from attraction to soil solids.

Constraints in water uptake

Happen when osmotic potential in soil solution is lower than in the plant root.

Saturated

Maximum retentive capacity; all pore space filled with water; water in largest pores will drain down due to gravitational force.

Permanent Wilting percentage

Plants have removed all of the water that they can; soil water potential is about -1500 kPa; soil is very dry.

Above field capacity

Water is available but ventilation is limited.

At field capacity

Water, ventilation, physical support, root growth potential are optimum.

Below field capacity

Water begins to be less available; about ½ way to wilting coefficient water becomes slowly available.

Easily available

Water that plants can readily obtain.

Slowly available

Water that plants can obtain but at a slower rate.

Available water holding capacity (AWHC)

Difference between θg at field capacity and at wilting coefficient; how much water that can be held and is available to plants.

Global stocks of water

97% ocean water, 1.8% frozen, and 1.7% groundwater; most groundwater is > 750 m below the ground; 'Other water' = Just 0.02% of the total global stock.

Water movement from Earth's surface

Atmosphere.

~65% of water falling to land

Becomes sorbed to soil particles at some point in cycle.

Soil-Plant-Atmosphere Continuum (SPAC)

Major component of hydrologic cycle; specifically examines water cycling between soil, plants, and atmosphere.

Movement governed by

Water potentials, rate of water supply to roots, plant transpiration rates.

Interception

• Precipitation caught by plant foliage or litter layers

• Returned to the atmosphere by evaporation

• Does not reach the soil surface

• Increases with higher temperature

• Decreases with greater humidity and rainfall intensity.

Runoff

Water that reaches the soil surface may be lost as surface runoff; over 50% in extreme cases; carries dissolved chemicals, nutrients, soil particles/sediment.

Urban watersheds

Affected by compaction, destruction of soil structure, profile truncation, and impervious surfaces.

Ways of increasing infiltration

Permeable pavers, green roofs, vegetated street medians.

Infiltration

Process by which water enters soil pore spaces and becomes soil water.

Percolation

Once water infiltrates the soil, the water moves downward through the profile by percolation.

Soil storage water

Water retained by soil after infiltration/percolation, which eventually moves upward through capillary rise.

Evaporation

Occurs from the upper 15-25 cm of soil and is affected by temperature, soil moisture, and plant characteristics.

Transpiration

Specifically evaporation from plant foliage, requiring movement from the soil, into the root, and through the plant.

Evapotranspiration (ET)

Combination of evaporation (E) and transpiration (T).

Potential Evapotranspiration (PET)

Rate at which water vapor is lost from a densely vegetated plant-soil system at an optimum soil water content.

PET formula

PET = 0.65 x Pan evaporation.

Surface Evaporation

Direct evaporation from soil surface, a large part of ET, especially in bare and wet surface soil.

Factors affecting Evaporation

Temperature, soil moisture, and plant characteristics (e.g., LAI).

Compaction

Influences runoff and infiltration by affecting soil structure.

Vegetation

Influences infiltration and runoff, with different types affecting water movement.

Soil properties

Affect infiltration and runoff, including texture and structure.

Capillary rise

Process by which water moves upward through soil.

Drainage

Occurs when water-holding capacity of soil is exceeded, leading to loss from the root zone.

Management practices

Include irrigation rate, furrows, cover crops, and controlling weeds to influence water movement.

Rain gardens

One method to increase infiltration and slow runoff in urbanized watersheds.

Permeable pavers

Another method to increase infiltration and slow runoff in urbanized watersheds.

Soil management

Practices that affect the infiltration rate and runoff, including tillage and crop residue management.

Hardpans

Soil layers that restrict water movement and increase runoff.

Biotic activity

Influences soil structure and water movement, including the role of earthworms.

Loose/open soils

Higher infiltration rates due to sandy texture and well-granulated structure.

Heavy/unstable soils

Higher runoff rates due to clayey texture and poor structure.