Hydrology Midterm II

Infiltration

The process by which water moves from the surface into the soil

Permeability/hydraulic conductivity

The rate at which water can pass through a soil or rock layer- influenced by factors such as porosity, structure, water content

Porosity

The relative proportion of soil volume made up of open space (air)

Vadose zone

The area between the ground surface and the water table

Phreatic zone

The area of and below the water table, where the empty space in rock and soil is fully saturated with water

Regolith

Rock that has broken from the bedrock into smaller fragments, including soil and organic matter content

Mechanisms for soil water flow

Gravitational and capillary

Gravitational flow

When water flows downwards through the pull of the Earth’s gravity

Capillary flow

When water flows through the forces of hydrophilia and attraction to surfaces (sticking to the walls of a pore), including through roots

Adhesion

The tendency of water to stick to unlike particles (in this case, the walls of pores)

Cohesion

The attraction of water to itself (like particles)

Hydrologic processes that infiltration contributes to

Deep percolation, groundwater flow, surface water (discharge)

Conditions that affect infiltration rates

Soil water content, texture, density, surface conditions, organic matter content, hydraulic conductivity, porosity, topography (flat land gives more time for water to infiltrate versus flow out), soil/rock structure

ADD QUESTIONS FROM OTHER INFLUENCES ON INFILTRATION

TBD

Field capacity

Occurs when as much water as possible has drained due to gravitational forces, and now gravity is neglible compared to capillary action. This water is held by capillary forces, and is available to be sucked up through roots for transpiration and plant growth

Wilting point

Occurs when soil water content is so low that roots cannot pull up water and lose their turgor, ALL soil water is hygroscopic water that cannot be pulled up by the roots

Hygroscopic water

Water that is held tightly by molecular attraction to surfaces, and cannot be evaporated or pulled up by roots; not available for evapotranspiration under normal conditions

Capillary water

Water that is being held by the capillary forces of adhesion and cohesion in the soil pores, keeping it from being pulled down due to gravity. Subject to natural evaporation and the primary source of plant water and thus transpiration

Gravitational water

Water than is moving downward in soil pores due to gravitational forces. It is unavailable to plants as it drains before the roots can suck it up due to capillary action and root suction, but evaporates normally

Thee cores of soil

Sand, silt, and clay

Most important natural variable influencing infiltration

Soil structure and soil properties

How does soil texture affect infiltration capacity?

Courser soils yield a higher infiltration capacity, because they have larger macropores that can accommodate more water at a time

Swelling clay

Clay soil whose volume expands significantly during wetting and shrinks afterward, leading to varying infiltration capacity that is lower during and after wetting as

Non-swelling clay

Clay whose soil volume does not expand during wetting, leading to a constant infiltration capacity that does not diminish during wetting

3 ways in which organic matter affects infiltration

1. Absorbs moisture and form porous spaces through which water can move

2. Forms organic glues that glue soil particles together

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Pedogenesis (MORE REVIEW)

The process by which regolith and organic material form into soil

REVIEW HOW DIFFERENT SOIL TYPES WOULD RESPOND TO INFILTRATION

TO DO

Soil development

The process by which surface materials respond to external conditions

Conditions affecting soil development: (MORE DETAIL LATER)

Parent material (C and R horizons), temperature, precipitation, vegetation, topography and time

Zone of elluviation

Upper zone of leaching where losses occur. Composed of organic material and inorganic minerals, which are picked up by water in leaching (including many ions) and absorbed as the water moves through the soil- composed of the Z horizon and above

Zone of illuviation (CHECK)

B horizon- the zone where inorganic minerals and ions accumulate from leaching from the horizons above, leading to mixture and formation of chemical compounds

O horizon (organic)

Layer of decomposing organic material at the surface

A horizon

Rich mixture of organic material and mineral grains; mineral ions here are brought further down through leaching

E horizon

Zone of eluviation and of maximum leaching, where the most leaching occurs of minerals and their ions to the B horizon

B horizon

Zone of maximum illuviation- leaching deposits minerals and ions down in this layer like clay, iron, and aluminum, salt- subsoil, rich in minerals

C horizon

“Cobble” horizon- composed of partially altered and broken pieces of the parent material, along with minerals leached from above. The lowest layer that is considered regolith

R horizon

“bedRock” horizon- composed of unweathered parent material

Aquatard

Material that prevents water from infiltrating downward directly, and thus directs it to flow horizontally until it is once again able to infiltrate downward

Soil properties used in classification reflect

• Degree of soil development

• Indications of process (LOOK UP FOR LATER OR ASK)

• Measures of time

• Relative landform stability (LOOK UP)

Observable soil properties that affect clay, organic content, and ability to hold water are:

• Horizonation- distribution and size of different soil horizons

• Color indicates mineral composition and organic content

• Texture- distribution of different particle sizes

• Structure- shapes in which the soil particles cluster together

Soil structures and how they affect infiltration

• Granular soils allow for quick infiltration as they are highly permeable

• Blocky soils allow water to flow through the cracks

• Platy soils are plate-shaped, often formed by mechanical compaction, and make it very difficult for water to infiltrate through

USDA Soil classification

Classifies soil based on relative composition of silt, sand, and clay particles, using a triangular diagram with cutoffs for different soil categories.

Loamy soils

Contain a mixture of all three soil particles- offering high porosity and permeability, the soil structure often allows for many connected pores which all absorb water

Sandy soils

Have a high infiltration rate due to high porosity

Clay soils

Very low infiltration, as there is very low porosity and permeability

Force that enables water to evaporate

Heat energy- water molecules absorb heat energy, allowing them to change phase from liquid into gas form as water vapor

True or false: evaporation and condensation can occur at the same time out of and into a body of water

True

Net evaporation occurs when

There is more evaporation out of the water than there is condensation of vapor into water

How much energy is required for water to evaporate? (Latent heat of vaporization of water)

540 calories/g at boiling point

Vapor Pressure Deficit (VPD)

The difference between the amount of moisture in the air and the maximum amount of moisture the air can hold when it is saturated

More specifically, it is the difference between:

• The ability of air to hold water based on heat and relative humidity

• The energy of the water body itself, largely thermal, which makes it so more water molecules can evaporate into the unsaturated air

Who developed the concept of the VPD?

John Dalton

How does VPD relate to fog?

When vapor pressure deficit is zero (air is fully saturated), any further water must condense, so net condensation occurs. This results in the formation of water droplets above the body of water

Three-stage diagram about VPD and evaporation

Stage 1- a covered water body where no vapor escapes or consenses

Stage 2- a VPD occurs where water from the uncovered water body may evaporate into air that is not fully saturated, but IT MUST HAVE HEAT ENERGY TO EVAPORATE, and evaporation is greater than condensation

Stage 3- fully saturated air. The VPD is zero when the air is fully saturated. Rates of evaporation and condensation (and both still occur) are equal, vapor pressure is at saturation

How can saturated air evaporate more water than condenses?

With air movement, such as wind

Evaporation over open bodies of water

Occurs with a virtually unlimited amount of water

Open water evaporation is used as a measure of

Potential Evapotranspiration (PET)

Factors that can affect open-water evapotranspiration

• LOWER barometric pressure caused HIGHER VPD and MORE evaporation

• DIssolved matter decreases evaporation as more thermal energy is required

• Shape, size, and location of the water body maximizing or minimizing wind- more wind, more evaporation

• Depth of the water body- SHALLOWER water body absorbs MORE heat and evaporates FASTER

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Two distinct phases of soil water evapotranspiration

Wetting stage and falling stage

Wetting stage

Occurs when soil is nearly fully saturated, and water evaporates nearly as quickly as in open-water evaporation at 90% of PET

Evaporation during wetting stage is limited by

Heat energy available

Falling stage

Occurs when the soil surface has dried, and fluctuating air pressure causes water vapor to diffuse up towards the surface, leading to evaporation, controlled by soil hydraulic conductivity through capillary forces, evaporation typically much SLOWER

Falling stage evaporation rate is governed by

Soil hydraulic conductivity through capillary action, which governs how much water vapor can pass through the soil barrier in a given time

Transpiration

ALL phases of the movement of water through plant roots, through the plant and the stoma, and its evaporation from the stoma into the atmosphere with the aid of heat energy from the atmosphere

True or false: evapotranspiration only properly describes the transpiration from leaf pores into the atmosphere

False

What does transpiration do for plants?

Provides nutrients and water to the plant, and cools the leaves as water passes through them

Necessary conditions for transpiration

1. The atmosphere must be under-saturated, creating a vapor-pressure GRADIENT for the water to pass through the leaf into the atmosphere

2. There must be enough heat energy

3. Wind creates air movement along the leaf that helps with this process, like with open-water evaporation

Basic process of transpiration:

• Water is absorbed by the roots

• Water travels up through the plant

• Water vapor lost from leaf pores called stoma

How does interception relate to transpiration?

Interception is when water falls on the leaves of a plant. It requires the same amount of heat energy to evaporate this water as it does to transpirate the same amount of water from a plant’s stoma. The energy that would be used for transpiration is thus used to evaporate intercepted water, leading to a 1-to-1 reduction in the amount of transpiration that occurs

Factors affecting transpiration rates

• Type of plant

• Wind helps increase transpiration

• Available water increases transpiration as it requires water, as long as there is enough energy

2 physical methods for measuring evapotranspiration

Evaporation pan and lysimeter

Evaporation pan

An open container filled with water. Operators fill it with water and observe how quickly the water evaporates over a period of time. Simple and cheap but a crude instrument

Lysimeter

• Simulates the growing conditions of a given crop and soil

• Consists of a container that is filled with undisturbed soil

• Measures the amount of water that infiltrates and then evaporates using weighing or collecting percolated water

• So basically collects a sample of evapotranspiration in the soil that grows around it

• PET in lysimeter is measured as PET = R + A - P, where R = rainfall, A = irrigation, P = percolation

Models for estimating PET

Mathematical models use data such as altitude, temperature, and and crop-specific coefficients to calculate the theoretical amount of PET for a given area

Three major PET models

• Thornthwaite

  • The crudest model, requires the least data and is calibrated to Eastern US

• Monteith

  • Good for short-term estimates, requires the most data

• Blaney-Criddle

  • Monthly or seasonal estimates, dependent on data and crop-specific coefficients