hysteresis

Summary

This video explains the phenomenon of hysteresis in soils, where the relationship between soil moisture content and suction varies depending on whether the soil is draining or wetting. It uses an experiment with soil and water tanks to demonstrate how different pore sizes control the drainage and refilling processes. Larger pores drain first as suction increases, while smaller pores refill first when suction decreases. The difference in behavior during these processes results in distinct soil moisture retention curves, which reflect the history of wetting and drying cycles.

Takeaways

• 🌊 Hysteresis occurs during the wetting and draining of soils, where the relationship between moisture content and suction (negative pressure) changes depending on the soil's history.
• 🧲 The term 'hysteresis' refers to this dependence on the drying and wetting history of the soil.
• 🌧️ Hysteresis significantly impacts runoff generation due to differences in soil moisture content during drainage and refilling.
• 🧪 The experiment demonstrates hysteresis by controlling suction in soil using the height difference between two tanks, which simulates soil draining and refilling.
• 🔬 Suction breaks down surface tension in soil pores during drainage, with larger pores draining first and smaller pores draining as suction increases.
• 🔄 Narrow pore constrictions control the process of drainage as water retreats until it encounters another constriction, illustrating a microscopic view of the soil.
• 🌿 During refilling, capillary forces draw water back into the soil, with larger pore diameters determining when water fills the pore space.
• 🚰 Refilling occurs step by step, with larger pores filling first, and smaller pores only filling when suction reaches a certain threshold.
• 🌀 The hysteresis phenomenon is caused by the different mechanisms controlling drainage (small pores) and refilling (large pores), leading to variations in moisture content.
• 📈 The resulting graphs, known as soil moisture characteristic curves, show the distinct drainage and wetting patterns, highlighting the hysteresis effect.

Q & A

What is hysteresis in the context of soil moisture content?

Hysteresis refers to the phenomenon where the relationship between soil moisture content and negative pressure head (suction) differs depending on whether the soil is undergoing drainage or wetting. The moisture content at a given suction depends on the history of drying or wetting.

What causes the difference in the moisture content between draining and wetting?

The difference in moisture content is caused by the fact that during drainage, narrow constrictions in soil pores control water movement, while during wetting, larger pore openings control the entry of water. This results in different moisture content at the same suction level, depending on whether the soil is draining or wetting.

How does suction influence soil moisture content during drainage?

During drainage, as suction increases (due to the water tank being lowered), larger pores in the soil drain first because surface tension forces in these pores break down more easily. As suction increases further, smaller pores also drain, leading to a decrease in overall soil moisture content.

What role do capillary forces play in soil moisture retention?

Capillary forces, resulting from the surface tension between water and soil particles, help retain water in soil pores. During drainage, these forces need to be overcome by increasing suction, which breaks the surface tension and causes the water to drain out of the pores.

How does the refilling process (wetting) differ from the drainage process in soils?

During refilling (wetting), larger pore openings control water movement, with water being drawn into the largest available pores by capillary forces. This is the opposite of drainage, where smaller constrictions dominate water retention and drainage.

What determines when a pore fills with water during the wetting cycle?

During the wetting cycle, the largest pore diameter in the soil structure determines when water enters. Once the suction decreases to a level corresponding to that large pore size, water fills the pore and is pulled upwards until it encounters the next larger diameter opening.

Why do some pores drain immediately after surface tension is broken during drainage?

Once the surface tension at a narrow constriction is broken, water retreats from the pore until it encounters another narrower constriction. This is because the pore is no longer able to sustain water through capillary forces, leading to immediate drainage.

What is a soil moisture characteristic curve, and what does it illustrate?

A soil moisture characteristic curve, also known as a soil moisture retention curve, illustrates the relationship between soil moisture content and suction during the processes of drainage and wetting. It highlights the differences in moisture content depending on whether the soil is drying or refilling.

How does pore size affect water movement during drainage and refilling?

Pore size influences water movement differently during drainage and refilling. During drainage, smaller constrictions control when water exits the pore. During refilling, larger pore openings control when water enters and how it moves upwards in the soil structure.

What is the significance of hysteresis in environmental processes like runoff generation?

Hysteresis can significantly affect environmental processes such as runoff generation. The differing moisture content between drainage and wetting cycles influences how water is retained or released by soils, which in turn impacts the rate and volume of surface runoff.

Outlines

00:00

🌍 Understanding Hysteresis in Soil Moisture

This paragraph introduces hysteresis, a phenomenon occurring during the drying and wetting of soils. It explains how the characteristic curve of moisture content and suction pressure differs between drainage and wetting phases. The term 'hysteresis' originates from the dependence on the soil's wetting and drying history, which impacts processes like runoff generation.

💧 Soil Moisture Experiment Setup

This part describes an experiment demonstrating hysteresis in soil. It involves two tanks: one containing soil and the other controlling the suction pressure by adjusting its water level. The goal is to show the relationship between soil water content and negative pressure during soil draining and refilling. The tank's manipulation causes changes in suction, affecting the soil's water content.

🔬 Microscopic View of Soil Pores

Here, the script zooms in to examine the soil's pore structure. The graph on the left shows the average relationship between pressure head and moisture content for all pores in the container. The pores, seen under a microscope, respond to changes in suction, showing how capillary forces and surface tension affect drainage and moisture content.

🌡 Capillary Forces and Soil Drainage

This section delves deeper into how capillary forces and surface tension impact larger and smaller pores during the drainage process. As suction increases, larger pores drain first, followed by smaller ones. The draining process is controlled by narrow pore constrictions, which break down the surface tension and cause water to retreat, as shown in the animation.

🌀 Refilling the Soil and Water Entry

The refilling phase is explained in this paragraph, focusing on how water re-enters the pores. The largest pore diameters control when water enters, and capillary forces pull the water upwards through the pore spaces. The water moves one pore at a time, stepping up to larger openings until it fills the entire pore.

🔄 Variations in Wetting and Draining Patterns

This section highlights differences in pore structures and how they influence wetting and drainage patterns. Larger pores refill first, while smaller constrictions control drainage. The paragraph emphasizes how these variations in pore size create differences in moisture content based on the soil's drying or wetting history.

📉 Soil Moisture Retention Curves

Finally, the script explains how graphs called 'soil moisture characteristic curves' or 'soil moisture retention curves' illustrate the differences in moisture content between wetting and drainage phases. These graphs visually represent the impact of hysteresis on soil moisture at varying suction levels.

Keywords

💡Hysteresis

Hysteresis refers to a system's dependency on its history of previous states. In the context of the video, it describes the differing behavior of soil during wetting and draining cycles, where the moisture content at a given suction level depends on whether the soil is being wetted or drained. The term comes from the soil’s response being influenced by its past experiences with moisture levels.

💡Soil Moisture

Soil moisture is the amount of water present in the soil. In the video, soil moisture content changes as the soil drains or refills. The relationship between soil moisture and suction (negative pressure head) is a key aspect of understanding how soils behave under different conditions of wetting and drying.

💡Suction

Suction, or negative pressure head, is the force that pulls water into the soil pores. In the video, the suction in the soil is manipulated by raising and lowering a water tank. This change in suction is critical to illustrating how soil pores drain and refill with water, influencing the moisture content of the soil.

💡Capillary Forces

Capillary forces refer to the forces that arise due to the surface tension of water and its interaction with the soil. These forces play a significant role in both the drainage and refilling of soil pores. In the video, capillary forces are responsible for holding water in the pores during drainage and drawing it back during wetting.

💡Pores

Pores are the small spaces in soil that hold water and air. The video highlights how the size and structure of soil pores influence the drainage and wetting process. Larger pores drain more easily under suction, while smaller pores retain water for longer periods. Understanding pore behavior is essential for analyzing soil moisture dynamics.

💡Soil Matrix

The soil matrix refers to the solid framework of soil, consisting of minerals and organic matter, which creates the spaces (pores) that hold water. In the video, the soil matrix is shown as having varying pore sizes, which control the movement of water during the wetting and draining processes, contributing to the phenomenon of hysteresis.

💡Drainage Curve

The drainage curve represents the relationship between soil moisture content and suction as the soil dries. In the video, this curve is depicted as the soil loses moisture, and the larger pores empty first. The drainage curve is a critical concept for understanding how soil releases water over time.

💡Wetting Cycle

The wetting cycle describes the process of soil refilling with water after being drained. In the video, this process occurs when suction is reduced, allowing capillary forces to pull water back into the pores. The wetting cycle is contrasted with the drainage cycle, illustrating how soil behavior differs in each phase.

💡Surface Tension

Surface tension is the cohesive force between water molecules that allows water to be held within soil pores. In the video, surface tension plays a key role in determining how water moves within the soil, particularly when it comes to the breaking of surface tension in larger pores during drainage and its reformation during wetting.

💡Soil Moisture Retention Curve

The soil moisture retention curve, also called the soil moisture characteristic curve, describes how soil retains moisture at different suction levels. In the video, these curves are used to visualize the difference between drainage and wetting processes. The curves illustrate how moisture content changes depending on whether the soil is drying or being rewetted.

Highlights

Hysteresis is a phenomenon occurring during the draining and wetting of soils, where the relationship between moisture content and suction varies.

Hysteresis depends on the history of drying and wetting, significantly influencing runoff generation.

An experiment is conducted with soil in a tank to illustrate hysteresis, controlling soil suction by adjusting the height of a water tank.

The difference in height between the soil container and water tank dictates the suction in the soil.

A graph is used to represent the relationship between soil water content and negative pressure head during suction changes.

The soil is initially fully saturated, and suction increases as the water tank is lowered.

Larger pores drain first as suction increases due to the breakdown of surface tension forces, leading to decreased soil moisture content.

Further suction increase drains smaller pores, reducing soil moisture content even more.

Drainage of pores is controlled by the narrow constrictions in each pore, where surface tension must be broken for water to retreat.

During refilling, capillary surface tension forces draw water back into the pore spaces, but large pore diameters control this process.

Water enters the largest pore first and then moves upwards until it encounters the next larger opening.

Different pore structures affect water movement during the wetting cycle, as seen in the soil matrix's pore pathways.

The difference in controlling factors during draining (small constrictions) and wetting (large openings) results in varying moisture content at the same suction.

The moisture content depends on the history of draining or wetting, highlighting the nature of hysteresis.

Graphs illustrating the varying wetting and drainage patterns are known as soil moisture characteristic curves or soil moisture retention curves.

Transcripts

hysteresis is a phenomenon that occurs

during the draining and wetting of soils

the characteristic curve describing the

functional relationship between moisture

content and negative pressure head

referred to as suction is different

depending on whether the soil is

undergoing drainage or wedding

the name hysteresis is derived from

dependence on the history of drying and

wetting hysteresis can have a

significant influence on the generation

of runoff in this animation we

illustrate one mechanism for hysteresis

in a soil draining and refilling

experiment the tank on the Left contains

soil the suction in this soil is

controlled by lowering and then raising

the tank on the right the height

difference between the soil and the

water tank controls the suction in the

soil on the Left we will graph the

relationship between soil water content

and negative pressure head for the

experiment where suction is increased

and then decreased let's zoom in to view

the pore structure of the soil in the

soil container our graph on the Left

will show the relationship between

pressure head and moisture content as an

average of all the pores in the soil

container a macroscopic perspective our

view of the pore structure is a

microscopic perspective in the beginning

of the experiment the soil is completely

saturated as the water tank is lowered

the difference between the water level

in the two containers increases and the

suction in the soil increases there is a

relationship between pore size and the

suction that a pore can sustain through

capillary forces due to the surface

tension between the water and the soil

the increasing suction breaks down the

surface tension forces in the larger

pores they drain resulting in a decrease

in the soil moisture content as suction

increases the smaller pores drain and

moisture content decreases further the

drainage of pores is controlled by the

narrow

constrictions and the pores when the

surface tension at each narrow

constriction is broken water immediately

retreats from the pore until a narrower

constriction is reached as illustrated

in the animation remember we are

representing only four of millions of

pores in our soil sample the aggregate

effect over all the pores is the

draining curve during refilling when the

suction is being reduced due to the

water tank being raised capillaries

surface tension forces draw water back

into the pore space however now it is

the largest pore diameter that controls

when water enters a pore and immediately

is pulled upwards until the next larger

diameter opening is encountered as you

watch the wedding cycle notice an hour

cartoon of the soil matrix pores the

first two pathways have larger pore size

at the top and smaller at the bottom the

second two pathways have smaller pores

at the top and larger pores at the

bottom the water steps up one pore at a

time in the pore space on the left

however in the pore space on the right

entry of water into the lower pour does

not occur until the suction

corresponding to the large pore diameter

is reached then these pores fill to the

top immediately because there are no

larger pore openings above this

difference between the small

constrictions controlling drainage and

large openings controlling refilling

results in differences in moisture

content between draining and wetting at

the same suction the moisture content

depends upon the draining or wetting

history hence the name hysteresis the

graphs which illustrate the different

wetting and drainage patterns are called

soil moisture characteristic curves or

soil moisture retention curves