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.