Lecture 7 Soil Water Storage and Movement (Part 2)

Flashcards covering concepts related to soil water storage and movement, including definitions of key terms, different types of water flow, and influencing factors.

What is the key difference between adhesive and cohesive forces in soil water movement?

Adhesive forces describe the attraction of water molecules to soil particles, while cohesive forces describe the attraction of water molecules to each other.

Define soil saturation.

Saturation is the point when all soil pores are completely filled with water.

What is field capacity in soil?

Field capacity is the amount of water remaining in the soil after gravitational water has drained away, leaving macropores empty but micropores still holding water.

Explain the permanent wilting point.

The permanent wilting point is the moisture content at which plants can no longer extract enough water from the soil to prevent irreversible wilting.

What is gravitational (saturated) flow in soil water movement?

Gravitational flow is the rapid movement of water in larger soil pores (macropores) driven by gravity, occurring in saturated soils.

Describe capillary (unsaturated) flow.

Capillary flow is the slow movement of water restricted to smaller soil pores (micropores), driven by the combined effect of adhesive and cohesive forces, occurring in unsaturated soils.

Which type of water flow occurs in saturated soils, and which in unsaturated soils?

Gravitational (saturated) flow occurs in saturated soils, and capillary (unsaturated) flow occurs in unsaturated soils.

How is the plant available water or water holding capacity of a soil calculated?

It is the difference between the water present at field capacity and the water remaining at the permanent wilting point (Field capacity – Permanent Wilting Point).

List other factors, besides soil texture, that influence water holding capacity.

Percentage of soil organic matter, compaction, osmotic potential (presence of salts), and soil depth.

What conditions must be met for gravitational flow to occur?

Water infiltrates into macropores open to the surface, soil moisture content exceeds field capacity, and the soil possesses macropores 1 mm or greater.

When does saturated (gravitational) flow cease?

Saturated flow ceases when water application is reduced or stops, causing macropores to drain rapidly and moisture content to fall below field capacity.

What conditions initiate and are necessary for unsaturated (capillary) flow?

Capillary flow is initiated by a moisture gradient from wet to dry, and a micropore system must be present.

In which directions can capillary flow occur?

Capillary flow can occur in any direction (upward, downward, or laterally) in response to a moisture gradient.

How does soil texture generally influence water retention?

Finer textured soils (silt and clays) generally show greater water retention than coarse soils (sandy soils).

How does soil texture impact gravitational water flow?

Soils with large continuous pores will have a greater ability for gravitational water flow; sandy soils tend to have large macropores, but well-aggregated finer-textured soils also can show good infiltration.

Summarize the key characteristics of gravitational (saturated) flow.

It occurs when soil moisture is greater than field capacity, involves fast water movement through macropores, and water freely drains downward through soil under the force of gravity.

Summarize the key characteristics of unsaturated (capillary) flow.

It occurs when soil moisture is less than field capacity, involves slow water movement through micropores, and water can move in any direction through capillary force.