Soil Water Movement and Retention Functions of Soil

Soil Water Movement and Retention

Functions of Soil Medium for plant growth Regulator of water supplies Recycler of raw materials Habitat for soil organisms Engineering medium

Functions of Soil Medium for plant growth Physical Support Gas exchange Water Temperature Nutrient source

Functions of Soil Regulator of water supplies Infiltration Run-off Storage/Movement Distribution Purification Integral to hydrologic cycle

Water Movement

Two Forces Responsible for Water Movement in Soils Gravity Capillarity

Gravity

Capillarity Spontaneous movement of water into and through pore spaces in soil without the aid of gravity.

Adhesion and Cohesion

Cohesion

Adhesion and Cohesion adhesion H cohesion oxygen H H S U R F A C E

Adhesion and Cohesion droplet adhesion Cohesion (H-bonding) Surface

Adhesion and Cohesion Strong adhesion Weak adhesion

Weak Adhesion

Adhesion to Soil Particles Strong Adhesive Forces

Soil Pores Adhesion and Cohesion capillarity Adhesion to the tube or pore wall Cohesion between water molecules

Capillarity h = 0. 15 r Tube/Pore wall adhesion } Force down cohesion

Capillarity h = 0. 15 r Small pores h

Capillarity

Soil Pores and Pore Size Distribution Texture Density Structure

Texture Particle Size Large/coarse Sand Loamy Sandy Loam Silt Pore Size Capillarity Large/Macro Weak Medium Fine/Small Sandy clay Loam Silt Loam Clay Loam Sandy Clay Silty Clay Meso/Medium Micro/Small Moderate Strong

Soil Pores Sandy Silty Clayey

Capillarity Dominated Gravity Dominated

Density Depth in Profile Arrangement of Particles Compaction

Structure Macropores Micropores

Examples

Sand Water Clay

Initial Saturation Sand Clay

Initial Saturation Sandy Loam Uncompacted Compacted

Aggregates

Same Texture and Density Wet Moist

transpiration Relevance water

Quantification: Soil Water Energy

Potential Energy waiting to be used or exploited

Gravitational Potential Energy Water moves in response to differences in potential energy, from high potential energy to low potential energy. High potential Energy The greater the difference in height The greater the difference in Gravitational potential energy. Low potential Energy

Gravitational Potential ψg The potential energy of a unit quantity of water. Unit quantities: volume mass weight = mg Ψg = mgh Ψg = h (cm) mg The greater the height, the greater the potential energy.

Gravitational Potential Independent of soil properties Height (cm) 100 a ψga = 100 cm 50 40 ψgb = 40 cm b soil Ψg = 0 Reference level Difference in energy determines movement

Gravitational Potential Ψga = 60 cm Height (cm) 100 a 40 0 b Ψgb = 0 cm Reference level (Ψg = 0) Ψga – Ψgb 60 - 0 = 60 cm

Gravitational Potential 1. Gravitational potential energy is due only to the height of an object (water) above some reference point. 2. Gravitational potential energy is independent of soil properties.

Capillary Potential Energy (Matric Potential Energy)

Matric Potential “suction” potential - capillarity Narrow capillary tube – high capillary rise h = 0. 15 - strong force r - compared to free water Small particles, small pores Applies to unsaturated soils

Primary Factors in Matric Potential Texture, Density, Aggregation Pore Size Distribution Moisture Content Which Pores are Filled

Capillarity and Soil Texture Small pores Strong suction Strong capillarity Large pores Weak suction Weak capillarity

Capillary Potential Energy water Dry soil Suction potential energy Matric potential energy

Capillary Potential Porous block Suction (capillarity) Ψm = -100 cm (suction) 100 cm Dry soil Vertical distance between the surface of the water and the porous cup.

Soil Texture Sandy Soil Porous block suction 1000 cm ψm = -1000 cm (suction) Dry soil Vertical distance between the surface of the water and the porous cup.

Soil Texture Fine-textured soil suction Ψm = -10, 000 cm (suction) 10, 000 cm Dry soil Vertical distance between the surface of the water and the porous cup.

Soil Texture suction Clay suction Sand Unsaturated soils have negative matric potential energy

Submergence Potential

Submergence Potential (ψs) Equal to the distance below a free water surface Water Table 10 cm

Units of Potential Centimeters of water Bars Pascals 1 bar = 1020 cm water (4 o. C) 1 KPa = 10 cm water 1 bar = 100 k. Pa

Total Potential Energy is the sum of the gravitational, submergence, and matric potential energies. Ψg + ψ m + ψ s = ψ T

Gravitational Potential + Matric Potential = Total Potential Height (cm) 50 a Ψm = -65 cm Ψg = 50 cm ΨT = -15 cm 20 10 Ψg = 0 Reference level

Gravitational Potential + Matric Potential = Total Potential Height (cm) 50 a Ψm = -65 cm Ψg = 50 cm ΨT = -15 cm 20 10 b Ψm = -5 cm Ψg = 10 cm ΨT = 5 cm Ψg = 0 Reference level

Energy Differences Height (cm) 50 a ΨTa = -15 cm 20 10 b ΨTb = 5 cm Ψg = 0 Reference level ΨTa – ΨTb = (-15 cm) - 5 cm = -20 cm

Which way will water move? Height (cm) 50 a ΨTa = -15 cm 20 10 b ΨTb = 5 cm Ψg = 0 Reference level ΨTa – ΨTb = (-15 cm) - 5 cm = -20 cm

Determining the Direction of Water Flow 1. Sum the individual potentials at each point 2. Determine if there is a difference in potential 3. Water will move from the higher to the lower energy 4. Point A – Point B 5. Water moves from high to low energy Positive Point A to Point B Negative Point B to Point A

Next: Characterizing Water Status