Soil Water Chapter 5

Chapter 5 Outline I. General Properties of Water II. Capillary Action III. Energy Concepts IV. Flow of Water V. Specific Examples

Michael J. Singer and Donald N. Munns Soils: An Introduction, 6e

What makes H 2 O so amazing? I. General Properties of Water A. Polarity B. Hydrogen Bonding C. Cohesion/Adhesion D. Surface Tension

I. General Properties of Water II. Capillary Action A. Equation B. Comparison of textures III. Energy Concepts IV. Flow of Water V. Specific Examples

Capillary Action h=0.15/r In moist soils, the pores act like thin tubes, and water is held by capillarity. Pores are not straight and smooth like the tubes in the figure, and so water does not reach the same height in pores as it would in tubes of the same diameter. A soil with many small pores holds more water by capillarity than does a soil with few large pores. In addition, because of capillarity, water rises higher from a water table in a clay soil than in a sandy soil. In moist soils, the pores act like thin tubes, and water is held by capillarity. Pores are not straight and smooth like the tubes in the figure, and so water does not reach the same height in pores as it would in tubes of the same diameter. A soil with many small pores holds more water by capillarity than does a soil with few large pores. In addition, because of capillarity, water rises higher from a water table in a clay soil than in a sandy soil. Figure 5.6

I. General Properties of Water II. Capillary Action III. Energy Concepts A. Energy review B. Potential C. General Equation and terms 1. Gravitational potential 2. Osmotic potential 3. Pressure potential (matric and hydrostatic) D. Measuring water content (Ө) and potential (Ψ) IV. Flow of Water V. Specific Examples

A. Energy Review 1. kinetic vs potential energy 2. movement from high to low energy B. Potential 1. difference in energy level of water from one site to another 2. wet soil: most water far from soil particle surface (loosely held, high Ψ) 3. dry soil: most water located in small pores (tightly held, low Ψ) Water potential is a measure of the water's availability, or escaping tendency, or tendency to move. It is measured relative to water at a standard pressure and temperature at some reference elevation. High water potential means that water can easily move. It is loosely held. High water potential means that water can easily move. It is loosely held. Water potential is high when it is zero and low when it is kPa. Water potential is high when it is zero and low when it is kPa. More negative is lower potential. More negative is lower potential.

Ψ t = Ψ g + Ψ m + Ψ o + Ψ s + …. Ψ g =gravitational=pulls water downward always positive Ψ m =matric=attraction to solids (suction, tension) adhesion and capillary (movement moist [high] to dry [low]) Ψ o =osmotic= attraction to ions (lower potential energy) >concentration = concentration = < potential (impt for water uptake by plant) Ψ s =submergence=hydrostatic (saturated zones only)

What are the major components of water potential? What factors most influence water potential in soil? What are the major components of water potential? What factors most influence water potential in soil? The major components of water potential are the gravitational, osmotic, and matric potentials. The major components of water potential are the gravitational, osmotic, and matric potentials. Dry Soils In unsaturated soils, matric and osmostic potentials are dominant and decline respectively with decreasing water content or increasing salinity. In unsaturated soils, matric and osmostic potentials are dominant and decline respectively with decreasing water content or increasing salinity. Wet Soils In saturated soils and aquifers, gravitational potentials are dominant. In saturated soils and aquifers, gravitational potentials are dominant.

Table 5.1

D. Measuring water content (Ө) and potential (Ψ) How would you measure the water content and the water potential of a soil? How would you measure the water content and the water potential of a soil? Water content (Ө) of soil is most easily measured by weighing a sample wet and again after drying (oven to constant weight). Water content = weight loss/dry weight. Water content (Ө) of soil is most easily measured by weighing a sample wet and again after drying (oven to constant weight). Water content = weight loss/dry weight. Water potentials (Ψ) in soil materials can be measured with tensiometers. Water potentials (Ψ) in soil materials can be measured with tensiometers.

Water content calculation