Created by Dr. Michael Pidwirny, Department of Geography, Okanagan University College, BC, CA

evaporation

Soil and Water Ch 5 Continued

Soil Water Energy Concept

Retention and movement of water in soils

Uptake and translocation in plants

Loss to atmosphere

Energy related phenomenon

Potential Kinetic

m * g * h 0.5*m*v2

Free Energy

Free energy: All forms of energy available to do work

Water movement in a soil is from a zone where free energy of water is high to one where free energy is low

Forces Affecting Free Energy

Matric Force: Adhesion and cohesion

Osmotic Force: Attraction of ions and other solutes

Gravity Force: Causes water to flow

Total water potential tells us which direction water moves

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Free energy of soil water

Decrease due to matric effects

Decrease due to osmotic effects

Free energy of pure water

Free energy of pure water higher elevation

Gravitational potential

Osmotic potential

Matric potential

Note: Both osmotic and matric potential are negative and are referred to as suction or tension also

Brady,1984

The matric potential is always negative and

The submergence or pressure potential is positive

Atmospheric potential

Total Soil Water Potential

The difference in free energy from one contiguous site to another is of greater practical significance. This difference is termed as total soil water potential, which ultimately determines the soil water behavior.

Definition:

The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water.

Soil water potential is the difference between energy state of soil water and that of pure free water.

Total Soil Water Potential = Gravitational + matric + Osmotic

T = g + m + o + ……

Gravitational Potential: The force of gravity acts on soil water exactly as it does on any other body, the attraction being towards the center of earth.

g = g * h or h

What direction does water move?

High to low energy points

http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm

Soil Water Content Soil Moisture Content

Water that may be evaporated from soil by heating at 1050C to a constant weight

Gravimetric moisture content (w) =mass of water evaporated (g)

mass of dry soil (g)

Volumetric moisture content () =volume of water evaporated (cm3)

volume of soil (cm3)

= w *bulk density of soil

density of water g

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Bulk density of soil (Db) =mass of dry soil (g)

volume of soil (cm3)

Example: A soil is sampled by a cylinder measuring 7.6 cm in diameter and 7.6 cm length. Calculate gravimetric and volumetric water contents, and bulk density using the following data:

1. Weight of empty cylinder = 300 g

2. Weight of cylinder + wet soil = 1000 g

3. Weight of cylinder + oven dry (1050C) soil = 860 g

Volume of cylinder = *r2*h = 3.14*(7.6/2)2*7.6 = 345 cm3

Weight of wet soil = 1000 – 300 = 700 g

Weight of dry soil = 860 – 300 = 560 g

Bulk density = 560 g / 345 cm3 = 1.62 g cm-3

Gravimetric moisture content = (700-560g)/560g = 0.25 or 25%

Volumetric moisture content = (Db x w) / Dw =

(1.62 g/cc x 0.25)/1.0 g/cc = 0.41 or 41%

Calculating dry soil weight for analytical samples

1. Weigh an empty drying pan

2. Weigh a soil subsample + pan

3. Oven dry the subsample at 105C for 24 hr.

4. Weigh the dried soil + pan

5. Calculate the moisture content (w):w = (g moist soil – g dry soil)/(g dry soil – pan)

6. Rearrange the eqn to solve for dry soil wt.Dry soil wt = g moist soil / (1 + w)

Derivation of dry soil wt calc’nDefine moisture content as:w = (g moist soil – g dry soil) / (g dry soil)

(= g water evaporated) (separate your numerators)

w = (g moist soil / g dry soil) – (g dry soil / g dry soil) = g moist soil / g dry soil – 1 (add 1 to both sides)

1 + w = g moist soil / g dry soil (invert both sides)

1 / (1 + w) = g dry soil / g moist soil (multiply both sides by g moist

soil)

g moist soil / (1 + w) = g dry soil

Don’t oven-dry all of your soil!

Example for lab analyses

You’ve collected some soil, air-dried it, and sieved to <2-mm.

Calculate the moisture content (w) of the air-dry soil (e.g., 2% or 0.02)

E.g., Determine the dry weight basis of 5.00 g of air-dry (or moist) soil:

Dry soil wt = g moist soil / (1 + w)Dry soil wt = 5.00 g / (1 + 0.02)

Dry soil wt = 4.90 g