Lecture 7 bSoil Water – Part 2

Source: Dept of Agriculture Bulletin 462, 1960

Water Movement MovieUniversity of Arizona

Be prepared for exam questions from this movie!

Describe in your own words what happens to the water in the diagram below.

Water

SoilA horizon - Air Dry

Answer

The water moves sideways and downward at the same rate. This is because of adhesion and cohesion.

Would the movement be different if the soil was saturated? Yes. The movement would

mainly be downward due to gravity.

WATER

Water Movement

Water

Loam

Sand

Water Movement

Water front does not move into sand until loam is saturated

Water

Loam

Sand

t 1t 2t3t4

Water Movement

Water front moves into clay upon contact with clay, but because it moves slow water builds up above the clay layer.

Water

loam

clay

Summary Points from Water Movement Movie University of Arizona

1)      Pore size is one of the most important fundamental properties affecting how water moves through soil. Larger pores as in sand conduct water more rapidly than smaller pores in clay.

  2)      The two forces that allow water to move

through soil are gravitational forces and capillary forces. Capillary forces are greater in small pores than in large pores.

 3)      Gravitational and capillary forces act simultaneously in soils. Capillary action involves two types of attractions, adhesion and cohesion. Adhesion is attraction of water molecules to solid surfaces; cohesion is the attraction of water molecules to each other. Gravity pulls water downward when the water is not held by capillary action. Thus gravity influences water in saturated soils.

 4)      Sandy soils contain larger pores than clay soils, but do not contain as much total pore space.

5)      Sandy soils do not contain as much water per unit volume of soil as clay soils.

6)      Factors that affect water movement through soil include texture, structure, organic matter and bulk density. Any condition that affects soil pore size and shape will affect water movement.

7)      Examples include compaction, tillage, decayed root channels and worm holes.

 8)      The rate and direction of water moving through soil is also affected by soil layers of different material. Abrupt changes in pore size from one layer to the next affect water movement. When fine soil overlies coarse soil, downward water movement will temporally stop at the fine coarse interface until the fine layer above the interface is nearly saturation.

 9)      When a coarse soil is above a fine soil, the rapid water movement in the coarse soil is greater than through the clay and water will build up above the fine layer as the water front comes in contact with the fine layer. This can result in a build up of a perched water table if water continues to enter the coarse layer.

Calculating Soil Moisture

Gravimetric The mass of water in a given

mass of soil (kg of water per kg of soil).

Pw = Percent water by weight or Pw = wt. water ÷ wt. O.D. soil Weight of water = (wet soil)-

(O.D.Soil)

Pw = (weight of wet soil – weight of oven dry soil) X 100weight of oven dry soil

Calculating Soil Moisture

Volumetric The volume of water in a

given volume of soil (m3 of water per m3 of soil)

Pv = Vol H20 ml ÷ Vol soil ml

Pv = Percent volumetric Pv = Pw X bulk density

Calculating Soil Moisture Inches of water per depth of

soil …. or how many inches of water are in a specified depth of soil.

Pv = percent water by vol. Inches water = Pv x (depth of soil) … or .. depth of soil wetted = (inches of water) ÷

Pv Inches of water can be inches of rain added

What determines Plant Available Water Capacity (AWC)AWC = FC-WP Rooting depth a) type of plants, b) growing

stage Depth of root limiting layers Infiltration vs. runoff (more water entering

soil, more will be stored ) Amount of coarse fragments (gravel) Soil Texture - size and amount of pores

silt loam has greatest AWC, followed byloam, clay loam silty clay loam

Soil Water Classification – Available Water Capacity (AWC) = Water Between Field Capacity and Wilt Point.

3.8-2.4=1.6 = clay

SL = 2.2-0.6 = 1.6

AWC by Texture Texture Available Water Capacity in

Inches/Foot of Depth Coarse Sands 0.25 - 0.75 Fine Sands 0.75 - 1.00 Loamy Sand 1.10 - 1.20 Sandy Loams 1.25 - 1.40 Fine Sandy Loam 1.50 - 2.00 Loam 1.80- 2.00 Silt Loams 2.00 - 2.50 Clay Loam 1.80-2.00 Silty Clay Loams 1.80 - 2.00 Silty Clay 1.50 - 1.70 Clay 1.20 - 1.50

DYAD= a soil with 2 feet of ls over 2 feet of silt loam has how many inches of AWC if 4 feet of soil is at field capacity?

Sample Problem

A a soil with 2 feet of loamy sand over 2 feet of silt loam has how many inches of AWC if all 4 feet is at field capacity?

from table – ls = 1.2”/ft and sil = 2.5”/ft.

(2 ft x 1.2”/ft) + (2ft x 2.5”/ft) = 2.4 “ + 5.0” = 7.4 “ of AWC in 4 feet of soil

Sample Problem: Gravimetric determination of soil water

Wt. of cylinder + oven dry soil = 240g wt. cylinder at field capacity =350g wt cylinder at wilt point = 300 Wt cylinder on June 1 = 320 volume cylinder = 200 cc

Or Wet------------FC----------field June 1----wp----------air dry 350 320 300

BD = 240/200 = 1.2 g/cc % water by wt. at FC = ((350-240)÷240)x100 = 45.8% % water by vol at FC = ((350-240) ÷200) x100 = 55% and (%water by wt.) X (BD) = % water by Vol Or 45.8 X 1.2 = 55% % water by vol at WP = ((300-240) ÷200) x100 = 30%

AWC = FC - WP -0.33 bar - ( - 15 bar)

% water by vol at Field Capacity = %FC = 55%%water by vol at Wilt Point = % WP = 30%

% FC - % WP = % AWC

55%-30% = 25% & ( % water x inch soil = inch water)

For 4 feet of soil 25% AWC means that .25 x 48 inch.= 12 inches of water stored in 48 inches of soil.

0

4 ft.= 12 inches of water available/ 4 feet

Rainfall Infiltration

How deep will a 1 inch rainfall infiltrate the soil on June 1?

Soil will be wet to field capacity than water moves deeper. And (% water vol) x (soil depth) = inches of water or Inches of soil = amount of water ÷ %water vol % water by vol between June 1 & and Field Capacity = June 1 = 320g; Field Capacity = 350g soil volume = 200 cc

Or Wet------------FC----------field June 1----wp----------air dry 350 320 300

And (350-320)÷200 = 0.15 Or 1”rain/.15 = 6.67 inches of soil is the depth of soil

wetting Overall formulae for depth of soil wetting =

in. of soil wetted = (in. of rain) ÷[ (%water on June 1) – (Field Cap %)]

World Water Total

97.2 % Ocean 2.8 % Fresh

2.15 % glaciers 0.65 % ground

water 0.0001 %

streams 0.009 % lakes 0.008 % seas 0.005 % soil 0.001 %

atmosphere

Hydrologic Cycle is driven by the energy from the sun-Evaporation

Hydrologic Cycle is driven by the energy from the sun-Evaporation

Water is heated by the sun Surface molecules become

sufficiently energized to break free of the attractive force binding them together

Water molecules evaporate and rise as invisible vapor into the atmosphere

Hydrologic Cycle -Transpiration Hydrologic Cycle -Transpiration

Water vapor emitted from plant leaves

Actively growing plants transpire 5 to 10 times as much water as they can hold at once

These water particles then collect and form clouds

Hydrologic Cycle

Evaporation Transpiration Soil Water

Storage determines ground water recharge

Soil Water and Plant Use

Water Budget

http://wwwcimis.water.ca.gov/cimis/infoIrrBudget.jsp

Ap May June July Aug. Sept Oct

Recharge RechargeRunoff

Evapotranspiration

Precipitation

Soil moisture utilization

Actual ET

Potential ET

Wat

er a

mou

nt

ET > Precip = Soil moisture utilizationPrecip > ET = Recharge, surplus, and runoff

Water Balance Diagram

Range of % of the total AWC – from 0 to 85%