Lecture 7 b
Soil Water – Part 2
Source: Dept of Agriculture Bulletin 462, 1960
Water Movement Movie
University 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
A horizon - Air Dry
Soil
Answer
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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?
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Yes. The movement would
mainly be downward due to
gravity.
WATER
Water Movement
Water
Loam
Sand
Water Movement
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Water front does
not move into sand
until loam is
Water
Loam
saturated
Sand
t1
t2
t3
t4
Water Movement
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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
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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.
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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.
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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.
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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
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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 100
weight of oven dry soil
Calculating Soil Moisture
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Volumetric
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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
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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
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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 by
loam, 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
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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
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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.
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(2 ft x 1.2”/ft) + (2ft x 2.5”/ft) =
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2.4 “ + 5.0” =
7.4 “ of AWC in 4 feet of soil
Sample Problem: Gravimetric determination of soil water
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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
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Or Wet------------FC----------field June 1----wp----------air dry
350
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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
= 12 inches of water available/ 4 feet
4 ft.
Rainfall Infiltration
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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
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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
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97.2 % Ocean
2.8 % Fresh
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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
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
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Hydrologic Cycle -Transpiration
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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
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Evaporation
Transpiration
Soil Water
Storage
determines
ground water
recharge
Soil Water
and Plant Use
Water Budget
http://wwwcimis.water.ca.gov/cimis/infoIrrBudget.jsp
Water Balance Diagram
Water amount
Evapotranspiration
Potential ET
Soil moisture
utilization
Recharge
Runoff
Actual ET
Recharge
Precipitation
Ap
May
June
July
Aug.
Sept Oct
ET > Precip = Soil moisture utilization
Precip > ET = Recharge, surplus, and runoff
Range of % of the total AWC – from 0 to 85%