Lecture 7 b Soil Water Part 2

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Lecture 7 bSoil Water Part 2
Source Dept of Agriculture Bulletin 462, 1960
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Water Movement MovieUniversity of Arizona

• Be prepared for exam questions from this movie!

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Describe in your own words what happens to the
water in the diagram below.
Water
A horizon - Air Dry
Soil
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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
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Water Movement
Water
Loam Sand
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Water Movement
Water

• Water front does not move into sand until loam
  is saturated

Loam
t 1 t 2 t3 t4
Sand
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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
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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.

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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.

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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

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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

• 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

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Hydrologic Cycle

• Evaporation
• Transpiration
• Soil Water Storage determines ground water
  recharge

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Soil Water and Plant Use
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Water Budget
http//wwwcimis.water.ca.gov/cimis/infoIrrBudget.j
sp
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Water Balance Diagram
ET gt Precip Soil moisture utilization Precip gt
ET Recharge, surplus, and runoff
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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 100 weight of oven dry soil
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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

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Calculating Soil Moisture

• Inches of water per depth of soil . or how many
  inches of water are in a specified depth of soil.
• Inches water Pv x (depth of soil)
• or ..
• depth of soil wetted (inches of water) Pv

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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 by loam, clay
  loam silty clay loam

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Soil Water Classification

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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 all 4
  feet is at field capacity?

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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

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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--------------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
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• 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

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AWC FC - WP -0.33 bar - ( -
15 bar)
water by vol at Field Capacity FC water by
vol at Wilt Point WP 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
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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
  350-320200 0.15
• Or 1rain/.15 6.67 inches of soil is the depth
  of soil wetting
• Overall formulae for depth of soil wetting
• inches of soil wetted inches of rain (on
  June 1) (Field Cap )

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The End
Range of of the total AWC from 0 to 85