1. Lecture 7 b Soil Water – Part 2
Source: Dept of Agriculture Bulletin 462, 1960

2. Water Movement Movie University of Arizona
Be prepared for exam questions from this movie!

3. Describe in your own words what happens to the water in the diagram below.
A horizon - Air Dry
Soil

4. 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

5. Water Movement
Water
Loam
Sand

6. Water Movement
Water
Water front does not move into sand until loam is saturated
Loam
t 1
t 2 t3 t4
Sand

7. 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

8. 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.

9. 3) Gravitational and capillary forces act simultaneously in soils
 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.

10. 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.

11.  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.

12. 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

13. 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

14. 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

15. 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

16. Soil Water Classification – Available Water Capacity (AWC) = Water Between Field Capacity and Wilt Point.
=1.6 = clay
SL = = 1.6

17. AWC by Texture
Texture Available Water Capacity in Inches/Foot of Depth
Coarse Sands
Fine Sands
Loamy Sand
Sandy Loams
Fine Sandy Loam
Loam
Silt Loams
Clay Loam
Silty Clay Loams
Silty Clay
Clay
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?

18. 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 “ ” =
7.4 “ of AWC in 4 feet of soil

19. 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
BD = 240/200 = 1.2 g/cc
% water by wt. at FC = (( )÷240)x100 = 45.8%
% water by vol at FC = (( ) ÷200) x100 = 55%
and (%water by wt.) X (BD) = % water by Vol
Or 45.8 X 1.2 = 55%
% water by vol at WP = (( ) ÷200) x100 = 30%

20. 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.
4 ft.
= 12 inches of water available/ 4 feet

21. 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
And ( )÷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 %)]

22. World Water Total 97.2 % Ocean 2.8 % Fresh 2.15 % glaciers
0.65 % ground water
% streams
0.009 % lakes
0.008 % seas
0.005 % soil
0.001 % atmosphere

23. 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

24. 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

25. Hydrologic Cycle Evaporation Transpiration
Soil Water Storage determines ground water recharge

26. Soil Water and Plant Use

27. Water Budget

28. Water Balance Diagram Evapotranspiration Precipitation Recharge Runoff
Ap May June July Aug. Sept Oct
Recharge
Runoff
Evapotranspiration
Precipitation
Soil moisture
utilization
Actual ET
Potential ET
Water amount
ET > Precip = Soil moisture utilization
Precip > ET = Recharge, surplus, and runoff

29. The End
Range of % of the total AWC – from 0 to 85%