Untitled Document

Irrigation Water Management

Soil-Water-Plant Relationships

4-Soil Moisture

Index
Definitions
Graphical Representations
Soil Moisture Characteristic Curve
Units
Layered Soils
Crop Root Zone
Soil Moisture Measurement

Definitions

Saturation (Sat): All soil pore space is filled with water - no air.
Soil moisture tension is zero.
Field Capacity (FC): Soil moisture condition after post-irrigation free drainage has stopped.
Soil moisture tension is 0.1 bar, 10 cb. (For disturbed soil samples measured in the laboratory, tension may be 1/3 bar, or 33 cb.)
Permanent Wilting Point (PWP): Soil moisture condition at which plants permanently wilt.
Soil moisture tension about 15 bar, 1,500 cb.
Air Dry (AD): As dry as soil surface can become by exposure to air. Some moisture remains. Soil deeper than the surface layers will generally be wetter than AD.
Oven Dry (OD): No water left in the soil.
Available Water Holding Capacity (AWHC): Amount of water available to the plants. Water above FC is not available to plants (except for the relatively brief period of free drainage) because gravity drainage removes this water from the root zone. Water below PWP is unavailable because it is held so tightly by the soil particles that it won't move into plant roots. So AWHC is the water held between FC and PWP. Numerically, AWHC = FC - PWP.
Soil Moisture Content (SMC): Amount of water that exists in a soil at a particular time. Changes daily as plants take up water in their normal growth process.
Soil Moisture Depletion (SMD): Amount of water that has been used (depleted) from the soil. The amount by which the soil moisture content is below FC.
Numerically, FC = SMD + SMC.
Also, SMD = FC - SMC.
Management Allowed Depletion (MAD): The percentage of AWHC that management allows to be used up before an irrigation.
Maximum Soil Moisture Depletion (max SMD): The largest SMD is allowed to become before an irrigation.
Numerically, max SMD = AWHC x MAD

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Graphical Representations

Sat+ [no air]
   |
   |
FC + ---1/10 bar
   |   \
   |    |
   |    |
   |    |
   |    > AWHC
   |    |
   |    |
   |    |
   |   /
PWP+ ---15 bar
   |
   |
AD +
   |
   |
   |
OD + [no water] Sat+
   |
   |
FC + ---+---+---+--
   |   SMD  |   |
   |   -+- SMD SMD
   |    |  -+-  |
   |    |   | -+-
   |    |   |   |
   |    |   |   |
   |    |   |   |
   |   SMC  |   |
   |    |  SMC  |
PWP+    |   |  SMC
   |    |   |   |
   |    |   |   |
AD +    |   |   |
   |    |   |   |
   |    |   |   |
   |    |   |   |
OD + ---+---+---+--
day 1   2   3   4 Sat+
   |
   |
FC + -----+-----
   |      |
   |      |
   |  max SMD = AWHC x MAD
   |      |
   |      |
   |      |
   |======+== threshold
   |
   |   Plan
   |   irrigate before
PWP+   SMD exceeds
   |   threshold of
   |   max SMD = AWHC x MAD
AD +
   |
   |
OD + -----------

FC = Field Capacity
Sat = Saturation
PWP = Permanent Wilting Point
AD = Air Dry
OD = Oven Dry
SMD = Soil Moisture Depletion
AWHC = Available Water Holding Capacity
MAD = Management Allowable Depletion

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Soil Moisture Characteristic Curve

A soil moisture characteristic curve shows the relationship between soil moisture content (SMC), usually as measured in %, and soil moisture tension.

The SMC at Field Capacity (FC) is where the soil moisture characteristic curve crosses the vertical above 1/10 bar (10 cb) tension. The SMC at the Permanent Wilting Point (PWP) is where the soil moisture characteristic curve crosses the vertical above 15 bars (1,500 cb) tension.

The Soil Moisture Characteristic Curve can also be shown as a table:

Soil Moisture Characteristic Curve

Soil Moisture Tension
Soil Moisture Content
Note

Bars cb
%
in/ft

0 0 30   3.60
Saturation

0.1 10 25   3.00
FC

0.5   50 24   2.88

1.0   100 23   2.76

2   200 20   2.40

5   500 18   2.16

  10
  1,000 16   1.92

  15
  1,500
15   1.80
PWP

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Units

Field Capacity (FC), Permanent Wilting Point (PWP) and Available Water Holding Capacity (AWHC) may be measured in percent (%) or inches per foot (in/ft). Per cent refers to the per cent of the bulk soil volume that is actually water. Or, since we may convert volumes to depth by dividing by the area, per cent refers to the per cent of the soil depth that is water (depth).

Since there are 12 inches per foot, soil moisture %s can be converted to in/ft by multiplying by the conversion factor 12 in/ft.

Example 1

A medium textured soil is 37% water at FC, 22% water at PWP. What is AWHC for this soil?

AWHC (%) = FC (%) - PWP (%)
AWHC (%) = 37% - 22%
AWHC (%) = 15%

AWHC (in/ft) = AWHC (%) x 12 in/ft
AWHC (in/ft) = (15/100) x (12 in/ft)
AWHC (in/ft) = 1.8 in/ft

Example 2

Some time between irrigations, a soil moisture probe determines that this same soil has a moisture content of SMC = 30%. What is the SMD at this time?

SMD (%) = FC (%) - SMC (%)
SMD (%) = 37% - 30%
SMD (%) = 7%

SMD (in/ft) = SMD (%) x 12 in/ft
SMD (in/ft) = 0.07 x (12 in/ft)
SMD (in/ft) = 0.84 in/ft

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Layered Soils

Calculations for a layered soil must proceed layer-by-layer, based on the moisture holding characteristics of the soil in each layer and on the thickness of each layer.

Example 3

Layer
Depth
(in)
Layer Thickness
(ft)
Layer AWHC (in/ft)
Layer AWHC
(in)

1
0-12"
1 ft 0.75 in/ft
1 ft x 0.75 in/ft=0.75 in

2
12"-30"
1.5 ft
1.60 in/ft
1.5 ft x 1.60 in/ft=2.40 in

3
30"-42"
1 ft
2.20 in/ft
1 ft x 2.20 in/ft=2.20 in

4
42"-48"
0.5 ft
1.60 in/ft
0.5 ft x 1.60 in/ft=0.80 in

All
42"-48"
4 ft
-
0.75+2.40+2.20+0.80 = 6.15 in

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Crop Root Zone (RZ)

As long only soil information is known, soil moisture terms must be expressed in % or in/ft. However, once the crop and rooting characteristics are known, this can be combined with soil information to determine water conditions for the Crop Root Zone. AWHC for the root zone (in) is the AWHC for the soil (in/ft) times the depth of the root zone (RZ, ft).

Example 4

Strawberries with a 2 ft root zone (RZ = 2 ft) are grown on the soil of Example 1. [FC = 37%, PWP = 22%.] What is the AWHC for the crop root zone?

AWHC for the soil:

AWHC (%) = FC (%) - PWP (%)
AWHC (%) = 37% - 22%
AWHC (%) = 15%

AWHC (in/ft) = AWHC (%) x 12 in/ft
AWHC (in/ft) = (15/100) x (12 in/ft)
AWHC (in/ft) = 1.8 in/ft

AWHC for the root zone:

AWHC (in) = AWHC (in/ft) x RZ (ft)
AWHC (in) = 1.8 in/ft x 2 ft
AWHC (in) = 3.6 in

For a layered soil, count only those layers, or portions of a layer, that are included within the root zone.

Example 5

Strawberries with a 2 ft root zone are grown on the layered soil of Example 3. What is the AWHC for the strawberries' root zone? Note: the 2 ft root zone extends partially into layer 2 of the soil, but not all the way to the bottom of layer 2. To simplify the calculations, let's sub-divide layer 2 at the 24" depth where the root zone ends.

Layer
Depth
(in)
Layer Thickness
(ft)
Layer AWHC (in/ft)
Layer AWHC
(in)

1
0-12"
1 ft
0.75 in/ft
1 ft x 0.75 in/ft=0.75 in

2a
12"-24"
1 ft
1.60 in/ft
1 ft x 1.60 in/ft=1.60 in

2b 24"-30" 0.5 ft 1.60 in/ft 0 (not part of root zone)

3
30"-42"
1 ft
2.20 in/ft
0 (not part of root zone)

4
42"-48"
0.5 ft
1.60 in/ft
0 (not part of root zone)

Strawberry RZ = 2 ft
-
0.75 + 1.60 = 2.35 in

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Soil Moisture Measurement

Feel & Appearance Method

Observe and feel soil
Using reference tables
Determines SMD (in/ft) directly
Only method to give SMD directly

Tensiometers

Measure soil moisture tension
Like an "artificial root"
Not affected by addition of fertilizers or salinity
Place multiple instruments at one site, at different depths
Require off-season maintenance
Only reads tension up to about 80 cb
- Breaks suction at tensions higher than that

Resistance Meters

Gypsum blocks, Watermarks
Measures electrical resistance inside blocks
- More resistance, less water
Inexpensive
Must be calibrated
Somewhat affected by salinity
Gypsum blocks slowly dissolve, changing calibration curve
Gypsum blocks can read accurately over a wide range of tension
- 1/2 - 10 bars, or 50 - 1,000 cb
Watermarks can read accurately up to 2 bars tension
- 1/10 - 2 bar, or 10 - 200 cb
- Important for some crops - e.g. wine grapes

Capacitance meters

Frequency Domain Reflectometry (FDR) device
- Troxler is one brand name
Reads SMC directly (usually % water by volume)
- Can determine SMD indirectly, if FC is known
Not affected by salinity
Portable units exist, but they have some problems
- Are sensitive to contact with soil
- Often read only in relative terms

Other Devices

Infrared Thermometer (measures degree of plant stress)
Leaf Bomb (leaf water potential)
Temperature dissipation sensor (soil moisture)

Indicator-Threshold Method

All of these devices can be used in an indicator-threshold approach. Select any device and note its measurements (the indicator) daily. Based on research, judgment or personal experience, select a threshold value that you do not wish the indicator to go beyond. Project into the future to predict when next irrigation will be needed.

Regarding Deep Percolation

In general, these devices do not quantify nor identify the existence of deep percolation. For most soils, one day after an irrigation a root zone which has received exactly the amount needed to rise to FC will be at the same moisture content (i.e., FC) as another root zone which has received twice as much as needed.

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