Water & Transpirationor

Water, water everywhere and all the boards did shrink. Water, water everywhere and not a drop to drink.

J.G. Mexal

A/H 100

Soil Properties

Water Holding Capacity• Water Holding Capacity (WHC): amount a water

held by a soil; a function of texture.

• Field Capacity (FC): amount of water held against the force of gravity after irrigation.

• Permanent Wilting Point (PWP): amount a water at which the plant wilts permanently. (Does not hold well for trees & shrubs)

• Available Water Content (AWC): FC - PWP = AWC

• Maximum Allowable Depletion (MAD): ~50% AWC

Water & Transpiration

Important Definitions

• Field Capacity ~0.03 MPa– amount of water held against the force of

gravity; usually measured 24 h after irrigation

• Permanent Wilting Point ~1.5 MPa– amount of water resulting in wilting from

which plant will not recover; developed for agronomic crops

Water & Transpiration

Colligative Properties of Water

• ConversionsMPa = mega Pascal

1000 kPa

10 bar

1000 g/cm2

~ 1 atm (14.7 lb/in2)

~760 mm Hg

Water & Transpiration

Relative Humidity Comparison

R.H.(%)

Water Potential(Ψ = MPa)

Maximum pore size filledwith water (μm)

0 0 n.a.

99.999 - 0.001 300

99.990 - 0.010 30

99.930 - 0.100 3

99.270 - 1.000 0.3

92.700 - 10.000 0.03

50.000 - 50.000 0.003

20.000 -222.000 --

Soil Properties

Soil-Water Path

Irrigation Scheduling-nonstressed

0

0.4

0.8

1.2

1.6

2

2.4

Sand Sandy loam Loam Clay loam Clay

0

10

20

30

40

50

60

70

AWC

PAW

MAD

Soil Type

Available Water (in/ft) MAD (%)

Soil Properties

Water Holding Capacity

Texture F.C. P.W.P. A.W.C. mm water / 300 mm soil Sand 40 3 37 Sandy Loam 55 3 52 Loam 55 5 50 Clay 65 2 53 Peat 120 30 90

Water Holding CapacityF.C. P.W.P.

Water & Transpiration

Factors Influencing Transpiration

• RoT Water moves from Hi to Lo Concentration

• Water moves through the Soil-Plant-Atmosphere-Continuum in response to this concentration gradient.

Water & Transpiration

Factors Influencing Transpiration

• Plant species

• R.H.

• Stomata (number, size, and aperture)

• Leaf morphology

• Wind

• Temperature (air & soil)

• Soil moisture availability

Water & Transpiration

Factors Influencing Transpiration

• Component– Soil

– Root System

– Xylem

– Leaf Mesophyll

– Substomatal Cavity

– Stomata

– Boundary layer

– Atmosphere

• Water Potential (MPa)– > -0.1

– ~ -0.3

– ~ -0.7

– ~ -1.2

– ~ -2.0

– < -2.0

– < -3.0

– < -10.0

Water & TranspirationFactors Influencing Transpiration/Soil Water

Time (days)

Soil

Wilting

Plant Soil at soil-rootinterface

Field Capacity

Water & Transpiration

Factors Influencing Transpiration

< -3.0 MPa

~ -1.2 MPa ~ -2.0 MPa

Water & TranspirationFactors Influencing Transpiration/Soil Temp

Collards

Cotton

Watermelon

Water & Transpiration

Factors Influencing Transpiration

Frequency (no./mm2)SpeciesLower Upper

Area(% of leaf)

Corn 108 98 0.7

Sunflower 175 120 1.1

Onion 175 175 2.0

Cottonwood 226 187 3.0

Oak 340 0 ?

Stomata Frequency & Location

Water & TranspirationWater Potential vs. Ps and Growth

Corn Ps

Sunflower

Corn Growth

Water & TranspirationFactors Influencing Transpiration/ Wind

Wind Speed = 0.5 cm/sec = 1 mph

Wind Speed = 10 cm/sec = 22 mph

Irrigation Requirements/Harris et al. 1999

June

December

Water & Transpiration

Water Use Efficiency (WUE)

• Species– Alfalfa C3

– Cotton C3

– Corn C4

– Sorghum C4

• g H2O/g D.W.– 864

– 538

– 366

– 365

Things to Know

• Colligative properties• Hydrologic cycle• Irrigation• Solvent• Solute• Field Capacity• Permanent Wilting Point• Water Use Efficiency• Potential Evapotranspiration

• Transpiration• Turgor• Wilting• MAD• RH• AWC• Water Potential• Xeriscaping