Soil water content in soils

Rafael Muñoz-Carpena

Outline

Soil Hydrology

A soil and water “refresher”

Capillarity theory

Field capacity

Hydrological Methods

This refers to water balance methods.RO: RunoffP: PrecipitationF: Soil InfiltrationD: Deep percolation= Soil moisture

If RO=0 and all butET are measured we

can Estimate ET

Atmosphere

Soil

Aquifer

Soil is made of three components

Pore space=Va+Vg Porosity=Pore space/total

volume= (Va+Vg)/V Water content (in

volume) = Volume of water/total volume =Va/V

(in weight)=Ma/Ms Bulk density= Mass of

solids/total volume = Ms/V

Air

Water

Solids

SOIL MassVolume

Water is held in the soil pores

Soil particle size have an effect on soil water holding capacity

Clay

Silt

Sand Texture is made out by the the relative content of each of the soil particles

Pores are spaces between particles

…as does soil structure…

Block

Prismatic

Structure is the association of particles in larger lumps.

A paradox?

The coarser the soil the less water it contains

- The coarser the particles the larger the pores but the total amount of pores is small

On the other hand…

-The finer the particles the smaller the pores but the total amount of pores is large.

Also water flows slower in fine soils…

Does love make the world go around?

Energy, or rather differences in energy do…

The universe tends spontaneously to lower energy stages: “chaos” or “disorder”

Soil water movement follows the same pattern

Water in soil is related to energy

Water does not move freely as it does above the surface, but is held in the grasp of the soil which determines how it will move and how much energy (work) the plant roots have to invest to withdraw it .

(Drawing source: SoilMoisture, Inc.)

Air water soil root

Potential: Energy in the soil t = g +p +o

t: total

g: gravitational

p: pressure

o: osmotic

As soil dries more energy is needed

Increasing work is required to remove the water from the small sized pores compared to the large pores, as the soil dries out. Because of this, plants find it increasingly difficult to get adequate water as the soil dries. When remaining water is held only in extremely small pore spaces, the plants cannot exert enough force to withdraw it, and the plants wilt and die

(even when there is still water in the soil).

(Drawing: SoilMoisture, Inc.)

Wet soil Dry soil

Pressure (capillary) potential

P=g |hc|

Weight-unit volume

units p= hc

|hc| = 2 cos / (gr)

Moisture is related to suction “Soil Suction”(negative pressure potential) is the work that

plants have to do to get needed water, and the energy that determines which way moisture will move in the soil.

ClaySand

Suction,

W

ater

con

tent

(in

3 H2O

/in3 S

oil)

Water content in the soil is related to suction (energy)

Yes!, moisture is related to suction

Moisture holding is related to texture

Coarse soil releases moisture rapidly with less energy required.

ClaySand

Suction,

W

ater

con

tent

(in

3 H2O

/in3 S

oil)

Fine soils hold moisture longer, even at high energy (suction)

Water content in the soil is related to texture

Texture vs. Structure

ClaySand

Suction,

Wat

er c

onte

nt (

cm3 H

2O/c

m3 S

oil)

TextureStructure

Field capacity: Hydrology or Agronomy?

In 1949 Veihmeyer and Hendrickson “in 2-3 days after rain or irrigation in soils of uniform texture and structure soils”

When gravitational and capillary forces equilibrate after a water application event, the soil stops draining freely.

It is a static concept, while the system is dynamic (redistribution does not stop after FC). In sandy soils the concept is closer to reality (why?)

Ways to estimate it: 1/3 bar with Richards plate, centrifugue at 1000 rpm

Factors affecting FC?

Wilting point

Field capacity

W

ater

con

tent

Clay

(w/ organic matter)

(w/o organic matter)

(w/ organic matter)

(w/o organic matter)

Questions?