Soil Water

Introduction

• The amount of water associated with a given volume or mass of soil ("soil water" or "soil moisture")

• It is a highly variable property.  It can change on time scales of minutes to years.  

• Water use efficiency in Jordan is a very important issue.

Particle size & pore space

LargeParticle

Pore

2 x 2 x 2 = 8

radius = 4r

16 r

Particle size & pore space

MediumParticle

Pore4 x 4 x 4 = 64

radius = 2r

16 r

Particle size & pore space

SmallParticle

Pore 8 x 8 x 8 = 512

radius = r

16 r

Fig. 8.5. Classification of soil water (after Heaney, Crown and Palylyk, 1995).

Credit: Pedosphere.com

Soil Water

Saturated soil is when that soils pores are full of water.

Gravitational water is that water that moves out of the soil due to gravity. This water is generally in the larger Macro-pores.

Capillary water is that water that is held in the soil due to adhesion and cohesion against the pull of gravity. This water is generally held in the smaller Micro-pores and as a film around soil particles.

Soil Water

After a major rain event, once the gravitational water has left the soil, the soil is at Field Moisture Capacity.

The wilting point is reached when soil water levels decline to the point that all remaining water is held too tightly by soil particles to be removed by the plant.

Soil Water

Available Water Capacity

Available water capacity is a measure of the water available to plants.

Commonly defined as the difference between the amount of water at field moisture capacity and the amount at the wilting point.

This is the water a plant has a chance of utilizing.

Available Water Capacity

Soil water contentHow to quantify soil moisture content? Soil Moisture: Mass basis:1. Gravimetric water content, θw (Pw )

= mass of water/mass of dry soil= Mw/Ms = %, or gm/gm

dry weightoven

dry weightoven -dry weightair =

Soil water content, Volume basis2. Volumetric water content, θv (Pv )

= volume water/bulk volume of soil = Vw/V

= %, or mm/m or in/ft ranges from 0 to 60%θv = θw x ρb/ ρw

swv A.θ=θ

Expressing soil water in depth units, dw

Note: θv is a better term for quantifying the amount of moisture of the soil, as it can be expressed as an equivalent depth of water per depth of soil.

* This is necessary when determining appropriate depths of irrigation water to apply to replenish depleted soil moisture.

Example

A soil sample is taken with a soil container.

Sample volume is 100 cm3

Total wet mass (weight) including container = 188 g

Total oven dried mass including container

= 155 gContainer weight = 21 g

1- What is the bulk density?2- soil moisture, mass basis?3- soil moisture, volume basis?4- total moisture in mm/m and in/ft?

Wet mass of the soil = 188 - 21 = 167 gDry mass of the soil = 155 – 21= 134 gρb = mass of dry soil/bulk volume= 134/100=1.34 g/cm3

θ w=(Mw/Ms) = (167-134)/134 = 33/134 = 0.246 or 24.6

θv = (Vw/V) = (33/1)/100 = 0.33 or 33%θv = θw x (ρb/ρw) = 24.6 (1.34/1) = 33%

Fig. 8.10. Saturated and Unsaturated Flow

Fig. 8.4. Capillary rise and capillary retention

Credit: Brady & Weil, 1996; Kohnke, 1968

Fig. 8.6. Interaction of water molecules with clay surfaces, and cations and anions in soil

Credit: Pedosphere.com

Matric Potential

Matric Potential: Adhesion of water to surfaces through adsorption and capillarity; markedly reduces the energy state of adsorbed water molecules

Matric potential is universally important and is used in calculations of water movement