Average density of mineral grains in the soil.

 Pm = Mm / Vm

The value of tension for a given 

water content  also depends 

on 

the history of wetting and 

drying (hysteresis).

Bulk density:  Pb = Mm/Vs = Mm/(Va+ Vw + Vm) 

Where Vs is the total volume of the soil sample, and Va, Vw and Vm are the volumes are the air, water and mineral components of the soil. Typical values are 1000-2000 kg m-3

Porosity (proportion of pore space in a volume of soil):

Φ = (Va + Vw)/Vs = (1- ρb / ρm).     

Porosity is typically around 0.5.  

In many soils porosity decreases with depth due to compaction. 

ratio of water volume to total volume

θ =  Vw/Vs 

Equivalently θ = (Mswet – Msdry )/( ρw.Vs )

the proportion of pores that contain water,

is: S = Vw/(Va + Vw) = θ/ Φ

If the soil is saturated, then  S = 1 

the capillary 

tension increases,  the  water is 

held  more tightly to grains (the 

menisci have a smaller radius of 

curvature

Field Capacity

The maximum amount of water that a soil can hold after gravitational drainage.

is the water content at which plants can no longer extract water from the soil.

high porosity

high field capacity

high wilting point

low field capacity

low surface area capacity leading to low wilting point

 1) the ability of the soil to “conduct” water; (2) the magnitude and direction  of the slope, which is the 

gravitational  potential energy gradient per unit weight of flowing water (soil water wants to 

flow downhill, towards lower gravitational potential energy), and (3)  the magnitude and 

direction  of the horizontal gradient in soil water pressure (if soil water pressure decreases 

in the x direction, the water wants to flow in that direction)

Darcy’s Law states that the flow rate in the vertical depends on

 For a given hydraulic conductivity, the vertical flow rate depends on the sum of  the gravitational potential energy  gradient per unit weight of water (dz/dz =1) and the vertical gradient in soil water pressure

The hydraulic conductivity Kh

(θ)  depends on:

a)    The type of soil,  tending to be larger for coarse-grained soils (water path is less sinuous);

b)   The volumetric soil water content θ;  it increases with increasing  soil moisture to is maximum value at saturation( the saturation hydraulic conductivity  K*h

The pressure head ψ is being

more negative means

a)   Is negative for unsaturated soils, and increasingly negative the drier the soil.  It is positive for saturated soils.

b)    It depends on soil type.  For a given degree of saturation, it is more negative in fine-grained soils than in coarser-grained soils. 

Relationships between matric suction,  volumetric water content and hydraulic conductivity can also be expressed in terms of the hydraulic diffusivity, Dh(θ):

Surface Horizon

Subsoil

substratum

Rate (m s -1) at which water enters the soil from  

the surface

:Maximum rate at which infiltration can 

occur (also called infiltrability)

Water input rate from rainfall, snowfall or irrigation

•The saturated hydraulic conductivity of the soil profile

•The degree to which pores are already filled with water when the infiltration 

process begins

•Variations in hydraulic conductivity through the soil profile 

•The inclination and roughness of the soil surface

•Chemical characteristics of the soil surface

•Physical and chemical properties of water

The time rate of change in volumetric soil moisture for  a given  thin layer of soil  depends on the vertical rate of change of the hydraulic conductivity (itself a function of θ) and the vertical rate of change  of the product of (a) the hydraulic conductivity, and (b) the vertical rate of change of the pressure head ψ, (the matric suction gradient) the pressure head also being a function of θ. In this expression, z is taken to increase downwards.