Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 336

9.2.2.2 Porosity

It is the ratio of volume of voids or pores to the bulk or total soil volume (solid plus
pores). That is,

V p
V =

V p
V p +

η =

V s
It represents proportionate amount of pore volume and bulk volume.

9.2.2.3 Relation Between Porosity and Void Ratio

V p
V p +

1 1 e

η =

V s =

= =

V s
V p

1
e

1 + e

1 + 1 +

or

e

η = (9.1)

1 + e

9.2.2.4 Moisture Concentration

The moisture concentration is defined as the volume of water per unit volume of
soil.

9.2.2.5 Pore Water

Water that is held in the soil pores is termed as pore water.

9.2.2.6 Drainable Pore Space or Drainable Porosity ( P d )
Drainable pore space or drainable porosity is the air-filled pores present when the
soil is drained to field capacity (i.e., after gravity drainage). Alternatively, the macro-
pore spaces which releases or drains water due to gravity drainage is termed as
drainable porosity. This can be estimated as

P d (%) = soil porosity (%) − soil moisture at field capacity (%)

It is influenced by soil texture and structure. Coarse-textured (sandy) soils have
large drainable porosity, whereas fine-textured (clayey) soils have smaller drainable
porosities (Table 9.1 ) . This implies that for a certain amount of water drained, a
sandy soil shows a smaller water-table drop than that of a clay soil.