Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 337

Table 9.1 Typical drainable porosity in different textured soils

Soil texture Drainable porosity (%)

Sandy 20-30

Loam 12-18

Clayey (clays, clay loam, silty clays) 5-12

Physical meaning of drainable porosity ( P d )
By definition,

Drainable porosity (%) = (Volume of drainable pore spaces in total volume of soil)
×

100/total volume of soil

Another way of expressing drainable porosity is the quantity of water drained
( d d , mm) from a given drop in water table ( h ), expressed as percentage. That is,

d d
h ×

P d = 100 (9.2)

A P d of 8%means that draining 8 mm of water lowers the water table by 100 mm.

9.2.2.7 Drainable Water

Drainable water is that water which can be drained from a saturated soil by gravity
or free drainage. The amount of drainable water in the soil depends on the amount
of “drainable pore space” or drainable porosity. In drainage system, drainable water
is expressed in units of depth (meter or millimeter). Expressing drainable water in
this way assumes that its depth applied to a unit area (i.e., square meter or hectare).
The volume of water from this depth can be computed simply by multiplying the
depth of drainable water by the area of drainage (area of interest), making sure to
keep the units consistent.
In the field, the soil moisture is not constant but changing with time and varies
throughout the soil profile - from ground surface to a particular depth. Soil closer
to the water table is wetter than soil closer to the ground surface. This means that
as moving up from the water table, the soil pores contain proportionately less water.
The change in proportion of air-filled and water-filled pores between the ground
surface and water table (after the downward flow of excess water) is illustrated by
the curved line in Fig. 9.6 . At some height above the water table, the soil moisture
will have drained to field capacity. Poorly drained soils may have water tables at or
very close to the soil surface for prolonged period of time. Under such condition the
proportion of air-filled pores in the soil profile is very small, and that's why the soil
lacks proper aeration to support plant growth.
Suppose a pot with soil have no holes at the bottom for water to escape. The
pot is watered until water spills over the top. At this point the soil is saturated and
no air in the soil pores. If a hole is then made at the bottom of the pot, the free or
“drainable” water will drain out and the soil will be left at field capacity. In case of