Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 341

9.3 Theory of Water Movement Through Soil and Toward Drain

9.3.1 Velocity of Flow in Porous Media

Discharge velocity V is defined as the quantity (or volume) of water being dis-
charged from a soil column per unit cross-sectional area of the column per unit
time, i.e., V Q / A .

The discharge velocity is useful when we are mainly concern with the amount of
water flowing through a porous medium, but does not relate to the time of travel of
this water. The flow can occur only through the pores, thus the velocity across any
cross-section must be taken as some kind of average over the cross-section.
Let “ m ” be the ratio of the effective area of pore ( A p ) to the total area ( A )ofthe
indicated section, then

=

A p
A

m =

or, A p = A × m

Then,

Q = AV = A p V av = mAV av (9.5)

where V av is an average velocity through the pores of the cross-section.

9.3.2 Some Related Terminologies

9.3.2.1 Drainage Intensity/Drainage Coefficient/Drainage Requirement

Drainage intensity or drainage coefficient is defined as the depth of water (mm) that
is removed or to be removed per day (i.e., in 24 h) from a field for successful growth
of crop or land amelioration. Its unit is mm/d (millimeter per day).

9.3.2.2 Drainage Density

It is the length of drain per unit area.
Drainage density, D d (m/ha) = (total length of drain in an area, m)/(area of the land,

ha)

9.3.2.3 Head

It is the energy (potential) of water at a point of interest, expressed as water
height (m).

9.3.2.4 Water Table

Free water surface in soil, where soil-water pressure equals zero.