Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 334

9.1.7 Difference Between Irrigation Channel
and Drainage Channel

Drainage channel can be distinguished from the irrigation channel with the follow-
ing characteristics:

Irrigation channel Drainage channel

1. Irrigation channel network spreads from
the source toward the field

1. Drainage channel networks convergence
from the field toward the main drain

2. Irrigation channel is normally elevated
from the plot or at least at equal level to the
field plot

2. Drainage channel is certainly lower than
the field plot

3. In some instances, irrigation channel may
be used as drainage channel (e.g., channel
at lower part of field can be used as
drainage channel of upper part)

3. Drainage channel can not be used as
irrigation channel without structural
modification

9.2 Physics of Land Drainage

Drainage systems are engineering structures that remove water according to the
principles of soil physics and hydraulics. To understand the mechanism how
drainage influence the water balance in the soil and control the subsurface hydrol-
ogy, we should understand the basic concepts regarding soil (pore space) water and
their retention or release characteristics.

9.2.1 Soil Pore Space and Soil-Water Retention Behavior

The soil bulk volume consists of both solid and pore space (Fig. 9.4 ) . The proportion
of pore space in bulk volume depends primarily on soil texture. The typical range
of pore space is 30-55%. For practical implications, soil pores can be classified as
follows:

Pore type Pore diameter

Micro-pore <0.01 mm

Meso-pore 0.01-0.2 mm

Macro-pore >0.2 mm

Water is held within the pore space by weaker capillary forces. Around the soil
particles, water is held as “film” by stronger adsorptive forces. Within the saturation
range of a soil, the classification of total water is illustrated in Fig. 9.5 .