Page 229 - Biosystems Engineering

Design of Sustainable W ater Management Systems 207

6.6 Drainage Systems
Although the previous sections described the importance of supply-
ing the plant water needs by irrigation, it is also important to main-
tain proper aeration status in the root zone. Drainage systems allow
the rapid removal of excess water from within the zone. Biosystems
engineers are knowledgeable about the physical properties of soil
and are able to model the flow of water through soils. They are also
trained to identify the source of water problems and to design appro-
priate drainage systems.

6.6.1 Importance of Drainage
Waterlogged soils will adversely affect the plant roots by reducing
the aeration status of the root zone. The primary purpose of drainage
system is to remove the excess water from within the root zone or
prevent the water from entering. There are additional benefits to
drainage as well. Water has 5 times more heat capacity than dry soil
particles. As a result, it takes less heat to warm dry soils than wet
soils. In a temperate climate, solar energy is used to warm the soil
during the spring. Therefore, the soil that is drained warms up much
earlier than undrained soil. Early warming up will advance the plant-
ing season and allow the planting of longer season crops that have
higher yields. By lengthening the growing season, the number of
days available for machinery use is increased. This will allow more
land to be cultivated with the available machinery and labor.
Wet soils have a low weight-bearing capacity compared to dry soils.
As a result, farm machinery operating under very wet conditions will
result in puddling of the soil, leading to loss of soil structure. When
the water content of the soil is below that of the lower plastic limit,
the soil will be able to support machinery (Smedema et al. 2004).
Therefore, well-drained soils will allow soil tillage operations to begin
much earlier than undrained soils. In addition to early warming up,
the ability to allow early tilling and late harvesting operations lead to
extended time for machinery use. This increases the number of days
machinery can be used and reduces the cost of running the machin-
ery. These additional benefits have to be taken into account when
weighing the need for draining a particular farmland.

6.6.2 Methods for Dealing with Excess Water
The first task of the biosystems engineer assigned to solve a drainage
problem is to identify the source of the excess water. Aerial photo-
graphs taken under flooded conditions or a contour map of the land
surface can identify the extent of the problem. If the water is coming
from outside the boundaries of the field as surface water flow, it can
be prevented from entering the field by the construction of dikes to
keep the water out. Alternately, interceptor drains can be installed to