Page 232 - Biosystems Engineering

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accumulation within the root zone. As described previously, excess
irrigation water is used to wash the salts below the root zone, and this
excess water is removed from root zone by drainage systems. Envi-
ronmental regulations set the water quality standards of drainage
return flows and require desalination of the water before it is emptied
into a river or a lake. Alternatively, return flows could be impounded
in a constructed wetland where the salts are taken up by specific plant
species. Biosystems engineers design such engineered wetlands that
help clean up the water and prevent environmental degradation of
waters in lakes and rivers, which are described as follows.

6.7.1 Introduction to Salinization
Whenever irrigation water is applied to plants, the plants take up the
water for ET and leave behind salts within the root zone. Over time,
the accumulation of salts will begin to alter the physical properties of
the soil. Generally, clay soil particles tend to disperse in the presence
of sodium ions, leading to the destruction of the soil structure (U.S.
Salinity Laboratory Staff 1954).
Biosystems engineers have to determine the origin of the salts
when proposing a solution. The salts may already exist within the
parent material. As soil salinity increases, the electrical conductivity
of the soil also increases. Electromagnetic methods are available to
map the electrical conductivity from the surface without destroying
the soil profile (Wittler et al. 2006). Equipment is available to map soil
salinity to a depth of 6 m from the ground surface. This equipment
can be linked to global positioning systems (GPS) to georeference the
data. These data can then be mapped using geographic information
systems (GIS). The subsurface salinity can be mapped using GIS, and
hot spots of salt concentration can be identified.
6.7.2 Methods to Control Salinization

Dealing with Saline Parent Material
The water table should be kept below the root zone to avoid the
upward movement of water from the deeper saline layers of soil.
Drainage systems can help keep the water table much below the root
zone. Water is applied in excess of irrigation requirement to wash the
salts accumulating within the root zone below it. Drainage systems
can help remove this excess water.
Dealing with Saline Irrigation Water
The salinity of the water is assessed by measuring the electrical con-
ductivity (EC). Water with an EC greater than 4 dS/m is unsuitable
for irrigation (U.S. Salinity Laboratory Staff 1954). Water with an EC
less than 0.25 dS/m is considered to be of excellent quality for irriga-
tion. When the EC of irrigation water is greater than 0.25 dS/m,
excess water should be applied to wash the salts below the root zone.