Page 223 - Biosystems Engineering

Design of Sustainable W ater Management Systems 201

removed by seedlings. Thus, a certain level of soil moisture depletion
is allowed as part of the irrigation planning to help lengthen the irri-
gation interval. A longer irrigation interval will help maximize the
use of irrigation equipment and help cover a much larger area of land
for the given size of the equipment. As plants develop and mature,
the root zone volume also increases, allowing a larger water storage
volume. Thus, the irrigation system is usually designed to meet the
water demand during this stage.

6.4.2 Other Benefits of Irrigation
There are additional benefits to irrigation such as frost protection,
fertigation, minimizing soil salinization, crop cooling, and effluent
disposal, described here.

Frost Protection
During early spring, an unusual warming up may trigger bud forma-
tion in perennial trees. When cold weather returns, the newly formed
buds will be damaged. By spraying water to the tree canopy, the
microclimate can be cooled to delay bud formation. Another mecha-
nism protects plants against frost damage in early summer. When
temperature dips below freezing point in early summer, water is
misted onto the crop canopy to form a thin layer of ice that acts as an
insulator of the buds and young leaves. Water is sprayed onto the
plants until the atmospheric temperature rises above the freezing
point. Even though irrigation has been used for frost prevention,
atmospheric air inversion methods have been used to conserve water
in water-scarce areas. Giant fans are operated to create this air inver-
sion and keep the crop canopy warm.

Minimizing Salinization
All irrigation water has some salts in dissolved form, and this salt is
accumulated within the root zone during ET. Continued accumula-
tion of salts will change the soil’s physical properties and render them
unproductive. This salt accumulation can be prevented by applying
excess water, which washes the salts below the root zone. A drainage
system to remove the excess salt-laden water will help maintain a
favourable root zone environment. This excess water, beyond plant
water requirements, is designated as leaching requirement (LR). Further
description of leaching is presented at the end of the chapter.
Crop Cooling
During hot summer days, plants may show signs of wilting to mini-
mize ET. However, this will also affect the photosynthetic activity of
the plant, leading to a reduction in yield. By spraying water onto the
crop canopy, the temperature can be reduced by evaporative cooling.
The latent heat energy required to evaporate the water droplets will
be taken from the atmosphere causing the temperature to decrease