Environmental Engineering Reference
In-Depth Information
only concern. Unfortunately midday irrigation usually results in substan-
tial water losses due to low midday air humidity and increased evapor-
ation from warmer surfaces and in high drift because the wind is
strongest at midday. Early-morning irrigation, around sunrise, is there-
fore better. Water is applied on the coolest surface and sprayed into the
most humid and calmest air. In addition, irrigation water washes off the
dew droplets persisting on the leaves, causing turf to dry faster, thus
minimizing disease development.
Several other factors can be considered when irrigation is scheduled.
Because compaction from foot traffic increases substantially when soil is
wet, and running athletes rupture wet turf more easily, many sport fields
should not be irrigated within a day or two before a game. During games,
sport surfaces should be dry, but at the same time, turf should not be
water stressed. In other cases, irrigation scheduling is determined by
water availability. Many municipal areas allow watering only at certain
times of day or on certain days of the week. Usually, under temporary
water-conservation measures, turf is able to survive until restrictions are
lifted or eased, but if restrictions are too severe or water extremely
expensive, turf may be replaced with the so-called 'alternative landscap-
ing', which includes rocks and drought-resistant plants.
Irrigation Frequency
The first principle of irrigation management says: 'deep and infrequent'.
At each irrigation event, soil capillaries in the entire root zone should be
filled with water to soil field capacity and then gradually depleted to the
point at which turf-grass approaches light water stress. Excessive irriga-
tion results in water loss, whereas excessive irrigation frequency results
in development of shallow root systems (Fig. 7.3). Soil usually dries from
the surface, remaining moist at greater depth. Roots seek water where it
can be found and therefore often elongate to depths of 50 cm or even
more. Turf-grasses with deep roots are more resistant to water stress and
perform better overall. Considerable evidence also relates formation of
deep roots to a plant hormone called abscisic acid (ABA). Under dry
conditions higher production of ABA slows shoot growth and allows
more carbohydrates to be translocated to the roots, which induce root
growth.
The majority of soils in tropical climates must be irrigated with
10-15 mm of water every 3-4 days. Sandy soils have lower water-holding
capacity and may need lighter irrigation daily, especially in arid climates.
Heavy clays may have poor infiltration rates, which can result in run-off.
Therefore, heavy soils, like sands, may need lighter and more frequent
watering.
