Environmental Engineering Reference
In-Depth Information
knowledge of the methods needed for reaching correct decisions is there-
fore important.
The Time to Irrigate
Water stress occurs when the rate of water loss through evapotranspira-
tion exceeds the rate of absorption through the root system. Therefore,
from the physiological standpoint, the best time to irrigate is just before
the turf experiences water stress, which is manifested by wilting. In
reality this timing is difficult to achieve because the use patterns of
some turf-grass areas may prevent irrigation at the desired time and
also because of other issues related to 'turf health' that will be discussed
below. Because irrigation systems cannot be turned on, e.g. during foot-
ball matches or when golfers are playing on the course, turf is usually
irrigated only as near to the ideal time as is practical. Irrigation can
usually be somewhat delayed. The only critical demand is that it be
applied before permanent wilting in order to avoid serious injury and
long-lasting damage to the turf.
The simplest visual evidence that wilt is imminent is visibility of
prints on the turf canopy. The
foot-printing
technique involves walking
across the area and observing how long the turf-grass leaves take to
return to their original, upright position. Depending on the turf species,
turgid leaves return within one to several minutes, whereas wilted leaves
take 15-20 min or more. The foot-printing technique is not always con-
clusive for species with stiffer leaves, such as zoysia grass or cynodon. An
additional guide is turf colour. Water-stressed patches of turf turn bluish
green and can be easily distinguished from areas that have not yet
undergone water stress. Another method is observation of turf-grass
leaves. Water-stressed leaves roll or fold to conserve moisture. When a
considerable proportion of leaves are rolled or folded, turf should be
irrigated as soon as possible. Examination of soil is also quite reliable. If
soil sampled to a depth of 15 cm feels dry, the turf is probably experien-
cing water stress.
As mentioned above, turf is seldom irrigated exactly at the time just
prior to water stress. In addition to factors associated with use of the turf,
a major factor that influences irrigation timing is the need to restrain turf
disease. Many fungal pathogens require the continuous presence of water
droplets for 14-16 h for spore germination and the penetration of turf-
grass leaf tissue. Frequent irrigation or irrigation at times that permit
water droplets to remain on the leaf for more than 14 h results in
increased disease problems (see Chapter 11, this volume). Turf should
therefore not be irrigated in the evening or late afternoon. Midday irri-
gation would be the best if minimizing disease development were the
