Horticulture Principles and Practices

icarbonate, and iron. These chemicals can precipitate out of the water, causing clogging
of pipes and nozzles as in the case of physical pollutants, as well as corrosion of metal
parts if water pH is low. Also, chemicals like iron and manganese can stain walls and
sidewalks. Untreated water can carry a variety of microbes and algae. These problems
are associated with water wells and surface water sources of irrigation water.
Polluted irrigation water is more problematic for drip irrigation systems than
sprinker systems because of the tiny orifices of the emitters and tubes used in microirrigation.
Some filtration is required for such pollution-sensitive systems.
4.8.11 EVAPOTRANSPIRATION AND ITS
EFFECT ON IRRIGATION
Moisture in the root zone of the soil is lost through plants by the process of transpiration
through plant surfaces. Soil moisture is also lost from the soil when water vapor is lost
by evaporation from the soil surface. These two processes produce a combined effect,
called evapotranspiration, that is responsible for most of the water removal from the soil.
The efficiency of irrigation is dependent on evapotranspiration. To minimize this factor,
water must be applied at the right time of day by the most water-efficient method and
protected from rapid loss. Water applied to bare soil is rapidly lost by evaporation from
the soil surface. Mulching, where practical, may be applied to reduce this moisture loss.
Irrigation in the early morning allows water to seep into the soil and thereby reduces the
loss that occurs at high noon. Methods of irrigation vary in water use efficiency; drip irrigation
provides water in the most efficient manner.
4.8.12 BEST TIME OF DAY TO IRRIGATE
The critical considerations in watering plants in the landscape or garden are to deliver adequate
moisture to the root zone, reduce waste, and avoid the persistence of a microclimate
conducive to disease organisms. Water should be delivered at a rate at which it can
infiltrate the soil and for a long enough time to achieve deep wetting of the root zone. Such
watering can happen any time of day. In terms of waste, certain times of day, especially
around noon, provide the greatest opportunity for water loss from the soil and other surfaces
due to evapotranspiration (see next section). Contrary to popular belief, plant leaves
will not be scaled when they are watered at high noon. Rather, moisture loss is very high
at this time of day. The other important consideration is avoiding the persistence of high
humidity and high moisture, which predisposes plants to diseases. Watering plants such
that there will be time for leaves to dry before nightfall reduces disease incidence. Most
landscapes are watered in the early part of the day for the reasons given.
Precipitation Rate
The amount of rainfall
received by an area within
one hour.
4.8.13 PRECIPITATION RATE
It is important that an automatic lawn irrigation system be carefully designed for efficient
and effective high water use. The topography of the area, soil water-holding capacity, and
plant needs must be considered in the design. One factor of paramount importance is the
precipitation rate, the measure of the amount of rainfall received by the area within one
hour. A high precipitation rate is considered to occur when more than 1 inch (2.5 centimeters)
of rain falls within an hour; a low rate is less than 1 inch per hour. An area of
predominantly sandy soil has a high soil infiltration rate and thus requires a high
precipitation rate to satisfy plants’ water needs. Using this information, one can select
a system that will not be overworked (because of use for an excessively long period
at one time).
Sprinkler heads used in subirrigation systems may be divided into three groups according
to precipitation rates. Low-precipitation-rate sprinklers are used where the soil
can tolerate a large amount of water in a short period. Moderate-precipitation-rate sprinklers
are run for a longer time and can have variable output. They are located in areas
that are average in infiltration rate. It should be mentioned that although a homeowner
136 Chapter 4 Plant Growth Environment