Mar_Apr_PCC_2017_Web

Photo Credit: Dan Munk
Water meters are an essential system
evaluation tool.
Groundwater well development and
re-development has become more
common as groundwater levels decline
and surface water availability is
limited.
Subsurface drip irrigation line configuration
for vegetable crops including
onions and garlic.
Continued from Page 28
a non-uniform manner which commonly
leads to larger soil water deficits on low
water application areas and over-irrigating
areas of the field that have higher
than average application rates. To maintain
preferred soil moisture levels in all
areas of the field, water managers typically
compensate by over-applying water on
portions of the field which causes losses
to leaching or run-off both considered to
be non-beneficial uses of water. Properly
designed irrigation systems work
to achieve a high degree of uniformity
and deliver near equal amounts of water
Page 30 Progressive Crop Consultant March/April 2017
throughout the field.
One of the more challenging design
issues in drip irrigation systems
is achieving near uniform pressures
throughout the field. Even when fitted
with pressure compensating emitters,
fields that have large differences in line
pressure are susceptible to significant
differences in emitter output causing
non-uniform applications. This problem
is more commonly associated with long
field lengths which stretch the design
capacities of our current drip irrigation
products. And while many drip irrigation
fields have relatively high distribution
uniformity, maintenance and management
issues continue to leave many fields
with less than optimal efficiencies. Field
lengths of less than 800 feet are less prone
to this concern while it is a much more
common issue in field lengths exceeding
1100 feet. This emphasizes the need to
carefully consider the products being
purchased and the pressure requirements
of that product. Uniformity issues can
also be exagerated as the system ages or
in systems that are not properly filtered
and maintained to avoid biological and
mineral contamination.
Occasionally simple modifications in
surface irrigation systems can also result
in significant improvements in distribution
uniformity and application efficiency.
Many flood and furrow irrigated
systems experience slow water advance
times down the furrow or irrigated
strip before the irrigation is completed
resulting in long infiltration periods at
the head end of the field relative to the
infiltration period at the lower end of
the field resulting in higher amounts
of applied water at the head end of the
field. Solutions to this problem have been
achieved by increasing the on-flow rate
of irrigation water, reducing the size of
the irrigation check and by modifying
the surface soil roughness to allow water
to more freely move to the bottom of
the field. These relatively modest system
design changes can have a significant
impact on delivering water with greater
uniformity and efficiency.
Crop Development, Physiology and
Water Stress Management
Crop sensitivities to water stress are
not constant throughout the growing
season depending on crop type, growth
stage and atmospheric conditions. Most
field and row crops are highly susceptible
to the impacts of limited water availability
during the germination and seedling
development stages and require high
soil moisture availability in the surface
soil during this period. However, as the
early vegetative growth period is initiated
crops like cotton and small grains can
go many weeks before the first seasonal
irrigation water is required. This is
partially due to the rapid root growth
that occurs in these plants and soil water
is more easily extracted during these low
evapotranspiration days. Alternatively,
many vegetable crops including tomatoes,
carrots and the brassicas require
more frequent irrigation events early
season to relieve mild water stress during
this period needed to support a rapidly
expanding plant canopy. Later in the
season when roots are well established
and the lower portions of the soil profile
are exploited, many deep-rooted crops
are less sensitive to water stress and
water management strategies can be
incorporated that take advantage of deep
soil water reserves resulting in delayed
irrigation scheduling.
Similar issues can be observed with
the permanent crops as early season
root flushes occur at different intervals
allowing soil moisture to be exploited at
varied depths depending on crop type.
Generally, crop water stress sensitivities
in permanent crops are often more acute
during early leaf out periods if winter
rains have not fully charged the soil
profile and again during the rapid fruit
growth periods. Monitoring the developmental
stage of the crop often provides
insights to the physiological periods of
the crop that are more sensitive to water
deficits and points to periods when water
deficiencies are more likely to result in
impacts to yield and quality. Understanding
these more sensitive water stress periods
also allows us to better plan on the
time of year to increase crop water stress
monitoring by using tools such as the
pressure chamber or canopy temperature
tools that are used to evaluate the relative
degree of crop stress. And while water
stress limits may differ during the growing
season, using established water stress
guidelines when available, can provide
sound guidance on irrigation scheduling
decisions.
Managing and Measuring Soil
Water Storage
The measurement of soil water can