logger are provided: one example with excessive (“over”) irrigation (Fig. 2)
and one with adequate irrigation (Fig.4) using plasticulture. In Figure 2, the
irrigation events consisted of the application of a single daily irrigation event
of 4,718 gal/ac (equivalent to 0.18 in for overhead or seepage irrigation,
or 36 gal/100ft for drip irrigation with 6-ft bed centers in plasticulture.
After each irrigation event, there was an increase in the soil water content
followed by rapid drainage. Large rainfall events may lead to substantial
increases in soil moisture content. On day 2, right after the irrigation, a large
rainfall of 0.44 in occurred, which resulted in a second spike of soil water
content in the same day. The following irrigation (day 3) started when the
the irrigation event of the day 3 could have been safely skipped. Between
day 3 and 6, no irrigation was applied to the crop. The volumetric water content
decreased from 0.14 to 0.08 in3/in3. Due to the very low water holding
capacity of the sandy soils, skipping irrigation for several days could lead to
unneeded crop water stress especially during very hot days or very windy
stage. Between day 6 and 10, large daily irrigation events were repeated,
exceeding the “safe irrigation zone”, and leading to more water drainage
and nutrient leaching.
0.70
0.60
Irrigation events (inches)
0.50
0.40
FC
0.30
0.20
0.10
0.00
0.18
0.16
Volumetric soil water content (in3/in3)
0.14
0.12
0.10
0.08
0.06
0.04
Rainfall 0.03 in
Rainfall 0.01 in
Irrig. events
4,718 gal/ac
(0.18 in)
Drainage Zone
Rainfall
0.44 in
Safe
Irrigation
Zone
1 2 3 4
5 6 7 8 9 10 11 12 13 14
Day
Figure 2. Example of excessive (“over”) irrigation of the upper soil
layer (0 to 6 inch depth) moisture content for drip irrigation under
plastic mulched condition for sandy soils. Black line indicates volumetric
soil water content using soil moisture sensors. Grey line
indicates Irrigation event, single daily irrigation event with volume
capacity line. Arrows indicate rainfall events.
Conversely, Figure 3 shows “adequate” irrigation applications for a 10
day period. In this case, the irrigation event will start exclusively when the
volumetric soil water content reaches an arbitrary threshold. For this partic-
when the volumetric soil moisture content reached values below the soil
3/in3). However, to maintain the soil volumetric water
content in the “safe irrigation zone”, a previous determination of the length
of the irrigation is necessary, to avoid over-irrigation (additional information
about irrigation depths can be obtained in “Microirrigation in Mulched Bed
The example in Figure 3 received irrigation of 943 gal/ac (equivalent to
0.03 in for overhead or seepage irrigation, or 6 gal/100ft for drip irrigation
increase the volumetric water content to a given moisture without exceeding
the “safe irrigation zone”. On average, the volumetric soil water content
root zone. For this particular example, there was no deep water percolation.
In addition, with the information of the soil water status, the irrigation
manager might decide to not irrigate if the soil moisture content is at a
satisfactory level. For example, in day 8, due to a rainfall event of 0.04 in,
there was no need of irrigation because the soil moisture was above the
day 8 was skipped. On the other hand, this “precise” irrigation management
requires very close attention by the irrigation manager. For a given
reason (such as pump issue), the irrigation was ceased in day 5 and it
was resumed late in day 6. As a result, soil water storage decreased to a
certain level, and if the water shortage is prolonged, the plants would be
water-stressed.
Drainage Zone
Safe Irrigation
Zone
0.18
0.16
Volumetric soil water content (in3/in3)
0.14
0.12
0.10
0.08
0.06
0.04
0.70
0.60
Irrigation events (inches)
0.50
0.40
FC
0.30
0.20
0.10
0.00
Irrigation events
943 gal/ac
(0.03 in)
Rainfall 0.04 in
1 2 3 4
5 6 7 8 9 10 11
Day
Figure 3. Example of adequate irrigation management using soil
moisture sensors for monitoring the volumetric soil moisture content
the upper soil layer (0 to 6 inch depth), on drip irrigation under plastic
mulched condition for sandy soils. Black line indicates volumetric
soil water content using soil moisture sensors. Grey line indicates
irrigation event, single daily irrigation event with volume application
Arrows indicate rainfall events.
INSTALLATION AND PLACING OF SOIL MOISTURE SENSOR DEVICES
The use of soil moisture monitoring devices (volumetric or soil water
tension) has potential to save irrigation water in a given vegetable area by
reducing the number of unnecessary irrigation events. However, the effectiveness
of these sensors depends on proper installation in representative
water table levels in seepage irrigation.
Sensors should be buried in the root zone of the plants to be irrigated.
Most of the vegetable crops have 80% to 90% of the root zone in the
upper 12 in, which generally is the soil layer with higher water depletion by
evapotranspiration. For vegetable crops cultivated in rows and irrigated by
drip tapes, the sensors should be installed 2-3 in away from the plant row.
For single-row crops such as tomato, eggplant, or watermelon), the sensor
should be placed on the opposite side from the drip tape, for double row
crops (pepper, squash), the sensors should be placed in-between the drip
tape and plant rows.
Sensors need to be in good contact with the soil after burial; there should
excessively around the sensor. In plasticulture, after the installation, the
area above the sensor should be recovered back with plastic and sealed
with tape.
CROP WATER REQUIREMENT (ET)
Crop water requirements depend on crop type, stage of growth, and
evaporative demand. Evaporative demand is termed evapotranspiration
(ET) and may be estimated using historical or current weather data.
Generally, reference evapotranspiration (ETo) is determined for use as
14 2019 Vegetable Production Handbook of Florida