Table 3.3. Summary of irrigation scheduling guidelines for vegetable crops grown in Florida.
Irrigation system1
Irrigation scheduling component Seepage2 Drip3
1- Target water application rate Keep water table between 18 and 24 inch depth Historical weather data or crop evapotranspiration (ETc) calculated from
2- Fine tune application with soil
moisture measurement
Monitor water table depth with observation wells Maintain soil moisture level in the root zone between 8 and 15 cbar (or 8%
3- Determine the contribution of rainfall Typically, 1 inch rainfall raises the water table
by 1 foot
4- Rule for splitting irrigation Not applicable. However, a water budget can be
developed
5-Record keeping Irrigation amount applied and total rainfall received 4
Days of system operation
1
2
3 On deep sandy soils
4 Required by the BMPs
water tension (SWT) represents the magnitude of the suction (water potential,
negative pressure) the plant roots have to create to free soil water from
the attraction of the soil, and move it into the root cells. The drier the soil,
the higher the suction needed, hence, the higher SWT. SWT is commonly
expressed in centibars (cb) or kilopascals (kPa; 1cb = 1kPa; 7kPa = 1psi).
For most vegetable crops grown on the sandy soils of Florida, SWT in the
and 15 cb. Because of the low AWHC of Florida soils, most full-grown vegetable
crops will need to be irrigated daily. During early growth, irrigation may
be needed only two to three times weekly. SWT can be measured in the
measuring devices, consult IFAS circular ABE326 ‘Using Tensiometers for
Vegetable Irrigation Scheduling in Miami-Dade County’ at <http://edis.ifas.
>.
Within the category of volumetric sensors, capacitance-based sensors
have become common in recent years due to a decrease in cost of
electronic components and increased reliability of these types of sensors.
However, sensors available on the market have substantially different
accuracies, response to salts, and cost. Soil moisture sensors are detailed
in the publication, “Field Devices for Monitoring Soil Water Content” (http://
-
ume of water in a sample volume of undisturbed soil ft3/ft3 or percentage.
This quantity is useful for determining how saturated the soil is (or, what
it is expressed in terms of depth (volume of water in soil down to a given
depth over a unit surface area (inches of water)), it can be compared with
other hydrologic variables like precipitation, evaporation, transpiration, and
deep drainage.
PRACTICAL DETERMINATION OF SOIL FIELD CAPACITY USING VOLUMETRIC
SOIL MOISTURE SENSORS
It is very important that the irrigation manager understands the concept
optimum soil moisture for plant growth, productivity, and reduction of fertilizer
nutrient leaching. Figure 1 represents volumetric soil water content (VWC) at
depth of 0-6 in measured by a capacitance sensor during a period of 4 days.
irrigation depth that resulted in saturation of the soil layer, in this particular
case 0-6 in. The depth of irrigation applied was 4,645 gal/ac (equivalent to
0.17 in for overhead or seepage irrigation, or 34 gal/100ft for drip irrigation
reference ET or Class A pan evaporation
and 12%) available soil moisture
Poor lateral water movement on sandy and rocky soils limits the contribution
of rainfall to crop water needs to (1) foliar absorption and cooling of
foliage and (2) water funneled by the canopy through the plant hole.
When plants are small and fully grown, irrigation events greater than 12
and 50 gal/100ft, respectively (or 30 min and 2 hrs for drip tapes with
Irrigation amount applied and total rainfall received 4
Daily irrigation schedule
with 6 ft bed centers in plasticulture) in a single irrigation event. Right
after the irrigation events, there was a noticeable increase in soil moisture
content. The degree to which the VWC increases, however, is dependent
upon volume of irrigation, which is normally set by the duration of irrigation
event. For plastic mulched drip irrigation in sandy soils, long irrigation events
result in a relatively large increase in soil moisture in the area below the
drip emitter. The spike in soil moisture appears to only be temporary, as the
irrigation water rapidly drains down beyond the 6-inch zone (observed by the
decrease in VWC). This rapid spike in soil water content indicates that the
VWC rapidly reaches a point above the soil water holding capacity and the
water percolated down to deeper soil layers. Between the end of day 1 and
day 3, the VWC declined at a constant rate due to some soil water extraction
by drainage, but most extraction was due to evapotranspiration during the
day. For sandy soils, the change in the slope of drainage and extraction
lines, in other words, changing from “rapid” to “slower” decrease in soil water
has moved out from the large soil pores (macropores), and its place has
with water and will supply the plants with needed moisture.
Figure 1.
0-6 inches soil depth after irrigation event using soil moisture sensors.
EXAMPLES OF IRRIGATION SCHEDULING USING VOLUMETRIC SOIL
MOISTURE SENSOR DEVICES
In this section, two examples of irrigation management of vegetable
crops in sandy soils using soil moisture sensor readings stored in a data
2019 Vegetable Production Handbook of Florida 13