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Copyright 2005, Board of Trustees, University
GULF COAST RESEARCH AND EDUCATION CENTER
n WATER MANAGEMENT SERIES
Institute of Food and Agricultural Sciences, University of Florida
Institute of Food and Agricultural Sciences, University of Florida
GCREC Bradenton Research Report
Water Management for Drip Irrigated Strawberries
Gary A. Clark
Water Management for Drip Irrigated Strawberries
Gary A. Clark, P.E.
Assistant Professor of Agricultural Engineering
University of Florida
Gulf Coast Research and Education Center
Drip irrigation offers many advantages for production of strawberries.
Water is applied below the plant leaf surfaces and at the root zone of the
plant through controlled discharge emitters which are either embedded into,
embossed onto, or attached to plastic transmission tubing. This method of
water application reduces the potential for foliar diseases and increases
application efficiency. In addition, liquid fertilizer may be injected and
applied as needed to the plant. Thus, prescription water and fertilizer
applications may be made in response to plant needs on a daily or weekly
basis. These applications of drip irrigation are particularly beneficial on
sandy soils which have low water holding capacities and low nutrient cation
Studies performed to evaluate fertilizer amounts, placement, and
supplemental injection have indicated improved results with fertigation using
drip irrigation systems. In addition, water application efficiency can be
improved with drip irrigation as compared with sprinkler irrigation systems or
when scheduling techniques are used. This article will present some general
guidelines for water management of drip irrigated strawberries.
Irrigation System Characteristics
Drip irrigation tubing discharges water from small emission points or
orifices. Tubing flow rates are provided as either gallons per hour (gph) per
emitter or gallons per hour (or minute, gpm) per 100 feet of tube length, and
are related to a certain operating pressure. Most drip tube emitters are not
pressure compensating. That is, the water discharge from an emitter varies
with operating pressure. Thus, it is important to know the recommended
operating pressure and the corresponding emitter water discharge.
Common emitter discharge rates range from 0.2 gph to 1.0 gph, however,
higher and lower values are available. Most field crop drip tubes will be in
the range of 0.3 gph to 0.5 gph per emitter. Discharge per unit length
depends on the emitter discharge rate and spacing with typical spacings of 8,
9, 12, 16, 18, and 24 inches. Closer spacings are available, but are commonly
used on floricultural crops and are limited to short lengths of run. Greater
spacings are also available, but these are commonly used on tree or shrub
crops. Choice of emitter spacing should be based on plant spacing, expected
root distribution and soil hydraulic characteristics. These will be discussed
in additional detail in the following sections.
The tubing discharge per unit length can be converted to tubing
discharge per acre which is useful for irrigation system design and scheduling
purposes. Data in Table 1 may be used to convert from bed spacing and number
of tubes per bed to linear feet of tube per acre. Data inTable 2 converts
emitter water discharge rate and spacing to gallons per minute per 100 feet of
length. The resultant values from Tables 1 and 2 may then be entered into
Table 3 to determine gross water discharge per acre.
Table 1. Conversion from bed spacing and number of tubes per bed to linear
feet of tube per acre.
Bed Spacing Number of tubes per bed
(feet) 1 2 3
Linear Feet of Tubing per Acre
3 14520 29040 ---
4 10890 21780 ---
5 8712 17424 26136
6 7260 14520 21780
Table 2. Conversion from emitter water discharge rate (gph) and emitter
spacing to gallons per minute per 100 feet of length.
Emitter Discharge Rate (gph)
0.2 0.3 0.4 0.5 0.6
Gallons per Minute per 100 Feet
8 0.50 0.75 1.00 1.25 1.50
9 0.44 0.66 0.88 1.10 1.32
10 0.40 0.60 0.80 1.00 1.20
12 0.33 0.50 0.67 0.83 1.00
16 0.25 0.38 0.50 0.63 0.75
18 0.22 0.33 0.44 0.55 0.66
20 0.20 0.30 0.40 0.50 0.60
24 0.17 0.25 0.33 0.42 0.50
Table 3. Conversion from gallons per minute per 100 feet of length and linear
feet of tube per acre to gallons per minute per acre.
Linear Feet of Tubing Discharge Rate (gpm/100 ft)
Tube per Acre
(feet) 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Gallons per Minute per Acre
7000 14 28 42 56 70 84 98
8000 16 32 48 64 80 96 112
9000 18 36 54 72 90 108 126
10000 20 40 60 80 100 120 140
11000 22 44 66 88 110 132 154
12000 24 48 72 96 120 144 168
14000 28 56 84 112 140 168 ---
16000 32 64 96 128 160 --- --
20000 40 80 120 160 --- --- --
25000 50 100 150 --- --- ---
Evaporative Demand and Plant Water Use
The rate and amount of plant water use corresponds to the evaporative
demand for water. Evaporative demand is influenced by solar radiation,
temperature, relative humidity, and wind. Solar radiation is the primary
driver of evaporative demand in humid regions. In more arid regions and
conditions of low humidity, the significance of wind effects on evaporative
demand are greater than in humid climate areas.
Evapotranspiration (ET) is the combined effects of water evaporation
from soil and plant surfaces, and transpiration of water through the plant
leaves and stomata. Potential evapotranspiration (ETp) is the rate of ET from
a well-watered, uniform height, actively growing, green turf or alfalfa type
of crop surface. Water use from other crops (ETc) such as strawberries, is
related to ETp by a crop water use coefficient (kc). The crop coefficient is
the ratio of ETc to ETp. Several methods are available to determine or
estimate ETp and require information such as temperature, solar radiation,
wind speed, and humidity. Crop coefficients depend on the specific method
used to calculate ETp and must be used accordingly.
Pan evaporation is another method used to determine evaporative demand.
Pan evaporation is a common measurement made at most weather stations. In
addition, this is an easy method to use at individual farm or field locations.
For individual use, a galvanized washtub available from a hardware store may
be used and calibrated for the local site. Place the washtub slightly above
the ground surface (a few inches) using a wood frame. Fill the tub with fresh
water and then using an accurate ruler or measuring device, monitor the depth
of water in the tub (pan) on a regular (daily, every other day) basis. The
depth of water evaporated out of the tub is similar to water use by the crop.
However, because of the nature of the pan or tub, such as an open water
surface and small size, the depth of water evaporated out of the tub will be
greater than the use by a well-watered crop.
Water use from a complete crop cover may be about 75 to 85 % of the
depth of water evaporated out of the pan. During the early stages of crop
growth or with crops that do not result in complete cover of the .entire
production area, actual crop water use will be even less. For strawberries,
water use may be on the order of 10 to 15 Z of pan evaporation during the
initial third of the season, increasing to 40 Z of pan evaporation during the
mid part of the season, and reaching 50 to 60 Z of pan evaporation during the
last third of the production season. These are some initial guidelines,
however individual field calibration and checks will be necessary.
These methods of water use are expressed as inches of depth over the
production area. However, row crop drip irrigation applies water in strips
and as gallons per unit length of row. Data in Table 4 may be used to convert
from required inches of application to time of application based upon the
discharge rates from Table 3. Also, remember to add time, depth, or amount of
application to account for irrigation system application inefficiencies. This
can be handled by dividing the required time from Table 4 by the application
efficiency of the irrigation system. Drip systems may average 80 to 90 Z in
water application efficiency. However, actual efficiency depends on system
design and operation.
Table 4. Irrigation system run times based on required depth of application
and system discharge (this table assumes an application efficiency of 100
System Discharge Application Depth (inches)
(gpm/acre) 0.05 0.10 0.20 0.30 0.50 0.75 1.00
Minutes of run time @ 100 Z Efficiency
20 68 136 272 407 679 --- --
30 45 91 181 272 453 679 ---
40 34 68 136 204 339 509 679
50 27 54 109 163 272 407 543
60 23 45 91 138 230 345 460
80 17 34 68 102 170 255 340
100 14 27 54 81 136 204 272
125 11 22 44 66 110 165 220
150 9 18 36 54 90 135 180
Soil properties and characteristics play an important role with respect
to irrigation scheduling and plant water management. The soil has a finite
capacity to hold and store water for crop use. Soils have a certain porosity
or pore volume associated with them. After a thorough saturation, the soil
drains within a day or two to "field capacity" (FC). This is the amount of
water which the soil can hold against the influence of gravity after drainage
has occurred. The plant then extracts water from the soil until the remaining
water is held so tightly that it is unavailable to the plant and permanent
wilting occurs. This is called the "permanent wilting point" (PWP).
The "available water-holding capacity" (AWHC) of the soil is defined as
the difference between PWP and FC. A demonstration typical of a sandy soil is
depicted in the Figure 1 where the saturation, field capacity, and permanent
wilting point levels are at 30%, 16%, and 9% of pore volume, respectively.
The AWHC of this example is [(16X 9%)] 7X by volume. At FC the soil holds
16% water by volume or 1.92 inches per foot of depth. However, at PWP the
soil holds 9% water by volume or 1.08 inches per foot of depth which remains
unavailable. Therefore, the AWHC is 7% or [(1.92-1.08)] 0.84 inches of water
per foot depth of soil at field capacity.
Soil PWP FC Soil
Pore 0% 9% 16% 30%
Volume <------------> <------- ---->< ---------------->1
Unavailable Available Drainage Occurs
Figure 1. Available water-holding capacity example of a "sandy" soil.
Available water-holding capacity ranges from 0.40 to 1.00 inches per foot
(0.75 average) in sands to 1.25 to 1.75 (1.50 average) in sandy loam and sandy
clay loam soils. Soils with greater proportions of clay or silt will have
even higher water holding capacities.
The volume of water available to the crop depends on the root volume of
the crop and the water holding characteristics of the soil. For example, a
crop with a uniformly distributed root zone 10 inches deep is grown on a soil
with 0.84 inches of water available per foot of depth. The available water
for this example is then equal to [(10/12 foot)(0.84 inches/foot)] 0.70
inches. An acre-inch is equal to 27152 gallons, therefore 0.70 inches is
equal to [(0.70)(27152)] 19006 gallons.
Irrigations are generally scheduled when a critical fraction of the
available water above PWP has been depleted. This is used to avoid potential
crop stress. Allowable depletion levels range from 33Z to 67% of the
available water with 50% used as an average. Therefore, if in the above
example a 50% allowable depletion was used, irrigations would be scheduled
when [(0.50)(0.70 inches)] 0.35 inches of water had been depleted from the
Limited Irrigation and Root Zones
Most of the previous discussion focused on irrigation applications which
were applied to the entire field area. When line-source micro-irrigation
tubes are used, only portions of the field are wetted by the irrigation
system. These types of irrigation systems deliver water in slow drips at
discrete locations along the irrigation tubing. The drip locations, emitters,
are located at regular intervals (e.g. 8, 9, 10, 12, 18, or 24 inches).
Sandy soils have poor water distribution characteristics. Lateral water
movement may only be on the order of a maximum of 10 to 12 inches from the
emitter, depending upon the application duration. Therefore, closely spaced
emitters will generally have greater uniformity of moisture distribution
within the soil profile. Greater emitter spacings will work well on the
heavier loam and clay type soils. Individual soils should be tested to
determine the lateral wetting capabilities to aid in selecting an emitter
spacing and in irrigation scheduling.
A Water Budget
A water budget is an accounting or balance procedure used to help
schedule irrigations. This method takes into account the amounts of water
currently in the soil and any additions or deletions which occur. This
information is used to determine if an irrigation is required to maintain the
current soil water level above the allowable depletion. The water budget
Current Storage (CS) Previous Storage (PS)
+ Effective Rainfall (ER)
+ Irrigation (I)
Crop Evapotranspiration (ETc). (1)
The values in Eq. (1) may be expressed in inches if the irrigation and
rooting systems are uniformly distributed over the production area. If it is
more convenient, they may be expressed in gallons per acre or gallons per
irrigated block. The latter method may be more useful for drip irrigated
strawberries grown on raised beds.
It is most convenient to start with a full profile of water in the soil
at FC. This would be the initial level for PS. Effective rainfall is actual
rainfall minus runoff and minus drainage out of the root zone. In some cases,
mulched bed systems for example, many rainfall events may have low
corresponding ER levels. This is because only a small portion of the actual
rainfall is stored in the active root zone of the crop. Irrigation water
additions must be that which is available to the crop by considering the
application efficiency of the irrigation system. The appropriate levels of
'PS', 'ER' and 'I' are added together to determine the available soil water.
Daily levels of ETc are subtracted from the available soil water to
obtain the current storage level. When current storage reaches the allowable
depletion level, an irrigation is scheduled to replenish the profile to field
capacity. After irrigation, the accounting procedure is started again.
Soil Moisture Management
Many times, roots are not uniformly distributed within the bed area and
may be in greater numbers near the plant. Monitoring soil moisture levels can
help to determine if the soil moisture status is within a favorable range and
if the current irrigation management level is appropriate. Tensiometers can
be used to monitor soil moisture levels in the root zone of the crop.
Tensiometers positioned in the active root zone of the crop can indicate if
irrigation water is being placed in the intended locations and if levels are
excessive or sufficient. They can also be used to determine when soil
moisture levels have been depleted to levels which require replenishment.
The use of tensiometers will vary with soil type. The readings which
indicate favorable moisture conditions on one soil will be different on
another soil. In general, field capacity on very sandy soils will be at
tensiometer gauge readings of 7 to 9 centibars (cb), while on loam or clay
soils FC may be 30 cb. Irrigations should generally be scheduled when
readings correspond to the drier end of the available water range which may be
10 to 20 cb on sandy soils and up to 70 or 80 cb on clay and loam soils.
Fertilizer Management Concerns
Fertilizer management is a serious concern when using drip irrigation.
Excessive applications of water will leach soluble nutrients out of the root
zone of the crop. This practice is costly by wasting fertilizer and is an
environmental concern. Fertigation is the application of liquid fertilizers
to the crop through the irrigation system. A good fertigation program can
provided the required nutrients to the crop as needed and in readily usable
amounts. Thus, large amounts of "leachable" nutrients are not present in the
bed (soil). In addition, these prescription applications can be adjusted in
form as well as amount to correspond to current crop requirements.
When using liquid fertilizers it is very important to use only soluble
forms and those which remain in suspension. Precipitated fertilizer sources
will clog the drip tube. It is also recommended to inject solutions which
contain only nitrogen (N) and potassium (K). Phosphorus fertilizers will
precipitate when calcium or magnesium are present in the water supply and
should either be applied preplant as granular, or may be injected in liquid
form as phosphoric acid in a separate injection cycle. This latter practice
will provide some additional cleaning of the system.
Fertilizer injection cycles should not be so long that they exceed the
nominal run time of the irrigation cycle. This can be prevented by proper
sizing of the fertilizer injection pump. Sometimes multiple injection cycles
may be necessary to provide the needed fertilizer even when large injectors
are used. Another key consideration with injectors is to place the injection
point as close as possible to the intended irrigation zone as possible. This
practice minimizes travel time within the irrigation system main and submain
pipes and reduces the overall injection period.
This publication presents information on the scheduling of irrigations
for drip irrigated strawberry production. Topics of discussion included crop
water requirements, soil characteristics, limited irrigation zones, a crop
water budget, and management of soil water. Proper irrigation scheduling must
incorporate the application characteristics of the system with the
requirements of the crop and the water holding characteristics of the soil to
achieve an operable and effective system.
The steps to follow are:
1. Determine the water requirements of the crop and place into units
which are consistent with the method of irrigation management
(inches, gallons, ...).
2. Determine the water-holding characteristics of the soil for
incorporation into the water budget analysis. This can be
expressed as inches per foot of depth, gallons per acre, gallons
per 100 linear bed feet, or some other convenient unit.
3. Be sure to realize the limitations of the root zone of the crop
and of the delivery capabilities of the irrigation system.
4. Develop a water budget by looking at the crop water demands, the
storage amounts, and external replenishment sources (rainfall).
From this budget determine the necessary irrigation schedule to
maintain soil water storage with the allowable depletion level for
5. Finally field checks of soil moisture levels can be used to adjust
irrigation schedules to conform to actual field conditions.
Tensiometers may be a useful tool for these measurements.