LowVolume
Irrigation
Scheduling of Citrus
IS001 Quick
Answer
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and Water Resources
University of California
Davis, CA 95616, USA
N.V. O'Connell, Farm Advisor
UCCE Tulare County
Ag Bldg Co Civic Ctr
Visalia, CA 93291, USA
Soil Volume and Saturation
In water balance irrigation scheduling, the soil water content is
either
measured or estimated and the soil is irrigated before the depletion of
water
below field capacity exceeds the yield threshold depletion. In general,
soils
about half solid and half pore space. When saturated with water, then
the soil
is about half solid and half water. If a soil is thoroughly wetted and
then
allowed to drain for a few days, there is a sharp decrease in the rate
of
drainage when it reaches field capacity (q_{F}). If plants are allowed to further deplete
water from
the soil, the water content will decrease until it reaches the
permanent
wilting point (q_{P}), which
is typically about half of q_{F}. The water held in the soil between field
capacity
and the permanent wilting point is called available water (q_{A}). Note
that the values q_{F}, q_{P}, and q_{A }are all
soil water holding capacities with the units inches of water per inch
depth of
soil. To determine the field capacity (FC), permanent wilting point
(PWP), and
available water (AW) content in inches for a known depth of soil, the
water
holding capacities are multiplied by the soil depth (d) in inches
(e.g., FC=q_{F} ´ d, PWP=q_{P}_{´}_{ }d and AW=q_{A}_{´}_{ }d). The
variables FC, PWP, and AW are soil water contents rather than water
holding
capacities.
For irrigation scheduling, the water content of the rooting depth is
needed.
For a soil with uniform water holding characteristics, multiply q_{A} by the
rooting depth to determine the plant available water (PAW). This is the
quantity of water held between field capacity and the permanent wilting
point
within the crop rooting depth. For a soil that changes q_{A} with
soil depth, calculate the available water (AW) by uniform soil layer
and sum
the results to get the plant available water (PAW= SAW) for the rooting depth. Sample q_{A} ranges by soil texture are given in the
following
table in inches per inch and in inches per foot.
Range of available water content by soil texture
Soil Texture 
Available Water (inches in^{1}) 
Available Water (inches ft^{1}) 
Sand 
0.040.06 
0.480.72 
Loamy sand 
0.060.09 
0.721.08 
Sandy loam 
0.070.12 
0.841.44 
Loam 
0.080.15 
0.961.80 
Clay loam 
0.100.18 
1.202.16 
Clay 
0.120.21 
1.442.52 
For example, assuming a rooting depth of 2 ft for a uniform soil having
q_{A}=2.0
inches of water per foot depth of soil, multiply 2.0 ft by 2.0 to get
PAW=SAW=4.0 inches. For soils that are not
uniform with depth, the AW is
calculated by layer and summed to obtain the PAW. A sample calculation
is shown
below.
Sample SAW calculation
with variable q_{A}
Layer 
Depth (in.) 
q_{A} 
AW (in.) 
PAW (in.) 
1 
6 
0.16 
0.96 
1.44 
2 
6 
0.17 
1.02 
1.98 
3 
9 
0.16 
1.44 
3.42 
4 
9 
0.13 
1.17 
4.59 
5 
6 
0.12 
0.72 
5.31 
PAW Correction for Wetted Volume
If not fully wetted, then the PAW calculation needs a correction for
the wetted
volume. If the entire field is wetted, then the full volume (V_{F})
is
calculated as the product of the surface area and the rooting depth.
For a
partially wetted surface area, multiply the wetted surface area by the
rooting
depth to estimate the wetted volume (V_{W}). If the water is
known to
spread laterally below the surface, increase the wetted surface area to
compensate for the lateral spread of the water.
The correction factor (C_{V}) for the wetted volume is
The PAW is calculated as
Example:
If the SAW for the fully wetted soil is 3.6 inches
and a
microsprinkler system wets only 1/3 of the soil volume, then
inches
Yield Threshold Depletion (YTD)
Although plants can potentially withdraw most of the PAW from a soil,
they will
start to experience water stress, which reduces plant growth as well as
photosynthesis and transpiration rates. The water content where a crop
is
expected to start experiencing yield reducing water stress is called
the yield
threshold (YT) and the difference between FC and YT is called the yield
threshold depletion (YTD). The YTD is often defined in terms of the
allowable
depletion (AD), which is the percentage of plant available water
corresponding
to the YTD.
For many crops, the allowable depletion is about 50% of the PAW.
However, the
AD really depends on plant, soil, and weather factors. Generally, an AD
less
than 50% is selected for water stress sensitive crops or crops with
limited
root length density. An AD greater than 50% is chosen for drought
tolerant
crops with an extensive root system. Under conditions of high
evaporative
demand (i.e., high ET_{o} rates), the AD should be
reduced. AD
can be raised under conditions of low evaporative demand. In general,
an AD=50%
should be adequate for citrus.
Example:
Using the PAW=1.33 inches that was calculated earlier and AD=50%, then
inches
Application efficiency (AE)
The net application amounts calculated in the previous example are the
amounts
of water that would be applied to the field if the water were applied
uniformly
over the field and at the correct depth. However, no irrigation systems
apply
water with perfect uniformity and; therefore, some overirrigation is
necessary
to insure that all parts of the field receive adequate water. This
means that
some of the field will receive too much water and the irrigation
application is
less than 100% efficient. For scheduling purposes, both the
distribution
uniformity and the application efficiency are important. The
distribution
uniformity (DU) is a measure of how evenly water soaks in across the
field and
application efficiency (AE) is a measure of how much of the water
applied
contributes to ETc.
The following figure conceptually shows how the application efficiency
varies
when a hypothetical system with perfect distribution uniformity
(DU=100%)
is used to underirrigate, overirrigate, and apply the correct amount.
The
four arrows indicate the depth of irrigation water applied is the same
for all
four quarters of the field. In the upper figure, the application amount
is less
than the soil water depletion (SWD). In this case, all of the water
applied is
stored in the rooting zone, so the application efficiency is AE=100%.
In the
middle figure, all four quarters of the field receive twice as much
water than
the soil can hold, so only half of the water applied is stored in the
rooting
zone and AE=50%. In the bottom figure, all four quarters of the field
receive
an amount exactly equal to the SWD and AE=100%. The bottom figure would
be an
ideal irrigation where all of the field would receive exactly the
amount needed
to bring the water content back up to FC.
The following figure conceptually shows how the AE varies for a
hypothetical
irrigation system with a nonperfect distribution uniformity
(DU=80%).
In the upper figure, none of the four quarters receives an amount
greater than
the SWD, so the AE=100%. In the middle figure, the low quarter receives
120% of
the SWD, so the other quarters also receive more than needed. In this
situation, less than 80% of the water applied is stored in the rooting
zone, so
the AE<80%. In the bottom figure, the low quarter application
exactly
matches the SWD, and approximately 80% of the water applied is stored
in the
rooting zone, so the AE=80%. Note that 3/4 of the field is over
irrigated when
the application to the low quarter equals the SWD before irrigating.
For a welldrained field, if there is no runoff from the field
and if
the gross application (GA) is equal to NA divided by the DU (expressed
as a
fraction) then the AE is approximately equal to the DU. The goals in
good
irrigation management are to apply
and to design and maintain the irrigation system with the highest DU
possible.
This will result in the highest possible AE while insuring that the
actual
application amount to the fourth of the field receiving the least water
is
equal to NA. In general, this should provide the best possible
irrigation
management. However, if salinity is a problem, then the NA should be
increased
to include a leaching requirement before calculating GA.
Example
If NA=0.84 inches and the DU=0.90, then
inches
Since 0.67 inches corresponds to the YTD that was calculated earlier, a
GA=0.74
inches is the maximum gross application that should be applied.
Distribution uniformity (DU)
The distribution uniformity is found by performing a system evaluation.
Sample
flow rates are measured from a representative sample of emitters. A
volume of
water is collected from each sample emitter over a fixed time period
and the
flow rate in gallons per hour per emitter (or liters per hour per
emitter) is
calculated. Then the samples are ranked from smallest to largest and
the mean
of the low quarter (M_{LQ}) is calculated as the mean of the
1/4 of the
samples receiving the least volume of water. Then the overall mean (M_{ALL})
is computed and the DU is calculated as
The DU is usually multiplied by 100 and reported as a percentage.
However, use
the DU as a fraction to calculate the gross amount to apply (GA).
The following table illustrates the procedure used to calculate the DU.
Column
2 contains the water collected in gallons per hour in the order
sampled. The
same data are ranked from smallest to largest in column 3. Because
there are 20
samples, the mean of the five smallest numbers is the mean of the low
quarter
(M_{LQ}). In this data set, the MLQ=0.89 gph. The overall mean
is MALL=
1.06 gph. Then, the DU is calculated as the M_{LQ} divided by
the
overall mean (M_{ALL}) or DU=0.84=84%.
Runtime (RT)
After computing the gross application, the runtime (RT) needed to apply
the GA
is found by dividing GA by the application rate (AR). The AR is equal
to M_{ALL}
from the system evaluation, but it is first converted to inches or
millimeters.
To convert from gph per emitter to inches applied to the field per
hour,
multiply M_{ALL} by the number of emitters per acre and divide
by 27154
gallons per acreinch.
inches per hour
Example
If M_{ALL}=5.88 gph per emitter and there are 90 emitters per
acre then
inches per hour
Therefore, a 24hour runtime would result in a gross application of
inches
and, for a welldrained soil, if the AE=DU=0.90, then this corresponds
to a net
application of
inches
Therefore, any net application bigger than 0.43 inches will require
more than a
24hour irrigation
Management Allowable Depletion (MAD)
The management allowable depletion (MAD) is the soil water depletion
value that
is used to time irrigation events. The MAD is selected to fit with
other
management constraints, but it should always be smaller than the YTD to
avoid
yield reducing water stress.
For illustrative purposes, a net application (NA) corresponding to a
24hour
set will be used for the MAD. In our earlier examples, the NA for
24hour set
was 0.43 inches, which is smaller than the YTD values 0.67 inches.
Therefore,
no problem with yield reduction is expected if MAD=0.43 inches is used.
Irrigation Scheduling
The calculated YTD is useful to time irrigation. Crop
evapotranspiration (ETc)
is calculated on each day and the cumulative ETc provides an estimate
of the
soil water depletion (SWD). The crop should be irrigated before the SWD
exceeds
the YTD.
Example
The following is a scheduling example for a microsprinkler irrigated
citrus
crop. Assuming the SAW=4.0 inches, AD=50%, and C_{V}=0.33,
the PAW=1.33 inches and the YTD» 0.67 inches. The application
rate is 0.020 inches per hour and a 24hour set results in a GA=0.48
inches. A
system evaluation was performed and the DU=0.90, so the NA
corresponding to a
24hour set is NA=0.43 inches. For the schedule, a MAD=0.43 is used.
Assuming
the SWD=0 on day 0, the following table shows how the irrigation is
timed during
a typical summer period.
Irrigation scheduling example for citrus
Day 
ETc 
SWD 
NA 
GA 
RT 

Inches 
Inches 
Inches 
Inches 
Hours 
1 
0.17 
0.17 



2 
0.16 
0.33 
0.33 
0.37 
18.5 
3 
0.15 
0.15 



4 
0.17 
0.32 
0.32 
0.36 
18.0 
5 
0.14 
0.14 



6 
0.15 
0.29 
0.29 
0.32 
16.0 
7 
0.16 
0.16 



8 
0.17 
0.33 
0.33 
0.37 
18.5 
9 
0.17 
0.17 



10 
0.16 
0.33 
0.33 
0.37 
18.5 
Note that irrigation is applied on the day before the SWD exceeds the
MAD. It
is assumed that an application will return the soil water content to
FC, so the
SWD is reset to zero on the irrigation date. The SWD on the next day
following
an irrigation equals ETc on that day. In all cases, GA is calculated by
dividing NA by the DU and RT is computed by dividing GA by AR. This
procedure
is repeated throughout the season.
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