Download design, scheduling and operation of turf irrigation systems

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Design, Scheduling and Operation of
Tuti Irrigation Systems
Fedro S. Zazueta and Allen G. Smajstrla
AgriculturalEngineering Department
hxitute
of Food and Agricultural Sciences
University of Florida
Gainesville, FL 32611
Introduction
Increasing urban growth in Florida has placed additional demands on the State’s water
resources. In 1974 there were over 900,000 acres of turf in the State with an industry value
of $524 millionl. Current estimates indicate that the industry has increased in value to well
over $1.5 billion2, with a large percentage of this being small areas under some form of
irngatiom Urban growth has resulted in localized water shortages due to demand exceeding
supply. It is clear that water use efficiency needs to be improved throu@ the use of better
strategies and procedures for desi~ schedulkg and operation of turf irrigation systems.
Design
The design of a turf irrigation system may be defined as the art and science of
selecting and properly applying all components of the system. Because numerous
components comprise an irrigation systeq and because many state and local regulations
may be applicable, designers are encouraged to consult applicable standards to ensure that
the- system-will be safe, economical and functional.
Standards for Turf Irrigation
System Design
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A comprehensive set of standards for the design and installation of turf and
landscape irrigation systems has recently been developed by the Florida Irrigation Society,
1 Florida Turf Survey. Florida Crop and Livestock Reporting Service. State of Florida.
1974.
2 i&E. Dudeck personal communication. 1989
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FIS3. These standards provide a wide range of technical guidance for irrigation
practitioners, water supplies, resource regulators, and inspectors. In developing these
standards, the goal of the FIS was to increase the reliability, life-cycle cost effectiveness, and
water use efficiency of turf and landscape irrigation systems.
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The design criteria section of the FE irrigation standards provides guidance in the
selection of a water source, system components and layout. It also gives methods for the
estimation of the system application rate, uniformity of water applicatio~ and pressure
losses in the system. Guidelines are also given for the selection of pipe sizes and pressure
ratings, sizing thrust blocks for pipelin~ and selecting pumps, valves, and backflow
prevention devices. Sections of the standards are itemized so that this document can be
used as a checklist to ensuxe that all required system components are properly considered
when turf irrigation systems are designed.
Design Software Tools
Software is available that simplifies some of the tasks involved in the design of turf irrigation
systems4 . The following examples are brief descriptions of some of the programs that are
part of the Water Management Utilities package. Due to space limitations only examples
of the some of the 38 programs contained in the package are be given here:
IRREQ:
Irrigation requirements estimator. This utility allows long-term irrigation
water requirements to be estirniited from a water balance and a salt balance
in the crop root zone. Monthly and seasonal irrigation requirements are
calculated from inputs of monthly evapotranspiratio~ monthly rainfall, soil
water storage, and salt concentrations of the irrigation water, drainage water,
and rainfall.
HAZEN:
Hazen-Williams equation calculator.
This utility allows the dynamic
redefinition of any of the parameters in the Hazen-Williams equation as a
dependent variable and continually updates the dependent variable when the
value of any independent variable is changed.
TURF:
Turf irrigation system design This utility calculates flow rates, pressures and
pipe diameters of open pipe irrigation networks with prescribed flows. The
program considers irregular geometries and changes in elevation.
3 Florida Irrigation Society. 1989. Standards for the Design and Installation of Turf and
Landscape Irrigation Systems. Florida Irrigation Society, Inc. Orlando, FL
4 Zazuet~ F.S. and AG. Srnajstrla. Water Management Utilities. Computer Support
Office, IFAS Bldg. 120, University of Florid% Gainesville, FL 32611. Tel. (904) 392-7853.
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PUMP:
Selection of irrigation pumps. This utility computes parameters for pump
selectio~ including total dynamic head, NIT@ water and brake horsepower.
It also computes operating costs for different types of power units.
Scheduling
Water Use and Evapotranspi.ration
Water is used by turf grass in several ways: 1) assimilation into the pkmt in the
growth processes, 2) &rect etiporation horn the soil; plant or other surface;, especially
when these surfaces are very wet such as immediately after rain or irrigatio~ 3)
transpiratio~ which is evaporation from plants, primarily through the stomata in plant
leaves, and 4) other uses such as leaching of salts or crop cooling during plant establishment.
Less than 1%, of the water used in turf growth is assimilated into plants. Other uses,
(category 4, above) maybe signitlcan~ depending on water quality or for limited times such
as during plant establishrnen; but they depend on factors other than maintaining adequate
soil water content thus they are not considered in this publication.
Most of the water applied to meet the water requirements of a crop is used in
evaporation and transpiration Evaporation and transpiration are important in cooling of
plant surfaces, to maintain temperature in the range that permits photosynthetic activity and
growth to occur. Transpiration is also required to transport nutrients into and within plants.
Because it is difficult to separately measure evaporation and transpirat.io~ the two processes
are normally combined and called evapotranspiration (ET).
Evaporation is the change of state of water from a liquid to a vapor. Energy is
required for this process. If turfgrass is well watered the amount of water vaporizing and
moving into the atmosphere in a humid region such as Florida is primarily determined by
the amount of energy available from solar radiatio~ although air temperature, humidity and
wind also affect ET rates.
Because climate factors largely determine ET, several climate-based equations have
been developed to estimate ET. A summary and discussion of several ET equations and
their modifications were presented by a committee of II?AS research scientists. They
stated that those ET eq;ations whi;h can be applied on a daily basis for imigation
scheduling require that solar radiation be measured. They concluded that the Penman
equation was the most accurate climate based ET estimation method and recommend its
use for Florida conditions.
5 Jones J.W., IX. Alle~ S.F. S~ J.S. Roge~ LC Hammond, A.G. Smajstrla and
J.D. Martsolf. 1984. Estimated and Measured Evapotranspiration for Florida Climate,
Crops and Soils. BuL 840 (Tech.) IFAS, Univ. Fla., Gainesville, FL
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As an alternative to radiation-based ET equations, a simple physical estimate of ET
can be made by measuring the water which evaporates from an evaporation pan. When an
evaporation pan is located in the area being irrigate~ it is subject to the same climate
factors as the irrigated are% and its rate of water depletion has been demonstrated to be
proportional to ET rates of crops. The correction factor required to estimate the actual ET
depends on the type of crop and pan used. When the National Weather Semite Class A
Evaporation Pan is use~ daily ET from turfgrass can be estimated by multiplying the daily
measured pan evaporation by 0.7,
The use of an evaporation pan has the advantage of ease of data measurement, but
regular maintenance of the pan is required. The use of ET estimation equations requires
climate data (especially solar radiation) that are not always available to individuals.
Therefore, irrigation schedules are often based on historical climate conditions (historical
records of ET) with modifications of schedules made for extreme weather patterns and for
rainfall.
The Water Budget
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The water budget procedure is also called a water balance or book-keeping
procedure. It is similar to keeping a bank account balance. If the balance on a starting
date and the dates and amounts of deposits and withdrawals are kno~ the balance can be
calculated at any time. Most importantly, the time when all funds (or water) would be
withdrawn can be determined so that an overdraft is avoided (or an irrigation can be
scheduled).
The water budget for irrigation scheduling on a daily basis can be writteu
S= R+
I-ET -(D+RO)
(1)
where:
s
R
I
ET
D+RO
=
=
=
=
=
change in soil moisture storage (inches),
rainfall depth (inches),
irrigation depth (inches),
daily evapotranspiration (inches),
drainage plus runo~ calculated as rainfall in excess
of that which can be stored in the soil to field
capacity (inches).
The soil water content on any given day (i) can be calculated in terms of the water
storage on the previous day (i-l), plus the rain and irrigatio~ and minus the ET, drainage
and runoff that occumed since the previous day as:
S(i)=
S(i-l)+
R+
I- ET-(D+RO)
(2)
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The starting point for water budgeting for irrigation scheduling is normally after a
thorough wetting of the soil profle by irrigation or rain. This brings the soil to field
capacity, which is the depth of water that can be held against the force of gravity. This
method requires that the amount of water av”~ble for plant use at field capacity and the
amount that can be depleted before an irrigation is scheduled be known. Often 50 percent
of the available water is “allowed to be depleted before an irrigation is scheduled. Daily
measurements or estimates of ET are subtracted horn the available soil water unti the soil
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water has been depleted to the level at which an irrigation is scheduled.
Methods for irrigation scheduling
Two questions must be answered when scheduling irrigation
1) When to irrigate? Water should be applied to turf only when soil-water in the root
system has been depleted to an unacceptable level, usually when 1/2 to 2/3 of the stored
soil-water has been depleted. There are several ways to determine when the soil-water
reservoir has been depleted beyond an acceptable level. These are:
a) Visual inspection of the turf.
Becoming familiar with the way that turf reacts when water is becoming scarce in the
soil is a common method of deciding when to apply irrigation. Common symptoms
of water stress include leaf color changes to a bluish-gray tint, footprints linger long
after being made, and leaf blades may curl or fold.
b) Estimations based on climatic records.
Past history of climate can be used to estimate how often irrigation should be applied
to turf. Fig. 1 shows the expected number of days that soil-water will last for a
typical Florida sandy soiI that can hold 1 inch of water per foot of soil and that is
irrigated to a depth of one foot. If your root depth and soil-water holding capacity
are different, m~tiply the number of days in Fig. 1 by your root depth a& h-en b;
your soil-water holding capacity.
c) Direct measurement of soil moisture.
Using sensors for measuring the amount of available water in the soil is another way
to determine when to irrigate. One such sensor is the tensiometer (See Fig. 2). As
the soil dries water moves out of the tensiometer through the ceramiq causing a
suction that is measured by the vacuum gage. Tensiometers can be used to
automatically schedule irrigation.
t,
6 Smajstrl~ AG., D.S. Harrison and F.X DurarL 1984. Tensiometers for Soil Moisture
Measurement and Irrigation Scheduling. Ext. Cir. 487. IFAS, Univ. l%., Gainesville, FL
2) How much water should be applied?
The amount of water applied to turf should not
exceed that which will refill the soil occupied by
the root system (poor quality water may require
- higher voluxries for leaching of salts). For an
established tufi the amount of water applied at
each irrigation should always be the same,
regardless of the time of year (Irrigation will not
be required as often during low demand periods).
Typi&d Florida soils will b> able to hold-about 1“
Nigure 1.
of water per foot of soil. The amount of water to
apply to a 1 ft deep soil water reservoir that has
been depleted to 50%, and has a water holding
capacity of 1“/ft is 2/3” of water. This assumes
an irrigation efficiency of 7570. The total volume
requir~d will depend on the size of the area
being irrigated and the soil-water holding capacity.
Number of days soil
water stored in the root
system wiU last before
irrigation
or rain is
require&
Software tools
SCHED:
Irrigation scheduling. MaintairJs a
water budget of a soil to predict
the duration and timing of
irrigation. This utility includes ET
estimation by the pan evaporation
and Penman
methods,
and
estimation
of leaching
requirements.
Figure 2.
Tensiometer.
Operation
Overview of Operation Methods
Operation of an irrigation system consists of those actions required to apply water to the
irrigated turf on a day-to-day basis. Systems can be operated using a variety of approaches
depending on when and how much water is to be applied.
a) Manual.
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This type of operation requires that the manager of the system manually open the
valves to the irrigation system and apply irrigation for the proper duration according
to the management strate~ followed, For manually operated systems the best
recommended strategy is to base irrigation frequency on visual indicators of plant
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stress. This type of operation may be very inefficient if close attention is not paid
to irrigation duratioq however, when proper amounts are applied
it ranks as one of
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the best methods.
b) Scheduled.
This we of system uses a clo& such as an irrigation timer-controller to oDerate the
irrigation system Normally clocks are set to figate using long-term climatic data
based schedules, with corrections or interruptions being made to account for weather
variatio~ particularly rainfall.
A
c) Soil-moisture sensor interrupted schedule,
This type of system is essentially the same as the previous @e, however, operation
of the irrigation system as specified by the settings of the clock is interrupted if the
soil-moisture has not reached a preset threshold. -Avaiiety of commercially available
sensors can be used, such as swi~chingtensiometers, cond&tivity probes, c~pacitance
probes, and others.
d) Sensor driven schedule.
This type of system uses a data acquisition interface to determine the soil-water
status. The soil-moisture status may be determined indirectly or directly.
.- Indirect methods.
-
These methods estimate the amount of water available to the turf at any time by
calculating a water balance of the root system, Inflows, such as rainfall and irrigation
are measured. Outflows, such as ET and drainag~ are estimated by using measured
climatic data and some model to predict these parameters. For example, the
estimation of ET can be made by using the pan evaporation method or the Penman
equation. These methods require higher level hardware support than clocks and
timers, usually a microprocessor-based controller.
.- Direct methods.
These methods measure the distribution of soil moisture throughout the irrigated
zone using soil moisture sensors Using these da@ the amount of water available
can be determined. In additioq the system actuates the irrigation system when the
allowable depletion level has been reached. These methods also require a
microprocessor-based controller.
e) Heuristic and sensor driven schedule.
These methods incorporate both the direct and indirect approaches and also use realtime ~ert system technology to make decisions that will result in system operation.
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These systems incorporate experience (expert rules) into the water management
process, allowing for considerations other than soil moisture level maintenance, such
as disease-environment relationships.
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Comparison of Water Management Strategies
A turf irrigation demonstration conducted by the IFAS Cooperative Extension Service
demonstrated the effects of water management strategies and operation on total water use.
Six plots with St. Augustine and Bahia grass where used to measure water use for the
following management strategies: 1) Sensor interrupted schedule (St. Augustine only), 2)
visual indicators of stress, 3) scheduled using a long-term water balance, and 4) no
irrigation (Bahia only). Table 17 demonstrates that water applications were reduced by
about 1/3 when irrigations were scheduled based on tensiometer or visual appearance rather
than Iong-term average climate conditions. J%ISO,
both tensiometers for soil-moisture sensing
and visual appearance were equally effective”for irrigation scheduling.
TABLE 1
Summary of preliminary results for
the turf irrigation management trials.
(inches)
ORANGE
(inches)
FIAGIXR
(inches)
BREV*
(inches)
St. Augustine
(Tensiometer)
20.0
“19.7
15.7
N/A
St. Augustine
(Scheduled)
26.6
25.2
36.4
N/A
St. Augustine
(Appearance)
17.6
18.2
17.7
N/A
Bahia
(No irrigation)
9.2
8.1
10.7
N/A
Bahia
(Scheduled)
21.0
25.2
36.4
N/A
Bahia
(Appearance)
16.5
18.2
17.7
N/A
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7 Project design: F. Zazueta (irrigation) and B. Mcarty (turf management). Project
execution: F. Jassa (Brevard 0.), D. Schrader (Flagler Co.), R. Swanson (Lake Co.) and
T. MacCubbin (Orange Co.). Project funded by St. Johns River Water Management
District.
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Software tools
Several computer control systems having a wide range of features and cost are available
commercially. In additio~ software is available horn the Cooperative Extension Semite.
IFAS, University of Florida. This software for digital computer ;ontrol of irrigation syste~
controls devices on a real-time basis. It includes a manual operation mode, a timer mode,
and an automatic mode that operates using soil moisture sensors. This software has been
implemented only on VIC-20, Commodore-M, and IBM PC compatible computers.
Evaluation
Standards for Irrigation System Evaluation
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The Florida Irrigation Society ~tandards for Des ~ and Installation of Turf ad
dscape Irrigation Svsterns presents standard practices ;or evaluating turf and landscape
typ;s of irrigation systems. Standard evacuation practices irdude periodic visual inspections,
leakage testing pressure distribution testing, application uniformity testing, and backflow
prevention assembly testing. These evaluations will help to ensure that an irrigation system
can be operated safely and effkiently.
Visual inspections are required to verify that coverage is adequate, that over-spray
onto paved or non-vegetated areas is avoided and that controllers cycle properly in
automated irrigation systems. Lxdcage testing can be done visually as well as by measuring
pressure losses to detect and repair defective pipes or other components. Measuring
pressure distributions within zones during system operation and comparing them with design
vahes will allow hydrauIic problems to be identified. Measuring water distributions with
catch cans will enable both application rates and uniformity of water application to be
calculated. Testing backflow prevention assemblies to ensure that they function properly
will help to ensure that the irrigation water supply will be protected.
Field evaluations
A Mobile Irrigation Laborato~ (ML) was designed to evaluate currently installed irrigation
systems with respect to adequacy of desi~ installation scheduling practices, operatio~ and
maintenance. The major components of the MD consist of a van equipped to carry out the
following tests:
1)
2)
3)
4)
Visual inspection of the irrigation system
Pressure distribution test.
Uniformiw of application test.
Potential efficiency estimation.
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Ip additio~ the MIL is equipped to provide custom reports that include recommendations
to the irrigation manager. The report may include corrective actions, maintenance
recommendations, scheduling recommendations, and best management practices for the type
of turf irrigated. Table 2 shows a summary of the performance of various systems that have
been testecL
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TABLE 2a
Summary of results fro”mfield tests using the
Mobile Irrkation Laborato#.
u
d
DU
Cu
(%)
(%)
Discharge
Variation
(%)
Pressure
Variation
(%)
Matched
Discharge
(%)
1
Reside&1
66
81
160
27
42
0.18
2
Residential
139
38
94
49
69
0.31
3
Residential
34
10
76
56
64
0.34
4
Residential
157
43
108
41
58
0.13
5
Residential
47
3
106
61
72
0.41
6
Residential
9
19
91
38
50
0.18
7
Commercial
33
2
33
66
78
039
Site
-
Av.
App.
Rate
(in/hr)
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8 Field tests Carried out by J. Creighton and S. Burtle. Project funded by Southwest
Regional Water Management Authority.
TAB X 2b (Continued)
Summaiy of esults from field tests using
the Mob [e Irrigation Laboratory.
Allowable
depletion
(%)
Site
Adequaq
level
(%)
Potential
Efficienq
(%)
Deficit
Fraction
(%)
1 Residential
I 60
50
63
51
2 Residential
I 60
60
80
38
3 Residential
60
4 Residential
50
5 Residential
I 50
31
6 Residential
50
40
7 Commercial
60
22
27
40
I 78
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The pressure and discharge variations in Table 2a were calculated as the difference between
maximum and minimum as a percentage of the average, The matched discharge is the
sprinkler discharge weighted by the size of the arc it serves (Sprinkler arc/360). DU, the
distribution uniformity, was calculated from catch-can volumes as the average of the lowest
quarter as a percentage of the mean. CU, the coefficient of uniformity was calculated from
catch-can data as the average deviation horn the mean as a percentage of the mean. The
adequacy level in Table 2b is the portion of the irrigated area where the irrigation
requirements are met. The potential efficiency refers to the maximum theoretical efficienq
that can be achieved operating the irrigation system under the test conditions. The potential
efficienq is determined from the application rate pattern in the tested area and the
irrigation requirement at a selected adequaq. The deficit fkaction is the percentage of the
water volume in the under-irrigated area that is required to bring the under-irrigated area
to field capacity.
Software
tools.
The software used in the mobile irrigation laboratory will be made available to the general
public through the IFAS Computer Support Office. The software requires a IBM-PC
compatible computer with a hard disk and VGA color graphics. In addition if the reporting
and database capabilities are desired WordPerfect (Tin) and DBXL (Tin) are required.
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Summary
Proper design, scheduling and operation of turf irrigation will help to ensure efficient water
use for turf irrigation. Guidance for system design is provided in recently published
standards, and assistance with hydraulic calculations is available horn computer software.
Turfgrass evapotranspiration can be estimated from climate conditions, or direct soilmoisture measurement. A water budgeting approach allows long-term climate records to
be used to develop standwd schedules, plus it allows the amount of water to be applied to
be determined. Turf irrigation systems can be operated using methods ranging from manual
system control to computer controllers which use decision making software. In field plot
studies, scheduling based on visual appearance and soil moisture sensors greatly reduced
irrigation applied as compared to schedules based on long-term climate conditions.
Computer software which is available for irrigation ~stem scheduling and control was
presented.
Irrigation standards provide guidelines for irrigation system evaluation as well as design and
installation. Results of evaluations of residential and commercial irrigation systems
conducted by a mobile irrigation laboratory demonstrate the measurements that must be
made to evaluate the efficiency of turf irrigation systems. Computer software was presented
that performs the required calculations and makes recommendations for corrective actions,
if confections are required.
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