Paper Number: 022254
An ASAE Meeting Presentation
Sprinkler irrigation systems design using ISADim
Vitor Miguel Abreu,
Centro de Estudos de Engenharia Rural/ISA/UTL, Tapada da Ajuda 1349-017 Lisboa
Portugal, vitorabreu@ip.pt
Luís Santos Pereira,
Centro de Estudos de Engenharia Rural/ISA/UTL, Tapada da Ajuda 1349-017 Lisboa
Portugal, lspereira@isa.utl.pt
Written for presentation at the
2002 ASAE Annual International Meeting / CIGR XVth World Congress
Sponsored by ASAE and CIGR
Hyatt Regency Chicago
Chicago, Illinois, USA
July 28-July 31, 2002
Abstract. ISADIM is a software tool intended to design and/or simulate set sprinkler irrigation
systems. The irrigated area topography is modeled as a set of triangular surfaces, allowing the
sprinkler positions to be calculated as the layout of the irrigation system is drawn on the screen.
ISADIM calculates head and discharge distributions along laterals and mainlines, based on the
available head at the system inlet.
ISADIM aims to be user-friendly. Irrigated areas, laterals and mainlines can be easily drawn with a
few mouse clicks. Pipe diameters, sprinkler characteristics and other required data are gathered
interactively during the drawing process. The resulting layout provides a visual image of the system,
which simplifies modifications and error detection.
ISADIM runs under WINDOWS® 95 or later.
Keywords. sprinkler, design, irrigation, simulation.
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reflect the official position of the American Society of Agricultural Engineers (ASAE), and its printing and distribution does not
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Introduction
Simulation models can be quite useful for sprinkler irrigation design and analysis. Examples of
these models are the software in Portuguese language developed by Correia (1997) and
SPRNKMOD (Andrade and Allen, 1997). The later has a wide application and includes not only
the sprinkler system but also the pumping equipment.
ISADim is a computer aided design software for set sprinkler irrigation systems. The application
allows the design of a simplified layout of the irrigation system, including fields, sprinklers,
laterals, manifolds and mainlines. With the collected data, ISADim computes the flow rates and
heads along the system, as well as a set of parameters quantifying the performance and the
quality of the designed (or analyzed) irrigation system.
This paper includes a brief description of the calculation methods and basic user instructions of
the main functionalities of the application. Although ISADim is not an open source project, in the
methods description use is made of the application source code in order to provide an accurate
explanation.
Model concept
To achieve a well-designed irrigation system using ISADim, one must provide a suitable
description of the fields and of the selected irrigation equipments. The resulting layout must also
comply with the following generally accepted standards:
1. The head variation between two sprinklers operating simultaneously within the same field
must be smaller than 20% of the average head available for the sprinklers (Christiansen
rule).
2. The application rate should always be less than the rate at which the soil can infiltrate water
to avoid surface water ponding, redistribution and runoff (Pereira and Trout, 1999).
3. Given the operating characteristics of the irrigation system, the application rate and time are
enough to apply the required irrigation depth in peak evapotranspiration periods.
4. The sprinklers’ radius or throw is adjusted to the wind conditions estimated for the field.
Small droplets from sprinklers are easily blown by wind, distorting wetting patterns and
reducing irrigation uniformity (Pereira and Trout, 1999). With winds up to 5 km/h, the watting
radius listed in the manufacturers’ brochures should be shortened by 10% to obtain the
effective radius of throw. A reduction of 1.5% for each one km/h over 5 km/h wind speed is
proposed for the usual range of wind conditions under which sprinklers are operated (Keller
and Bliesner, 1990, Pereira and Trout, 1999).
5. The Christiansen’s uniformity coefficient (computed in ISADim for an average grid-cell
consisting of four sprinklers operating at the average head, on an horizontal plane, with the
sprinkler spacing and lateral spacing employed in the layout, under no wind conditions, and
supposing a triangular water distribution profile) generally should be between 75% and 85%
(Raposo, 1994).
6. The flow velocity inside the pipes is between 1 and 2 m/s, ensuring a certain degree of
economical suitability (Lencastre, 1991), reducing deposits of sediments carried by the
irrigation water (which increase with low velocity), and preventing water hammer problems
(which increase with high velocity).
The flowchart of a typical design sequence using ISADim is given in Figure 1. The user may
modify or correct the system layout at any stage of the design process, namely after running the
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simulation and analyzing its results. The user shall analyze the simulation results concerning the
pressure head and discharge variation, flow velocity and distribution uniformity and then modify
the pipe diameters, the sprinkler spacing or the selected sprinklers until the results are in
agreement with the user design conditions.
Figure 1 – Design sequence flowchart
Main aspects of the Information Technology employed.
ISADim was developed using Microsoft Visual Basic 6.0 with Service Pack 5. The described
real objects (pipes, sprinklers, laterals, mainline nodes, etc.) were implemented as Class
Modules or as User Controls in relation to their need of an interface capable of triggering
events. The main form and the dialog boxes were implemented as Forms.
Figure 2 shows the object model implemented in ISADim to describe the real objects under
analysis, arranged by their access context. Throughout the routines while referencing a specific
sprinkler,
expressions
with
the
following
structure
are
used:
Sra_Prm(a).Sra_Mpr(b).Sra_Spr(c).Sra_Rmp(d).Sra_Asp(e).
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Figure 2 –ISADim object model
Fields topography
Each field is described as a set of triangular surfaces with known elevation vertexes. When a
set of sprinklers is drawn on the screen, the elevation of each sprinkler is automatically
computed using that representation.
QCotar function (Routine 1) is one of the main routines in the field modeling components, since
it evaluates if a given point belongs to a specific field and computes its elevation.
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Routine 1 – QCotar function of the fields’ class (Top_PAR)
QCotar function includes a For cycle where the surfaces (Top_TRG class, trg(i) variable)
forming the field (Top_PRL) are iterated. Each surface is defined by tree vertexes (Top_VTC
class, vtc(i) variable).
Inserting a field (Figure 3) comprises the following steps:
1. From the Insert menu, choose Field.
2. Click on the drawing where you want the first vertex of the field.
3. Type the first vertex elevation and then click the OK button.
4. As you move the mouse, a dashed line is drawn between the first vertex and the pointer
position. Click on the drawing where you want the second vertex of the field.
5. Type the second vertex elevation and then click the OK button.
6. As you move the mouse, a dashed line is drawn between the second vertex and the pointer
position. Click on the drawing where you want the third vertex of the field.
7. Type the third vertex elevation and then click the OK button.
8. If you want to add more triangles to the field description, click the Yes button.
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9. Choose the first vertex of the new triangle clicking one of the existing field vertexes.
10. As you move the mouse, a dashed line is drawn between the first vertex of the new triangle
and the mouse pointer position. Click on the drawing where you want the second vertex of
the new triangle.
11. Type the second vertex elevation and then click the OK button.
12. Choose the third vertex of the new triangle clicking one of the existing field vertexes.
This process is repeated until one has completed the set of triangular surfaces that describes
correctly the topography of the field.
In case a particular field has an irregular topography, it should be defined as a large set of small
triangles, even when its shape is simple. To attain valid results, the difference at any point within
a field between the real elevation and the model elevation should be smaller than 1 m.
Overlapped triangles must not exist in any case.
Figure 3 – Drawing a field in ISADim
Sprinklers and system layout
During sprinkler characterization, the sprinkler radius and flow rate are set for a given selected
pressure. The sprinkler discharge equation is then calculated assuming that the discharge
exponent is 0.5.
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Routine 2 – Cj function of sprinklers’ class (Sra_ASP)
The Cj function (Routine 2) computes the actual flow rate (Cj) of a sprinkler operating at a given
pressure head (h). Before the actual flow rate computation, the sprinkler characteristic
coefficient (k) is calculated based on the nominal pressure (HNM) and nominal flow rate (QNM).
The instruction executed when the head (h) is less then 1m (if h<1 then Err.Raise...) produces
an error message.
As it can be seen in this function, the instructions executed when the head is extremely low
depends on the execution status of the application. During the 1st calculation stage
(CurOpStatus<>IdmAnrShowRes), when the operating range and characteristics of the laterals
are computed, the executed instruction make the flow rate equal to zero but doesn’t produce an
error message to the user.
Drawing a manifold with laterals and sprinklers comprises the following steps:
1. From the Insert menu, choose Sprinklers.
2. ISADim shows a dialog box asking basic parameters on the sprinklers disposition: grid
dimensions type (measured on the field or on an horizontal projection); spacing between
laterals; spacing between sprinklers; manifold axis (in a straight line or fitted to the field
shape); distance between the manifold and the first sprinkler of a lateral; and the minimum
distance between a sprinkler and the field boundaries. After completing this form, click the
Next button.
3. A dialog is shown asking sprinklers data: model name; nominal pressure; nominal flow rate;
nominal radius of throw; and riser height. When nominal data are not specified, choose a
pressure head in the operating table and then use the flow rate and the radius of throw for
that pressure head. Sprinklers data collected in this step is used exclusively to compute the
coefficients of their characteristic equations. All operating data used throughout the
application are then computed using those equations.
4. Click on the drawing where you want to start the manifold alignment.
5. A preview of the sprinklers is drawn while the mouse pointer is moved. Click on the drawing
where you want to end the manifold alignment.
6. Once the manifold alignment is drawn, one starts drawing the laterals alignment on one of
the manifold sides in relation to its axis. Move the mouse and click on the drawing where
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you want to end the laterals alignment. The laterals alignment must have the length of the
lengthiest lateral. Smaller laterals are cropped to the field shape (although the preview grid
doesn’t show that adjustment).
7. Move the mouse and click on the field position where you want to end the laterals alignment
from the other side of the manifold in relation to its axis. If the manifold is intended to have
no laterals on this side, the laterals alignment must be smaller than the predefined distance
between the manifold and the first sprinkler of the laterals.
8. Click on the manifold alignment where the inlet is to be placed. The manifold inlet must be
placed inside the field, since the procedure performing the laterals length adjustment
depends on that position to select the field where the sprinklers are laid on.
9. A dialog box is displayed asking lateral pipes data: model name; pipe roughness (mm); and
inside diameter (mm). After completing this form, click the Next button.
10. The manifold form asks the following data: model name; pipe roughness (mm); inside
diameter (mm); usage of a pressure regulator at the inlet; and if a pressure regulator is
used, the maximum pressure regulator downstream head. After completing this form, click
the Finish button.
When the finish button is clicked, ISADim draws the sprinklers layout (Figure 4) based on the
defined lateral spacing, sprinkler spacing, alignments and field shape.
If the field has a steep slope and the disposition grid has been measured on the field, the
spacing between sprinklers and between laterals seem to be smaller than in the preview grid,
since the projection fitting to the field topography is done only in this final step.
When sprinklers are drawn outside a field, their layout is a rectangle with all sprinklers having an
elevation equal to their riser height.
Figure 4 – ISADim after inserting sprinklers
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Mainline
The mainline is modeled as a set of pipes starting at the system inlet, deviating at the mainline
nodes and ending on the manifolds. Important simplifications are assumed while using this
methodology, namely the non existence of an explicit representation of bends and contractions,
implying that the localized head losses must be estimated as a fixed percentage of the friction
head losses. However, a differentiation is made between two different types of pipelines:
mainline vs. manifolds and laterals.
The friction head loss is calculated using the Darcy-Weisbach equation (Routine 3) with the f
factor calculated with the Churchill equation.
The input variables in the hc function (friction head loss in m) are: flow rate (Q in m3/s), pipe
inside diameter (d in m), pipe length (L in m), the absolute pipe roughness (K in mm) and pipe
usage type (mainline vs. manifold or lateral). Water cinematic viscosity (curVIS) is estimated
through linear interpolation as a function of water temperature.
Routine 3 – Generic friction head loss function
The CurENV coefficient is used to adjust pipe roughness, which increases with aging
(consequently increasing the friction head loss). The CurPCS coefficient is used to estimate the
localized head loss. Both coefficients are set separately for the mainline and for manifolds and
laterals, since usually the localized head losses are much larger in manifolds and laterals than
in the mainline.
The system inlet and the mainline nodes must be inserted before the mainline segments. The
system inlet and the mainline nodes are inserted in a similar way. Inserting the system inlet
consists of the following steps:
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1. From the Insert menu, choose System Inlet.
2. Click on the drawing where the system inlet is to be placed.
3. A dialog box appears asking the system inlet elevation.
4. After clicking on the Ok button, the system inlet is placed on the layout as a red mark.
Mainline segments insertion must start at the system inlet and then continue downstream until
the manifolds. Inserting a mainline segment (Figure 5) comprises the following steps:
1. From the Insert menu, choose Mainline.
2. Click on the system inlet (or mainline node) where you want to start the mainline segment.
3. As you move the mouse, a dashed line is drawn on the screen linking the mainline segment
source node to the mouse pointer. Click the mainline node (or the manifold inlet) where the
mainline segment ends. The mainline segment is drawn on the screen.
4. A dialog box is displayed asking the user to input the pipe model name; pipe roughness (in
mm); and pipe inside diameter (in mm).
5. After clicking finish, a gray mark is drawn at the middle of the mainline segment. When
inspecting or editing mainline segments this gray mark is the interface area (although that
mark doesn’t show when printing the layout).
Figure 5 – ISADim after inserting a mainline segment
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Simulating the irrigation system operation
The pressure and the flow rate distribution along the irrigation system are computed in two
stages. During the first stage the system is analyzed in the upstream direction (from the
sprinklers to the system inlet) in order to compute the operating range of each component
(mainlines, manifolds and laterals), then storing the base values that will be used by the natural
cubic splines: Hj(Hm), downstream head as a function of the upstream head; and Cj(Hm), flow
rate as a function of upstream head.
In the second computation stage, the irrigation system is analyzed in the downstream direction,
starting from the available head at the system inlet and using the splines Hj(Hm) and Cj(Hm) to
estimate the heads and the flow rates along the system.
The PreCalcular procedure (Routine 4) implements the first stage of heads and flow rates
computation in the laterals class. The Asps variable used in procedure PreCalcular is a function
without parameters returning the total number of sprinklers in a lateral.
Routine 4 – PreCalcular procedure from the laterals’ class (Sra_RMP)
To simulate the irrigation system operation the following procedure is used:
1. From the Project menu, choose Simulation. Complete the simulation parameters dialog:
manifolds in operation; head available at system inlet; water temperature; estimated % for
the localized head losses along the mainline; estimated correction coefficient for mainline
pipes roughness; estimated % for the localized head losses along manifolds and laterals;
and the estimated correction coefficient for manifolds and laterals pipe roughness. Click on
the Next button.
2. The results dialog is displayed, when ISADim finishes the calculations (Figure 6). The
default results grid shows the irrigation system operating parameters, which comprise for
each set of sprinklers linked to a manifold: maximum head (m); minimum head(m);
maximum flow rate (l/h); minimum flow rate (l/h); average head (m); average flow rate (l/h);
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head variation (% of average head); flow rate variation (% of the average flow rate);
Christiansen’s uniformity coefficient; total flow rate (l/h); and application rate (mm/h).
Figure 6 – Simulation results
To enable further verification and analysis, the head, flow rate and elevation distributions of all
the operating sprinklers are also displayed, as well as the employed simulation parameters.
Row headings used in the distribution grids have the following meaning: PRx stands for the
manifolds; M0 and M1 are for the manifold side in relation to its inlet; Sx is for the manifold
outlet; and R0 and R1 are for the laterals in relation to the manifold axis.
As backup, one may go back to the main form, move the mouse over the marks and read the
information displayed in the status bar about a particular component.
Accuracy of the simulation results
According to a limited observation and analysis of simulation results obtained with the default
simulation parameters, the head variation in % of the average head may be considered
accurate within an error boundary of +/- 5% of the average head. This error is mainly due to the
following aspects:
1. Differences between field elevations and model elevations; even if those differences are
smaller than 1 m they may produce an error of 3.5%.
2. Errors related with aging and manufacture variation of irrigation equipments may produce an
error of 3.5 %.
3. Errors related with the simplifications employed in the calculation of the localized head
losses may produce an error of 2 %.
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Conclusion
A well-designed irrigation system is an essential requirement for a profitable and environmental
friendly irrigation. A poorly designed irrigation system, even if well managed, often results in
crop losses or water source contamination with fertilizers and simultaneously high operation
costs.
ISADim is a software tool intended to design and simulate permanently fixed or temporarily fixed
(solid set) sprinkler irrigation systems. It enables a significant working time reduction in
developing this type of projects, although with a few limitations on the description of the
irrigation equipments. The background and knowledge required for the task using ISADim is
somehow less than the one required to produce the same results manually or using generic
software tools as spreadsheets and drawing programs. Thus, ISADim will enable a broad
audience (obviously familiarized with the irrigation techniques) to develop a project with quality
at an acceptable cost.
References
Andrade CLT, Allen RG. 1997. SPRINKMOD - Sprinkler Simulation Model User's Manual. Utah
State University (USA) and EMBRAPA (Brazil).
Available at: www.usu.com. Accessed on 10 March 2001.
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Universidade Técnica de Lisboa.
Cheney W., Kincaid D. 1999. Numerical Mathematics and Computing. Pacific Grove, California
(USA): Brooks/Cole.
Correia S.M.M.C. 1997. Modelo de simulação de rega por aspersão em instalações em
quadrícula. MSc diss. Lisboa, Portugal: Instituto Superior de Agronomia / Universidade
Técnica de Lisboa.
Keller, J., Bliesner, R.D., 1990. Sprinkle and Trickle Irrigation. New York, Van Nostrand
Reinhold.
Lencastre A. 1993. Hidráulica Geral. Lisboa, Portugal: (edição do autor).
Microsoft 1999. MSDN Library (Visual Studio 6.0 release). Redmond, WA (USA): Microsoft
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(Eds.) CIGR Handbook of Agricultural Engineering. vol. 1, Land and Water Engineering).
St Joseph, Mich. ASAE, pp. 297-369.
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