Although this module appears last in the Main menu, the user will, most probably, begin all projects of reasonable size with it. The program analyses the information available on climate, crops, soils, and available operating time and costs, in order to provide the user with data concerning irrigation requirements, scheduling and energy costs. These data are then used for planning the project layout.
The menu consists of eight functions:
2 IRRIGATION SYSTEMS
3 CROP FACTORS
4 COSTS AND TECHNICAL INFORMATION
5 CLIMATIC DATA
6 CROPS, IRRIGATION SYSTEMS AND SOILS
8 INTEREST VS. ENERGY SENSITIVITY – GRAPHICAL
The total cost of an irrigation project generally consists of the following components:
c) running cost : energy (electricity, fuel)
In a new project, the first two cost items are immediate expenses, implying that the investor will either have this money available, or be creditworthy for the amount.
The third item, i.e. running cost, is a periodical expense during the life of the project. This cost may vary annually, due mainly to inflation and maintenance.
To handle all the variables that influence the above cost items, a few assumptions can be made to simplify the problem, without losing too much in accuracy. Let’s accept the following:
a) Installation cost can be expressed as a cost per length, per pipe size. This implies that the installation cost of a certain size can be added to the selling price of the pipe; the minimum project cost will then provide for installation. Normally, however, where the buyer is the farmer doing the installation with his own labour, installation cost is neglected.
b) Project maintenance and management costs are not relevant in design considerations. The practical aspects around these items are much more determining, which makes them specifications rather than considerations.
c) Within the design standards generally used, the purchase cost of water will have a negligible effect on the total cost difference between a bad and a good design.
The total cost of a project reduces to the following:
These two expenditures are not independent: the higher the capital cost, the lower the running cost, and vice versa. Joel Dean puts it as follows: “Because today’s capital expenditures make the bed that the company must lie in tomorrow, today’s decisions must be based on definite assumptions as to what tomorrow will be like”.
Let’s look at each of the above mentioned costs in more detail.
For a certain project layout, it is possible to calculate the minimum capital cost at a certain source pressure, in order to meet the pressure and flow demands at the various discharge points in the network. The mathematical calculations for arriving at a minimum cost become very complex, so that shortcuts are normally taken, producing less accurate results.
The problem becomes even more complex, if pressure demands at the discharge points are not fixed, such as centre pivots, which can operate at various pressures, depending on their design. In such cases the total capital cost, at a certain pressure, is the cost of the irrigation system, plus the Main pipe network.
To get a full picture of capital cost, various source pressures must be investigated; graphically the result will appear as follows:
Figure 7-1: The graph of the relation between capital cost and water source pressure
Although easier to calculate than minimum capital cost at a certain pressure, this calculation normally includes far more affecting factors.
These factors are:
Working pressure at source;
Available irrigation time;
Electricity or fuel cost;
Efficiency of irrigation system;
Efficiency of pump equipment;
Fortunately only the first of these factors is a true variable, since the others are normally specifications.
Furthermore, there is a linear relation between the working pressure at the source and the energy cost, so that only one calculation needs to be performed to get the full picture, unlike the calculation of capital cost.
Graphically this can be illustrated as follows:
Figure 7-2: The graph relation between energy cost and water source pressure
To compare costs, it is immediately clear that one of the above costs must be converted so that dimensions will be similar. One cannot compare annual cost, which may be subject to inflation, with immediate cost. Therefore energy cost must be discounted over the total lifespan of the project.
Look at it differently: to invest in a certain project, the developer must have a certain amount of money – first to cover the equipment cost, and then to cover the running cost. He can invest the money for the running cost at a certain interest rate in a savings account. At the end of the project lifespan this investment (for running cost) will have decreased to zero.
TOTAL PROJECT COST
The total project cost is the sum of the capital and running costs, at each operating pressure; this can easily be derived from the two above graphs. The minimum is, of course, that point where the collective graph is at a minimum.
Figure 7-3: The graph of relation between the total cost (pipes and pumping) and the water source pressure
A DESCRIPTION OF ENERGY CALCULATION FACILITIES IN WCADI
The design of Main pipe networks, with the WCADI software is done in two stages:
a) Calculate energy cost.
b) Define network and run optimization.
There are two ways to do this calculation, i.e.
a) With the Planning menu, which is a comprehensive method consisting of approximately 65 items of information, to be entered before the analysis or calculation can be done.
b) In the Main pipe Design menu, in energy cost calculation, where a short procedure is offered.
The following paragraphs deal with the comprehensive method performed by this menu.
General information required for this menu is entered through this function.
1.Number of irrigation systems 2
2.Number of crops 2
3.Number of simulating crops 2
5.Purchase cost of water ($/m^3) 0.5
6.Estimated purchase cost pump equipment ($/kw) 0.2
Table 7-1 : Planning parameters
Description of parameters 5 and 6 :
Purchase cost of water ($/m3)
This cost will not influence the energy cost of a project. It is used only to calculate the total water cost, should it be appropriate in a project, i.e. water is supplied by a Water Board.
Estimated purchase cost of pump equipment ($/kW)
This cost has an effect on the energy cost, but not much. See it this way: if it is a relatively expensive pump, it will be preferable to keep it small, thus low pressure. This will, of course, affect pipe sizes. The role this plays is explained later in more detail.
7.2 IRRIGATION SYSTEMS
This function prepares a table, listing all the commonly used irrigation systems. Systems not used in the specific project, can also be included in the table. The number applied to the system is an index number used only in an analysis and curve of this menu.
System# Irrigation System
Table 7-2 : Irrigation systems
7.3 CROP FACTORS
In this function, a table is entered for all types of crop to be grown in the project, and the crop factor for each month is entered. (The crop factor is the ratio between the evaporation and the evapotranspiration for a certain crop, in a certain period.)
Crops not cultivated at the particular stage when the information is entered, can also be included in the table. The index number applied for each crop is used in an Analysis of this menu.
Crop# Crop Name CROP FACTOR
Jan Fab Mar Apr May Jun Jun Aug Sep Oct Nov Dec
1 GRAPES 0.25 0.25 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.25 0.25
2 FRUIT 0.40 0.35 0.35 0.30 0.20 0.20 0.20 0.25 0.30 0.40 0.45 0.50
Table 7-3 : Crop factor for different crops
7.4 COSTS AND TECHNICAL INFORMATION
To calculate project energy costs, energy tariffs and other project information are entered through this function.
Each item to be entered in the function is now explained in more detail:
To provide for a wider application, costs can be entered here for two tariff structures, i.e. small and large demand. From a certain level of demand, which may be 100 KVA, the large demand tariffs will be applicable. Since the designer does not know the demand at the planning stage, he can enter both tariffs; the software will then select the appropriate tariff according to the calculated demand.
It may happen that other tariff structures are applicable for a project, in which case the user will have to simulate these to express the tariffs used here.
The tariffs used in the software are the following:
* Unit cost ($/kwhr).
* KVA demand cost ($/KVA/Month)
this is a monthly charge to pay for peak demand during this month.
* Estimated fixed installation charge ($/KVA/Month)
a monthly charge levied by the power company to cover the cost of installing equipment and connecting the project to the main distributing network. The charge is constant for each month, calculated according to the peak requirement encountered during the year.
* Meter-reading fee ($/month)
Mindependent of demand, charged for reading the consumer supply meter.
* Reduction in installation charge ($/KVA used/month)
a discount given by the power company on the fixed charge, determined in accordance the maximum monthly power consumption of the user. The discount cannot exceed the fixed installation charge for a month.
Large demand starts from (KVA)
The demand above which large demand tariffs will be used
Efficiency of electric motor (%)
Losses between driver unit and pump (%)
Power factor of electric motor
Rating, generally specified on the body of the electric motor. If not specified, a value of .85 can be used
Number of parallel pumps at pump station
The assumption is made that all pumps are of the same size.
Efficiency of pumps (%)
Approximate length of critical pipe section (m)
This length may be the longest, or the length leading to the highest discharge point in the network. It does not have a big effect on the energy cost and is used only to add the cost of the pump to the energy cost.
Estimated friction loss to design for (%)
This entry, too, does not influence energy cost much, and a good guess is sufficient here.
Static head difference (m)
The height difference between the water source and the critical point in the network (not sensitive to energy cost).
Design working pressure at valves (m)
Only the working pressure at the critical point is needed here (not sensitive to energy cost).
Available irrigation time
This time will determine the flow-rate at the water source.
Interest rate (%)
Estimated average interest rate during the life of the project.
Inflation rate (%)
Estimated average energy inflation, relative to crop price rises, during the life of the project. For example, if electricity is expected to increase annually by 14%, and the crop price is to be increased annually by 11%, then the inflation rate to be entered here is 3%.
Life time of equipment
This is the time for which the project will operate, under all the entered conditions. Two tables are applicable in the options submenu :
Energy Cost Small Demand Large
Unit cost ……………………….( /kWhr) 0.180 0.090
kVA demand cost …………….( /kVA /month) 0.000 25.000
Estimated fixed install charge .( /kVA /month) 0.000 4.000
Meter reading fee ………………( /month) 200.000 65.000
Reduction in install charge ( /kVA used/month) 0.000 3.000
Table 7-4 : Costs data
Large demand starts from (kVA) 100
Efficiency of electric motor (%) 95
Losses between driver unit and pump (%) 5
Power factor of electric motor .87
Number of parallel pumps at pump station 1
Efficiency of pumps (%) 75
Approximate length of critical pipe section (m) 1000
Estimated friction loss to design for (%) 1
Static head difference (m) 90
Design working pressure at valves (m) 15
Available irrigation time : days per week 6
Available irrigation time : hours per day 22
Analysis starts from interest rate of (%) 12
Analysis stops at interest rate of (%) 18
Interest intervals for analysis (%) 1
Inflation rate (%) 0
Life-time of equipment : pump equipment (years) 12
Life-time of equipment : Main pipes (years) 15
Table 7-5 : Technical data
7.5 CLIMATIC DATA
Enter historical average monthly rainfall and evaporation data. The evaporation is as measured in a Standard Class A pan.
# MONTH EVAPORATION(mm) RAINFALL(mm)
1 JAN 295.0 17.0
2 FEB 211.0 20.0
3 MAR 163.0 27.0
4 APR 89.0 54.0
5 MAY 70.0 89.0
6 JUN 64.0 107.0
7 JUL 78.0 103.0
8 AUG 114.0 87.0
9 SEP 156.0 56.0
10 OCT 229.0 36.0
11 NOV 254.0 23.0
12 DEC 288.0 24.0
Total: 2011.0 643.0
Table 7-6 : Average monthly evaporation and rainfall data
7.6 CROPS, IRRIGATION SYSTEMS AND SOILS
Enter information about the crops, irrigation systems and soil properties for all sub-areas to be included in this planning.
The index numbers given in crop factor and cost and technical in this menu are used for designation of crops and irrigation systems.
The items to be entered are listed below (* denotes a data input in the previous functions):
* Water holding capacity of the soil (mm/m) – *
* Allowable water extraction from soil (%) – *
* Water leaching requirement (%)
over irrigation requirement due to bad soil or water qualities
* Monthly rainfall depth to neglect (mm)
part of rainfall considered not to reach the plant roots (due to immediate evaporation, runoff, rainfall interception by leaves, etc.)
* Percentage of remaining rainfall that is effective (%)
further provision made for insufficient rainfall
* Lateral water distribution in soil (degrees from vertical) – *
* Irrigation system
refers to the list of system types entered in Crop factor of this menu
* Crop #
refers to the list of crops entered in Crop factor of this menu
* Crop name
the name of the previously entered crop # will be displayed
* Effective root depth (mm) – *
* Area (ha)
the area of the sub-area
* Wetted diameter (m)
* Emitter wetted diameter
* Efficiency (%)
efficiency of the irrigation system, i.e. if the efficiency of the system is 90%, then (100/90)x(irrigation requirement) must be applied to meet the demand
* Emitter row (m) – *
* Emitter spacing (m x m) – *
* Emitter discharge (l/hr) – *
* Working pressure (m)
* These are soil or system properties that will not affect energy cost. They do, however, influence the irrigation scheduling of the project.
Water holding capacity of soil (mm/m) 120.00
Allowable water extraction from soil (%) 50.00
Water leaching requirement (%) 0.00
Monthly rainfall depth to neglect (mm) 20.00
Percentage of remaining rainfall that is effective (%) 50.00
Lateral water distribution in soil (Degrees from vertical) 45.00
Table 7-7 : Soil properties and effective rainfall
# Irr Sys(#) Crop(#) Crop Name Effect. Rt. Depth Area(ha) Wett. diam.(m) Efficiency(%) Emtr Row(m) Space Emtr(m) Emtr Disch(l/hr) Working Press.(m)
1 1 1 GRAPES 400.0 53.20 20.00 80.00 12.00 12.00 1000.00 30.00
2 2 2 DEC.FRUIT 500.0 25.60 4.00 85.00 4.00 3.00 70.00 20.00
TOTAL AREA 78.80
The total discounted energy cost for the project, as well as the irrigation requirements, are calculated according to the input Functions. The output from this function displays all the data entries, as well as the calculated values.
The analysis gives the discounted energy cost as a cost per unit of head. This figure must be carried forward to the design of the Main pipelines, where the program will optimise between this figure, representing the running cost of the system, and pipe material costs.
Apart from the gross irrigation requirements for all the crops, tables showing the recommended scheduling of the project are also displayed. This information is useful in grouping sub-areas together for irrigation scheduling.
Since some entries are difficult to estimate, e.g. inflation rate, and lateral water distribution in the soil, it is recommended that different values for these variables be entered to determine their influence on resulting costs.
1. Effective Rainfall (mm); ER
*****(Rainfall – Rainfall to neglect) X (% Remainder that is Effective)
ER = —————————————————————————–
2. Net Irrigation Requirement (mm); NIR
NIR = Evaporation x Crop factor – ER
3. Maximum duration of irrigation per crop (hours)
This is a calculation of how long it will take to apply the quantity of irrigation, in order to get the soil back to field capacity, based on the irrigation system, water holding capacity, and minimum allowable water content
4. Maximum cycle length (days)
This calculation shows the estimated cycle lengths, as it varies through the year, according to evapotranspiration.
5. Recommended area to irrigate simultaneously (ha)
In this analysis the total discharge into the system is divided among the different crops, and the result gives an indication of the size of blocks each field should be divided into.
7.8 INTEREST VS. ENERGY SENSITIVITY – GRAPHICAL
This function graphically displays the relation between the interest rate and discounted energy cost. The interest rate is varied according to the information entered in Climate data of this menu.
The following figure shows the graph for the previously entered example:
Figure 7-4: Relation between the interest rate and discounted energy cost