1. Make an inventory of resources
• Visit the field site personally if at all possible, and talk with the farmer.
• Get data on soil, topography, water supply, crops, farm schedules, climate, energy, etc.
• Be suspicious of parameter values and check whether they are within reasonable ranges.
2. Calculate a preliminary value for the maximum net irrigation depth, dx.
3. Obtain values for peak ET rate, Ud, and cumulative seasonal ET, U (Table 3.3).
4. Calculate maximum irrigation frequency, fx, and nominal frequency, f'.
• This step is unnecessary for automated fixed systems and center pivots. 5. Calculate the required system capacity, Qs
• first, calculate gross application depth, d
• for center pivots use dn/f = Ud, and T « 90% of 24 hrs/day = 21.6 6. Determine the "optimum" (or maximum) water application rate.
• a function of soil type and ground slope (Table 5.4).
7. Consider different types of feasible sprinkle systems. 8. For periodic-move and fixed (solid-set) systems:
(a) Determine Se, qa, nozzle size, and P for optimum application rate (Tables 6.4 to 6.7)
(b) Determine number of sprinklers to operate simultaneously to meet Qs(Ns= Qs/qa)
(c) Decide upon the best layout of laterals and mainline (d) Adjust f, d, and/or Qsto meet layout conditions
(e) Size the lateral pipes
(f) Calculate the maximum pressure required for individual laterals 9. Calculate the mainline pipe size(s), then select from available sizes.
10. Adjust mainline pipe sizes according to the "economic pipe selection method" (Chapter 10),
and check velocity limits too.
11. Determine extreme operating pressure and discharge conditions. 12. Select the pump and power unit.
13. Draw up system plans and make a list of items with suggestions for operation and maintenance of the hardware.
III. Summary
• Note that MAD is not a precise value; actual precision is less than two significant digits; this justifies some imprecision in other values (don't try to obtain very precise values for some parameters when others are only rough estimates).
• When determining the seasonal water requirements we subtract Pe from U. However, to be safe, the value of Pe must be reliable and consistent from year to year, otherwise a smaller (or zero) value should be used.
• Note that lateral and sprinkler spacings are not infinitely adjustable: they come in standard dimensions from which designers must choose. The same goes for pipe diameters and lengths.
• However, buried PVC pipes in a fixed system can be cut to any length during installation, allowing for a great deal of flexibility.
• Note that design for peak Ud may not be appropriate if sprinklers are used only to germinate seeds (when later irrigations are by a surface method).
Sprinkle System Design
Design of a sprinkle system starts with layout of laterals and main pipelines. Pipe layout may be simple for small regularly shaped fields to complex for large and odd-shaped areas. Layout mainly depends on field topography, lateral settings, wind conditions and location of water source. Design of set move, solid set gun sprinkler and center-pivot are discussed briefly. For more details, the reader is referred to Keller and Bliesner (1990), Pair et al. (1975), Rolland (1982), and other sources.
Set-Move and Solid Set Systems
Main steps to design a set-move or solid set sprinklers are layout of main pipe lines and laterals, determining the number of laterals to be operated per irrigation, number of sprinkler heads to be operated, discharge required and pump characteristics.
System Layout
Layout of the system mainly depends on topography, field size and shape, location and source of water supply.
Required System Capacity
The capacity of a sprinkle system depends on the area to be irrigated; the gross depth of water required for each irrigation, time allowed completing irrigation during peak water use period, f and actual operating time per day. The capacity can be calculated by the equation as:
fT Ad K
Qs (4.1)
A = design area, ha (acres)
d = gross irrigation depth, mm (in)
f = operating time allowed to complete one irrigation, days T = actual operating time per day, h/day
For a given area, d, f and T have significant impact on the system capacity. Greater the irrigation depth, larger will be the system capacity. For a given depth, d, greater the operating time (IT), the smaller will be the system capacity and vice versa. The operating time, f should be one to two days less than the irrigation cycle during peak water use period. Similarly T should be at least 5 to 10 percent less than 24 hours.
Number of Sprinklers
Number of sprinklers operating at one time can be calculated as:
a s q Q s N (4.2) Where,
Ns = number of sprinklers operating Qs = total system discharge, L/s (gpm) qa = average sprinkler discharge, L/s (gpm)
Variation in the number of operating sprinklers should be kept to minimum but it may be sometime unavoidable at the end of an irrigation cycle. On the last day of irrigation, only a few laterals may be needed to complete the irrigation.
Number of Laterals Operated per Set
Length of laterals may be determined by deciding the final layout of the system. Dividing the lateral length by the spacing between the sprinkler heads on the lateral will give the number of sprinklers operating on the lateral.
Divide the total minimum number of sprinklers required by the number of sprinklers on one lateral to get the minimum number of lateral required. The above computations are finally adjusted as illustrated in the following example:
Sample Example 4.1: Computing system capacity requirements for a single crop in the design area.
Given: Field of corn, A = 16 ha Design moisture use rate, Ud = 5 mm/day
Moisture replaced in soil at each irrigation, dn = 60 mm Irrigation efficiency, Ea = 75%
Irrigation period, f = 10 days in a 12-day interval System operating time per day, T = 20 hr/day
Electrical conductivity of the irrigation water, ECw = 2.1 dS/m Calculations: Leaching requirements can be calculated by using the following equation:
100 / E LR) - (1.0 d 0.9 d a n in) (3.54 mm 90 100 / 75 0.20) - (1.0 60 0.9 d
Using the equation to compute the system capacity: Qs= 2.78 Ad fT gpm) (317 L/s 20.0 20 10 90 16 2.78
Sample Example 4.2:A side roll sprinkle system is to be designed for the field shown below;
laterals will operate on both sides of the main line.
Pump (in center) 1200 f t 2100 ft M ai nl ine
The following data is for the preliminary design:
qa = 3.8 gpm/sprinkler f = 5 days Se = 30 feet/sprinkler d = 1.11 inches
S1 = 40 feet/position T = (3 sets/day) x (4 hrs/set) 1. What is the preliminary system capacity (gpm)?
(There are 43,560 sq ft/acre)
2. How many sprinklers will be operating simultaneously? (Round to the nearest whole number)
3. How many sprinklers per lateral? 4. How many lateral(s) will be needed? 5. How many sets for each of the lateral(s)? 6. What is the adjusted system capacity (gpm)?
Solution: 1. 484.82gpm 5 12 1.11 43560 1200 2100 453 T.f Ad K Qsinitial 2. 127.58 128 3.8 484.82 q Q N a s s
3. Sprinkler per lateral = 35 30 1050
4. No. of laterals needed = 3.65 4 35
128
5. Settings of laterals = 30oneachsideof themain 40
1200
- If 1 lateral is operated, field will be irrigated in = 60/3 = 20 days - 2 “ 60/3 = 20 days
- “ 60/6 = 10 days - 3 “ 60/9 = 7 days - 4 “ 60/12 = 5 days
Therefore 4 laterals are needed (20 each side of the main line) Therefore setting of each lateral = 600/40 = 15
Layout of Laterals for Set Sprinklers