Exam: Section 6 - Designing system requirements

1. What are five factors that you should consider in the selection and design of an irrigation system?

Answer:

2. You need to construct a channel to convey irrigation water from a stream diversion to a field. You install a 90° V-notch weir in the stream and measure a head of 11 cm. a) What is the flow rate? You decide that a triangular-shaped canal will be the easiest construction for you and the farmer. With your Abney level, you measure a slope of 1.4% for the canal. b) What will the depth of water be for the triangular channel that will convey the measured flow downstream of the weir once uniform flow has been established? Use a rock-lined canal with a side slope of 1:1 (z = 1); c) What is the velocity of the water?

Answer:

a. Q = 0.014 H ^5/2 = 0.014 (11) ^5/2 = 5.6 liters/sec = 0.0056 m³/sec

b. n = 0.032
s = 0.014
z = 1

Q AR^2/3 S^1/2/n (metric)

For a triangular canal,

A = zd²
R = A/wp = zd² /2d(1 + z²)^1/2

For a triangular canal with z = 1,
d = 0.114 m = 11.4 cm c.

Q = velocity x area, or V = Q/area = 0.43 m/sec

3. A field has a slope of 5%. (a) If the channel is to be built in the same direction as the slope, the channel bottom is to have a 1% slope, and drops are 0.5 meters each, how many drop structures per 100 meters of channel would you need? (b) What type of drop structure would you use?

Answer:

The drops will have to compensate for 4% of slope, or 4 m/100 m. Thus, the number of drops is 4 meters/0.5 m = 8 drops per 100 meters.

4. A farmer knows that the water source is above her field, and she wants to irrigate the land. She goes to the market and buys 4 rolls (100 m/roll) of 1/2" ((diameter) polyethylene tubing. She installs the main line and, to her surprise, only a trickle of water comes out of the end of the tube. She calls you and asks for advice. What would you tell her, and how would you explain the problem?

Answer:

friction loss in tube
sizing of tube
elevational difference between water source and field

5. In land leveling, what is the main physical factor that determines if the practice can be done effectively? Explain your answer.

Answer:

topsoil depth
plant growth in minimum depth of 30 cm

6. What recommended flow rate would cover a 10 m x 10 m basin with sandy loam soil?

Answer: (5 L/sec (see Table 5.6, Irrigation Reference Manual)

7. What are the steps in constructing a contour furrow irrigation system?

Answer:

1. survey field
2. lay out guide furrows
3. make furrows between guide furrows

8. For furrows (a) What typical flow rate and length of furrow might you expect on a medium textured soil, down a 2% slope, and with 100 mm of water application? (b) If the furrow were half the typical length, what flow rate might you recommend?

Answer:

a) length = 120 m, Q = 20 L/sec
b) length = 60 m, Q = 10 L/sec

9. Which system generally requires a higher flow rate, borders or furrows?

Answer: borders

10. There is a 30 m elevational drop between the water source and a field that is to be sprinkler irrigated by gravity pressure. A 100 m distance separates them. A farmer wants to irrigate the field, and it will require a flow of 120 L/min. If the minimum operating pressure for the sprinkler system is 30 psi, what size PVC main line would you recommend to the farmer, 1", 1 1/2", or 2"?

Answer:

For the system to work, the elevation head must be greater than the pressure head plus friction losses. For 2" pipe, friction head = 2 m/100 m (Table 5.3, Irrigation Reference Manual), and the operating pressure of 30 psi = 21 m. Thus, elevation head (30 m) > 21 m + 2 m. The 2'' size would allow for up to 7 meters of additional friction losses in fittings, etc., and for slightly higher operating pressure.

11. A farmer comes to you and asks how he can eliminate air from the pressurized pipeline in his gravity flow sprinkler system. You accompany him, inspect the main line, and find all the unions to be watertight. Upon arriving at the stream and finding the farmer has placed the inlet of the main line in a fast-flowing, turbulent section of the stream, what is your recommendation?

Answer:

Construct a small diversion.
Put 50 cm head of water over tube inlet in non-turbulent pounded water.

12. Why do you need debris-free water when operating a localized irrigation system?

Answer: to prevent clogging of emitters

13. Name an appropriate technology localized irrigation system.

Answer: perforated polyethylene tubing, with appropriate sized holes, discharging into small basins around trees