Irrigation Engineering and Hydrology for SSC JE Civil: Duty and Delta, Canals, Headworks, Dams, Rational Formula and Hydrograph
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Irrigation: Basics
- 2.Methods of Irrigation
- 3.Water Requirement of Crops
- 4.Canals and Their Structures
- 5.Design Theories for Canals in Alluvial Soils
- 6.Dams
- 7.Hydrology
- 8.Solved Examples
- 9.Quick Tips
- 10.Quick Revision Points
- 11.Practice Questions With Answers
- 12.How to Practise This Chapter
- 13.Frequently Asked Questions
PDF Notes: Irrigation Engineering and Hydrology for SSC JE Civil: Duty and Delta, Canals, Headworks, Dams, Rational Formula and Hydrograph
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Irrigation engineering and hydrology together form a regular section in SSC JE Civil. The questions are based on definitions, the duty–delta relation, the types of canals and structures and a few formulas. This guide on irrigation engineering and hydrology for SSC JE covers all of them, with solved examples, practice questions and FAQs.
Irrigation: Basics
- Irrigation is the artificial supply of water to crops. It is needed because of the uneven rainfall and the need for assured crops.
- Gross Command Area (GCA) is the total area under the canal; Culturable Command Area (CCA) is the part that is fit for cultivation; Intensity of irrigation is the percentage of the CCA irrigated in a year.
- Seasons: Kharif (June–October), Rabi (October–March) and Zaid (summer).
- Crop ratio is the ratio of the area irrigated in the Rabi season to that in the Kharif season.
Methods of Irrigation
| Method | Description |
|---|---|
| Surface irrigation | Flooding, border strip, check basin and furrow methods |
| Sprinkler irrigation | Water is sprayed like rain; suitable for uneven land and sandy soils; saves water |
| Drip (trickle) irrigation | Water is supplied drop by drop at the root zone; the most water-efficient; suitable for orchards and arid regions |
| Sub-surface irrigation | Water reaches the roots from below |
| Lift irrigation | Water is lifted by pumps from wells or rivers |
Water Requirement of Crops
- Duty (D): the area (in hectares) irrigated by one cumec (cubic metre per second) of water during the base period.
- Delta (Δ): the total depth of water required by a crop during its base period.
- Base period (B): the period from the first watering to the last watering before harvesting.
- Relation: Δ = 8.64 B / D (Δ in metres, B in days, D in hectares per cumec).
- Duty is more at the head of a canal than at the field because of the losses by seepage and evaporation along the way.
- Consumptive use (evapotranspiration) is the water lost by evaporation and transpiration.
- Irrigation efficiency = (water delivered to the field / water diverted at the headworks) × 100.
Canals and Their Structures
| Type or Structure | Notes |
|---|---|
| Main canal | Takes water from the river headworks and feeds the branch canals |
| Branch canal and distributary | Carry water to the minors and then to the fields (watercourses) |
| Alignment | Watershed (ridge) canal is the ideal alignment since it can command both sides |
| Canal lining | Reduces seepage losses; materials: concrete, brick, shotcrete, asphalt and geomembrane |
| Weir | A barrier across a river that raises the water level; the crest is fixed; made for raising the level |
| Barrage | A weir with gates that give better control; a modern headwork |
| Aqueduct | A canal crosses a drainage or river (canal above the drainage) |
| Syphon (super passage) | A drainage passes over the canal |
| Canal fall | A drop in the level of the canal bed to reduce the slope |
| Escape | A structure that removes the excess water from the canal |
Design Theories for Canals in Alluvial Soils
| Theory | Main Points |
|---|---|
| Kennedy’s theory | Critical velocity V0 = 0.55 m D0.64; the silt is kept in suspension by the vertical components of eddies |
| Lacey’s regime theory | Silt factor f = 1.76 √d (d in mm); a channel is in the regime when it is stable; velocity V = (Q f² / 140)1/6; wetted perimeter P = 4.75 √Q; hydraulic mean radius R = 5V²/2f |
| Khosla’s theory | Design of weirs and barrages on permeable foundations using the method of independent variables and the exit gradient |
Dams
| Type | Features |
|---|---|
| Gravity dam | Resists the water thrust by its own weight; made of masonry or concrete; example: Bhakra Dam |
| Arch dam | Transfers the water load to the abutments by arch action; for narrow gorges; example: Idukki |
| Earthen (embankment) dam | Made of compacted soil; cheaper; example: Hirakud, Tehri (rockfill and earth) |
| Buttress dam | A sloping deck supported by buttresses |
| Rockfill dam | Made of loose rocks with an impermeable face |
- A spillway passes the surplus water safely; the energy is dissipated by a stilling basin.
- The conditions for the stability of a gravity dam: no overturning, no sliding, no tension in the base and no crushing.
- The base width of a low gravity dam is about 0.7 times the height.
Hydrology
| Item | Details |
|---|---|
| Hydrologic cycle | Evaporation → condensation → precipitation → infiltration and runoff → back to the sea |
| Rain gauge | Symon’s rain gauge is a non-recording gauge; a recording gauge is of the tipping-bucket, weighing or float type |
| Mean precipitation over an area | Arithmetic mean, Thiessen polygon and isohyetal methods (isohyetal is the most accurate) |
| Hyetograph | A graph of rainfall intensity against time |
| Hydrograph | A graph of the discharge of a stream against time |
| Unit hydrograph (Sherman, 1932) | The hydrograph resulting from 1 cm of effective rainfall in a given duration |
| Infiltration | Horton’s equation f = fc + (f0 − fc) e−kt |
| Rational formula | Q = C i A / 360 (Q in m³/s, i in mm/h, A in hectares) |
| Runoff coefficient (C) | Ratio of runoff to rainfall; high for roofs and pavements and low for sandy or forest areas |
| Groundwater | Darcy’s law: Q = k i A; wells are drawn to tap the aquifer |
Solved Examples
Example 1. A crop has a base period of 120 days and a duty of 1,000 hectares per cumec. Find the delta.
Δ = 8.64 × 120 / 1000 = 1.04 m.
Example 2. Find the peak runoff from an area of 50 hectares with C = 0.3 and an intensity of rainfall of 60 mm/h.
Q = 0.3 × 60 × 50 / 360 = 2.5 m³/s.
Example 3. Find the silt factor for a bed material of size 1 mm using Lacey’s theory.
f = 1.76 √1 = 1.76.
Example 4. If the area irrigated is 600 hectares and the CCA is 1,000 hectares, find the intensity of irrigation.
600/1000 × 100 = 60%.
Quick Tips
- Memorise Δ = 8.64 B/D and Q = CiA/360.
- Learn the structures: aqueduct is canal over drainage; syphon aqueduct has the drain under the canal.
- Remember Lacey’s silt factor formula f = 1.76√d.
- Remember that drip irrigation is the most efficient.
Quick Revision Points
- Duty is the area irrigated per cumec; delta is the depth of water.
- Δ = 8.64B/D.
- Rational formula: Q = CiA/360.
- Khosla’s theory is for weirs and barrages.
- Hirakud is an earthen dam; Bhakra is a gravity dam.
Practice Questions With Answers
- What is the relation between delta, duty and base period?
Answer: Δ = 8.64 B / D - Which theory gives the silt factor f = 1.76√d?
Answer: Lacey’s theory - Which type of irrigation gives the best water saving?
Answer: Drip irrigation - What is the rational formula?
Answer: Q = CiA/360 - Which structure is a canal over a drainage?
Answer: Aqueduct - Which theory is used for the design of weirs on permeable foundations?
Answer: Khosla’s theory - Which rain gauge is a non-recording type?
Answer: Symon’s rain gauge - Who proposed the unit hydrograph?
Answer: Sherman - What is the culturable command area?
Answer: The area of the command that is fit for cultivation - Which method of mean rainfall is the most accurate?
Answer: Isohyetal method
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Civil Engineering notes page, read the previous topic Steel Structures Design and the next topic Construction Management, CPM, PERT and Contracts.
Frequently Asked Questions
What is duty?
The area irrigated by one cumec of water during the base period.
What is delta?
The total depth of water required by a crop in its base period.
What is a weir?
A barrier across a river to raise the water level.
What is a barrage?
A weir with gates that gives control over the river level.
What is an aqueduct?
A structure in which a canal passes over a river or drainage.
What is a spillway?
A structure that releases the surplus water from a dam.
What is Lacey’s regime theory?
A theory for the design of stable channels in alluvial soil using the silt factor.
What is a hydrograph?
A graph of the discharge in a stream against time.
What is the rational formula?
A formula for peak runoff: Q = CiA/360.
What is a gravity dam?
A dam that resists the water thrust by its own weight.
Is this topic asked in SSC JE?
Yes. Irrigation is asked in the civil paper every year.
What is intensity of irrigation?
The percentage of the culturable command area irrigated in a year.
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Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.