Water use and flood control
Two jobs go together in water engineering:
- Water use: taking water from rivers, lakes, wells or reservoirs for crops, animals, homes and factories.
- Flood control: keeping extra water from harming people, houses and farms.
A reservoir behind a dam does both. In the wet season it stores extra rain, so the river downstream stays lower. In the dry season it lets water out for farms. Paddy fields and ponds help in the same way, because they hold rain on the land instead of letting it all run down at once.
Floods are controlled by storing water (dams, ponds, fields), by letting rivers spread safely (floodplains), and by giving water ways to leave (drains, embankments).
Irrigation and drainage
Irrigation means giving crops water that rain does not supply. Common methods:
- Surface (flood or furrow): water flows over the field or along rows. Simple, but a lot is lost.
- Sprinkler: pipes spray water like rain.
- Drip: thin pipes give water drop by drop at the roots. It saves the most water.
Drainage means taking extra water away. It is needed because:
- too much water leaves no air for roots (waterlogging);
- it lets tractors and harvesters work on firm ground;
- in dry areas it can wash away salt that builds up in soil.
Surface drains are open ditches. Underground (subsurface) drains are buried pipes with holes that collect water from the soil. Good farms plan irrigation and drainage together.
Irrigation facilities
Water travels through a chain of structures. Follow the 3D from left to right:
- Dam and reservoir: store river water.
- Weir or headworks: a low wall or gate that lifts the river level and sends some water into the canal.
- Main canal and branch canals: carry water across the land, getting smaller as they split.
- Pipelines and pumps: lift or push water where gravity alone will not do the job.
- Gates and sluices: adjustable doors that control how much water goes where.
- Farm ponds and tanks: small stores near fields.
- Drainage canals: take the extra water back to a river.
Canals work mostly by gravity, so they slope gently downhill. These structures need care. Silt, weeds and cracks reduce the flow, so farmers and local groups clean and repair them every year.
Multiple functions of agricultural water
Water used for farming gives benefits beyond crops. These are called the multiple functions:
- Flood help: paddy fields with raised edges hold rain water.
- Groundwater refill: some field water sinks down and recharges wells.
- Cooling: water evaporating from fields and plants cools nearby air in hot weather.
- Habitat: ponds, canals and wet fields shelter frogs, fish, insects and birds.
- Landscape and culture: green terraces and canals make pleasant places and keep village traditions alive.
If farms are abandoned or fields are paved over, these free services are lost too. That is one reason to keep farmland and water channels well cared for.
Try it: trace the water
Visit a canal, pond or ditch near you, or look at a map or photo. Draw the path from the source to a field and from the field back to a river. Mark the gates, pumps or drains you can find. Then ask a farmer or elder: when are the gates opened and shut?
Key formulas and definitions
- Water volume (m³) = depth (m) × area (m²)
- 1 mm over 1 ha = 10 m³
- Canal flow Q (m³/s) = cross-section area (m²) × speed (m/s)
- Volume = flow × time
- Saving (%) = (old use − new use) ÷ old use × 100
- Key terms: irrigation, drainage, reservoir, weir, canal, sluice, waterlogging
Worked examples
1. A 2-hectare paddy field needs 60 mm of water. How many m³ is that? (1 ha = 10,000 m²)
Area = 20,000 m². Depth = 0.06 m. Volume = 20,000 × 0.06 = 1,200 m³.
2. A canal carries 0.5 m³ of water every second. How much flows in one hour?
One hour = 3,600 s. Volume = 0.5 × 3,600 = 1,800 m³.
3. A canal is 2 m wide with water 1 m deep, flowing at 0.5 m/s. Find the flow Q.
Area = 2 × 1 = 2 m². Q = 2 × 0.5 = 1 m³/s.
4. Paddy fields over 100 ha have bunds that hold an extra 10 cm of rain water. How much flood water do they store? (1 ha = 10,000 m²)
Area = 1,000,000 m². Depth = 0.1 m. Volume = 100,000 m³.
5. A field has 80 mm of extra water. A drain removes 20 mm each hour. How long to clear it?
80 ÷ 20 = 4 hours.
6. Flood irrigation uses 900 mm of water for a crop, while drip uses 600 mm. What percentage of water does drip save?
Saving = (900 − 600) ÷ 900 × 100 = 33.3%, about one-third.
Common mistakes
- Thinking drainage is only needed in wet places. Many irrigated fields need drains to prevent waterlogging and salt build-up.
- Confusing irrigation (water in) with drainage (water out).
- Believing dams only give water. They also store flood water.
- Forgetting that 1 hectare is 10,000 m², which makes volume answers 10 times too small or large.