GRADE 8 Agriculture CONSERVING AGRICULTURAL ENVIRONMENT – Water harvesting and storage Notes

Water harvesting and storage

Topic: Conserving Agricultural Environment — Subject: Agriculture (Age: 13)

What is water harvesting?
Water harvesting means collecting rainwater and keeping it for later use on the farm. It helps farmers grow crops and give water to animals during dry months.

Why it is important (in Kenya)

• Provides water during dry spells (short rains Oct–Dec and long rains Mar–May vary each year).
• Helps small farms grow vegetables and fruit even when rain stops.
• Reduces soil erosion when water is stored and released slowly.
• Saves money and time walking long distances to fetch water.

Ways to harvest and store water

• Rooftop rainwater harvesting: gutters, downpipes and tanks (plastic, ferrocement, or metal).
• Farm ponds / water pans: dug holes that store runoff from fields.
• Sand dams: small concrete or stone walls in seasonal streams that trap sand and store water.
• Zai pits and micro-catchments: small pits on farms that collect and concentrate rain where crops grow.
• Contour bunds and terraces: slow down water on slopes so it sinks into soil and reduces runoff.

How to make a simple rooftop system (step by step)

1. Fix gutters along your roof edge to catch rain.
2. Connect a downpipe from the gutter to a tank. Put a mesh at the gutter to keep leaves out.
3. Fit a first-flush diverter or simple rock box to remove dirty water from the first rain.
4. Use a tank with a tight lid to stop mosquitoes and animals.
5. Put a tap at the bottom of the tank for easy water collection or connect to drip irrigation for crops.

How to dig a small farm pond

1. Choose a low spot where water flows naturally.
2. Mark the pond shape and clear the area.
3. Dig to a safe depth (often 1–2 m). Keep slopes gentle so people and animals can get out easily.
4. Line the pond with clay or plastic sheeting if the soil is too sandy.
5. Plant grass around the pond to stop erosion and fence it to keep animals and children safe.

Zai pits (quick guide)

Zai pits are small holes 20–30 cm wide and deep. Fill with compost and plant seeds. They catch water from the surrounding soil and help crops grow in dry areas of Kenya (e.g. eastern and northern parts).

Choosing a storage size — simple math

Use this formula: Volume (litres) = Roof area (m²) × Rainfall (m) × Runoff coefficient.

Runoff coefficient: about 0.9 for corrugated iron roofs, 0.8 for tiled roofs.

Example: House roof 50 m², rainfall in season 300 mm (0.3 m), roof metal.

Volume = 50 × 0.3 × 0.9 = 13.5 m³ = 13,500 litres.

So a 10,000–15,000 L tank would capture most of the season’s rain from that roof.

How to use stored water wisely

• Use drip irrigation or watering cans, not buckets, to save water.
• Water crops early morning or late afternoon to reduce evaporation.
• Mulch around plants (cover soil with straw or leaves) to keep soil moist longer.
• Prioritise vegetable seedlings and fruit trees during dry periods.

Safety and maintenance

• Cover tanks to stop mosquitoes and contamination.
• Clean gutters and tanks before the rainy season.
• Check for leaks and fix pipes quickly.
• If you use water for drinking, treat it (boil, filter or use chlorine) — stored roof water may collect dirt and bird droppings.
• Fence ponds and mark dangerous slopes to keep children safe.

Good practices for Kenyan smallholders

• Start small: one tank or one pond and expand when you have experience.
• Combine methods: use zai pits in the field and tanks for kitchen gardens.
• Work with neighbours: shared ponds or sand dams help whole communities.
• Keep records: note how much water you collect and how long it lasts — this helps plan for next season.

Try drawing your farm and mark where you can collect rain. Talk to local farmers or extension officers to learn what works best in your area.