Grade 8Agriculture

Water Harvesting and Storage

Shallow water pans, water ponds, water containers; storing harvested water for domestic use.

📖 3 min read · 3 worked examples · 8 practice questions

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The lesson

Today we're starting our unit with an overview of water harvesting and why it matters for homes across Kenya. First, water harvesting means collecting and storing rainwater or runoff for later use. It's a simple way to make sure we have water even during dry periods. Why is this important in Kenya? Many households rely on a single water source, and during droughts that source can run dry. Harvesting helps families keep enough water for drinking, cooking, and farming. This topic connects directly to our CBC strand on Conservation of Resources, where we learn how careful use of water protects our environment and supports community wellbeing.

Class, today we'll explore the three main water‑harvesting structures you'll learn about in Grade 8. First, shallow water pans – simple depressions in the ground that collect runoff after a rain. Think of the little mud hollows you see after a storm in the school yard; they hold water temporarily. Next, water ponds. These are larger, often lined with plastic or clay, and store water for the whole season. In many Kenyan farms you'll find ponds that provide water during the dry months. Finally, water containers such as household tanks, barrels, and jerrycans. They're portable, easy to install on rooftops, and perfect for collecting rainwater right where you live.

Class, we've reached the end of today's lesson. This slide is our Summary and Reflective Task. First, let's review the three harvesting structures we explored — the pan, the pond, and the container — and the situations each one is best suited for. Remember how we sized each structure based on the amount of rainfall we expected and the space we had available. For a short reflective activity: write two to three sentences about steps you could take at home to improve water storage, using the ideas we just covered. Take a minute to think, then share your thoughts with a partner or write them down. I'll circulate to hear some of your ideas.

Worked examples

– Designing a Shallow Water Pan

Class, let's walk through Worked Example 1 where we design a shallow water pan for a school garden in Nakuru. First, we need to identify the rainfall depth. In this region we get about 120 mm of rain each rainy season, so that's the amount of water we can capture. At this bar chart – the left bar shows the expected runoff from the rainfall, and the right bar shows the storage capacity we need, which is 200 litres per garden bed. From the chart we see the runoff exceeds the storage, meaning we must enlarge the pan. By dividing the required volume by the depth of water we can store, we calculate a surface area of roughly 1.7 square metres. To recap: we identified 120 mm rainfall, calculated 200 L storage per bed, confirmed with the chart, and determined we need about 1.7 m² of surface area for the shallow water pan. Any questions before we move on?

– Sizing a Water Pond for a Homestead

Let's work through Example 2, where we size a water pond for a homestead in semi‑arid Turkana. First, we consider the average household daily water need, which is about 50 liters per person. Next, we look at the rainfall capture period – we plan to collect rain over 90 days during the rainy season. We combine pond depth with surface area to calculate the total volume required: depth times area equals volume. Here's a table comparing pond sizes for 2, 4, and 6 family units. Notice how the required surface area grows as we add more families, while the depth stays constant.

– Choosing a Water Container

Let's dive into Worked Example 3, where we'll choose a water container for a Nairobi peri‑urban home. First, we assess the roof catchment area – in this case a modest 20 square metres, which tells us how much rainwater we can collect. Next, we need to estimate the first‑flush volume, about 30 litres, to discard the initial dirty runoff. Based on those numbers, a 200‑litre barrel fits perfectly – it's large enough to store useful water but still manageable for a household. Finally, we'll talk about maintenance: regularly clean the barrel, check for leaks, and ensure the inlet screen stays clear.

Practice questions

Remember, the three main types of structures we discussed are rooftop catchments, surface‑runoff ponds, and groundwater‑recharge pits. Anything that looks like an underground sand dam belongs to a different category, so keep that in mind for the first question.
When we design a system for a small school, the essential parts are clean gutters, a first‑flush diverter to keep debris out, and a way to move the water—often a pump. A large open evaporation pond would lose too much water, so it's not a must‑have component.
For the calculation question, use the formula: Storage (L) = Rainfall (mm) × Catchment area (m²) × Runoff coefficient × 1 L per mm·m². Plugging in 10,000 L, 120 m², and 0.
Finally, think about why storing water matters in Kenya. Many regions face long dry spells and unpredictable rain, so having stored water can mean the difference between a thriving school garden and a thirsty classroom.
First, always cover your containers tightly to prevent contamination from dust, insects, or animals.
Second, clean the containers regularly and remove any sediment that settles at the bottom.
Third, you can use simple filtration methods—like passing water through layers of sand and charcoal—to improve clarity.
Finally, set up community maintenance plans so everyone takes responsibility for checking and maintaining the storage systems.

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Water Harvesting and Storage

📖 The lesson

✏️ Worked examples