Two villages, one river: explaining rainfall and flood risk
Kondo and Mupfuti are two villages 18 km apart in the same district. Kondo sits at 1 450 m on the eastern side of a mountain ridge; Mupfuti lies at 900 m on the western side, in a broad valley. The prevailing winds blow from the east for most of the year. Kondo's weather station records 1 100 mm of rain a year, Mupfuti's records 620 mm. In the last week of January, Kondo recorded 240 mm in six days. Two days later the river that flows west past Mupfuti rose 2 m, swept away a footbridge and flooded maize fields. Villagers in Mupfuti say the rain 'came from Kondo'. A district officer has a small budget and must decide where to spend it on flood defences.
1. Explain why Kondo receives nearly twice as much rain as Mupfuti. Use the relief, the height and the wind direction in your answer.
Read the worked answer
Step 1 – Track the wind. Moist air arrives from the east, off the sea, and is forced to climb the eastern slopes where Kondo lies.
Step 2 – Apply the mechanism. As air rises it cools, the water vapour condenses, clouds form and heavy relief (orographic) rainfall falls on the windward slope. Kondo, at 1 450 m on that slope, gets the full benefit - 1 100 mm.
Step 3 – Follow the air over the ridge. By the time the air crosses the crest it has lost most of its moisture. As it descends the western side it warms and its relative humidity drops, so it holds on to what is left. Mupfuti, in the valley at 900 m, therefore gets little rain - 620 mm - in the rain shadow.
Step 4 – Check against the evidence. Windward village wetter, leeward village drier, difference of 480 mm a year: this is exactly the pattern the theory predicts, so relief rainfall is the best explanation. Height also matters - Mupfuti is 550 m lower, so the air reaching it is warmer and further from condensation.
Conclusion: Kondo is wet because it is windward and high; Mupfuti is dry because it is leeward and low.
2. Mupfuti had far less rain than Kondo yet suffered the flood. Explain how this is possible.
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Step 1 – Remember that a river carries water from elsewhere. Mupfuti's flood was not caused by rain falling on Mupfuti. The river gathers water from its whole catchment, which includes the wet eastern slopes above Kondo.
Step 2 – Do the arithmetic of the storm. 240 mm in six days over a catchment of even 20 km² is 240 mm × 20 000 000 m² = 4 800 000 m³ of water - millions of cubic metres arriving in one week.
Step 3 – Add the valley's disadvantages. Mupfuti is in a broad valley at 900 m, so the channel is the lowest point and water concentrates there. The ground is likely already saturated after earlier rain, so extra water runs off instead of soaking in. The valley sides funnel the flow, so the river rises fast - 2 m in two days.
Step 4 – Judge the villagers' claim. They are right that the water came from Kondo's direction, but wrong to think it fell on Kondo alone: it fell across the whole upper catchment. Floods are a catchment event, not a local one.
Conclusion: low local rainfall does not mean low flood risk when you sit downstream of a wet, steep catchment.
3. The district officer can fund only one thing now: a new concrete footbridge at Mupfuti, or a rain gauge and flood-warning system upstream at Kondo. Recommend one and justify it.
Read the worked answer
Step 1 – Identify what each option buys. A footbridge is a structural defence: it protects the crossing, but it does not protect fields, houses or lives, and a bigger flood can destroy it too. A rain gauge with a warning system at Kondo is non-structural: it buys time and information, and time is what saves lives.
Step 2 – Check the timing. The river took about two days to rise 2 m after the storm. That is a workable warning window - enough to move people, livestock and grain to high ground.
Step 3 – Judge against the pattern. Kondo is the wet headwater of the catchment, so a gauge there is measuring the actual cause of Mupfuti's floods. One gauge protects the whole downstream area; one bridge protects one crossing.
Step 4 – Accept the limits. A warning system only works if people understand it and have somewhere to go. So the recommendation must be conditional.
Recommendation: fund the gauge and warning system, linked to a village flood committee with agreed evacuation routes. Ask the ward to repair the footbridge with local labour and timber as an interim measure, and site any future permanent bridge higher and with wider spans so floodwater passes under it.