Rain, Rivers and the Reservoir
A tropical island relies on a reservoir in a steep, deforested catchment of 40 km². Mean annual rainfall is 1,600 mm, but 70% falls between June and November. The reservoir has silted up, losing 30% of its storage in 15 years. Downstream, a town of 20,000 people reports saltwater intrusion into coastal wells during the dry season, and farmers complain that the river no longer flows in April. Records show that peak river levels after storms are higher than in 1990, while dry-season flows are lower. A proposal would build check dams and replant the upper catchment.
1. Explain how deforestation in the catchment could produce both higher storm peaks and lower dry-season flows.
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Forest removal changes how rainfall reaches the river.
- Interception and infiltration: trees and leaf litter slow rain and help it soak in. Deforested ground has compacted, bare soil, so infiltration falls.
- Storm response: more rain becomes overland flow, reaching the river quickly. This raises peak levels after storms, matching the record of higher peaks than 1990.
- Storage: with less infiltration, less water is stored in soil and rock to feed the river slowly. Baseflow falls, so the river dries in April.
- Erosion: bare slopes deliver soil, which silts the reservoir, explaining the 30% storage loss.
So deforestation shifts water from slow storage to fast runoff, increasing flood peaks and reducing dry-season supply.
2. Assess the check dams and replanting proposal.
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Check dams trap sediment and slow runoff in gullies. They can reduce reservoir silting and small flood peaks. They are relatively cheap and quick. But they fill up, need clearing, and do little for dry-season flow unless they help water soak in.
Replanting addresses the cause: it raises interception and infiltration, cuts erosion, and over years increases slow release to the river. It is cheap per hectare but slow, needs protection from grazing and fire, and does not help the reservoir immediately.
Judgement: do both, but expect different timescales. Replanting is the long-term fix for both floods and dry-season flow; check dams are a short-term sediment control. Neither will restore 30% of lost storage, so dredging or a new supply may also be needed. Replanting should start on the steepest upper slopes and gullies, and be fenced and monitored.
3. Coastal wells are becoming salty. How does the catchment proposal relate to that problem?
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The salty wells are a coastal groundwater problem, not directly a reservoir problem, but they are linked.
- Cause of intrusion: over-pumping coastal wells lowers the freshwater table, letting seawater move inland. Dry-season river failure means less freshwater reaches the coast and less recharge.
- Link: if the catchment work raises dry-season flow and infiltration, more freshwater reaches the coastal aquifer, which helps hold back seawater.
- Limits: replanting will not stop intrusion if wells are pumped too hard. The direct fix is to reduce abstraction, spread pumping, and monitor salinity.
Judgement: the catchment plan is a useful supporting measure, but the wells need their own management. Relying on replanting alone would be too slow and uncertain for a town of 20,000.