A Rain-Shadow Town and Its Water Supply
A town of 45 000 people sits on the leeward side of a north–south mountain range that peaks at 2 400 m. Prevailing winds are westerly. The town receives 480 mm of precipitation a year, while a station 30 km west on the windward slope records 1 350 mm. The town draws water from a small river fed mainly by high-altitude snowmelt, whose flow peaks in late spring and falls sharply by late summer. Demand is 180 litres per person per day and rising as the town grows. A proposal would build a pipeline to a reservoir on the windward side, 40 km away and 600 m higher.
1. Explain why the two stations differ so strongly in precipitation, and what this implies for the town's local water resources.
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The contrast is orographic. Westerly winds carry moist air; forced up the windward slope, the air cools, relative humidity rises, and condensation produces cloud and heavy rain or snow - hence 1 350 mm. As the air descends the leeward side it warms and its relative humidity falls, giving clear, dry conditions in the rain shadow - hence 480 mm at the town.
The implication is that the town's local supply is structurally limited. Its own rainfall is modest and its small river depends on snowmelt from high ground, so flow is seasonal: a late-spring peak when snow melts, then a sharp decline into late summer when demand for irrigation and domestic use is often highest. A town of 45 000 at 180 litres per person per day needs about 8 100 cubic metres per day, roughly 3 million cubic metres a year; that must be met from a catchment whose reliable dry-season yield is uncertain. Growth raises demand against a supply that is already seasonal and climatically constrained, so local sources alone are a fragile base.
2. Evaluate the pipeline proposal against alternatives, using the numbers given.
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The pipeline's strength is that it taps the wet windward catchment, where rainfall is nearly three times the town's, so the source is far more reliable than the local river. The 600 m elevation difference is also an asset: gravity could drive flow, cutting pumping energy, though the 40 km length means significant capital cost and maintenance across mountain terrain, and the line crosses a range subject to snow and slope hazards.
Against it: a long single pipeline is a single point of failure, and the windward reservoir may already serve other users or have environmental flow obligations. Alternatives to compare: demand management (metering, leak reduction, tariffs) can cut the 180 litres per person per day toward 130–150, reducing the required yield by a fifth or more at low cost; local storage to carry spring snowmelt into late summer addresses the seasonal mismatch directly; rainwater harvesting and treated wastewater reuse for irrigation add smaller volumes.
The balanced judgement is a phased plan: first demand management and local storage to capture the snowmelt peak, then the pipeline as a strategic supplement sized to the residual deficit, with a second connection or storage for resilience. Recommending the pipeline alone ignores cheaper first steps; rejecting it ignores the structural rainfall gap.