Rainfall decline and water-supply choices on Gran Canaria
The Chira–Soria reservoir system on Gran Canaria supplies irrigation and some urban water. At the Soria gauge, mean annual rainfall was 480 mm for 1961–1990 and 410 mm for 1991–2020; the number of days with more than 30 mm has increased slightly, while the number of rain days has fallen. Reservoir storage in October was 32% of capacity, compared with a 1990–2020 average of 55%. Annual demand is about 60 hm³ and reliable supply about 55 hm³, and tourism is projected to grow by 2% per year. Saharan dust intrusions, which affect water quality and solar energy, have become more frequent in the records. The island water council is debating a new 20,000 m³/day desalination plant versus demand management and recycled water. You are asked to assess the evidence for a climate trend and to recommend an adaptation strategy.
1. Assess whether the rainfall record shows a statistically meaningful drying trend. What caveats must you state?
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The record shows a 15% decrease between the two 30-year averages, from 480 to 410 mm, and the drop in rain days alongside a slight rise in heavy events suggests a change in rainfall character, not just a random fluctuation. But two 30-year averages do not establish statistical significance. I would need the annual series and a trend test such as Mann–Kendall or regression with confidence intervals. A single gauge is also weak evidence on a mountainous island with strong orographic contrasts; gauge relocation, changes in exposure and missing years could bias the result. The 30-year windows are arbitrary, and multi-decadal variability such as the North Atlantic Oscillation can produce prolonged dry spells. The reservoir figure of 32% versus 55% is consistent with drought but also reflects demand and management. My conclusion is that the evidence is consistent with a drying trend and more intense rainfall, but it is not conclusive without better spatial coverage and attribution analysis.
2. Distinguish climate variability from climate change in this case. What role do the NAO and the trade-wind inversion play?
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Climate variability is the year-to-year and multi-decadal fluctuation around a mean; climate change is a sustained shift in that mean or in the distribution of extremes. The NAO strongly affects winter rainfall in the Canary region: a positive NAO is often associated with drier winters, while negative phases can bring more Atlantic fronts. A 30-year decline could therefore be partly NAO variability. However, the combination of fewer rain days, more heavy events and rising temperatures is more consistent with a climate-change signal, because warming increases evaporation and the atmosphere's water-holding capacity. The trade-wind inversion is critical: the subsidence inversion around 1,500 m separates moist marine air from dry free atmosphere. If the inversion strengthens or lowers, orographic rainfall on the windward slopes decreases and the mar de nubes may become thinner, reducing fog drip and laurel-forest water capture. Saharan dust adds another stress. Attribution requires formal detection and attribution studies, but the balance of evidence favours a change beyond simple variability.
3. Recommend an adaptation strategy and evaluate the desalination proposal using energy, brine and reliability criteria.
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The supply–demand gap is about 5 hm³/yr, or 8% of demand, and tourism growth of 2% per year will widen it. A 20,000 m³/day desalination plant would produce about 7.3 hm³/yr, enough to close the gap, and it is climate-independent, which is its main strength. But it is energy-intensive, typically 3–4 kWh/m³, and unless powered by renewables it increases greenhouse-gas emissions. Brine discharge can harm coastal ecosystems, especially if poorly diluted, and the plant creates a long-term dependency on energy and maintenance. I would recommend a portfolio rather than desalination alone: reduce leakage, price water to reflect scarcity, expand recycled water for golf courses and agriculture, promote efficient irrigation, and restore aquifer recharge with treated water. Desalination should be a backup sized to the residual deficit, coupled with renewable energy and a monitored brine diffuser. This strategy is more resilient because it combines demand reduction, recycling and a drought-proof source, rather than betting the island's water future on one plant.