Watering for Cool Comfort: How Soil Moisture Boosts Urban Tree Cooling

Redacción HC
24/09/2025

As cities around the world grapple with record-breaking heatwaves, urban planners and residents increasingly look to green infrastructure as a natural defense against extreme temperatures. Urban trees, long celebrated for their shading capacity, also provide a less visible but equally crucial service: evaporative cooling—the process by which water evaporates from soil and vegetation, cooling the surrounding air. Yet, the effectiveness of this natural air-conditioning depends heavily on soil moisture. A new study published in npj Urban Sustainability highlights how soil water availability can make the difference between a tree that merely casts shade and one that meaningfully lowers urban heat and improves human comfort.

Why Soil Moisture Matters in Urban Heat Mitigation

The research, led by L. Gobatti and colleagues from ETH Zurich and other European institutions, investigates how soil moisture influences the ability of urban trees to reduce heat stress. The team focused on Zurich, a city experiencing increasingly intense summer heat events. They asked a pressing question: to what extent do soil moisture and urban morphology—such as building density—shape the cooling power of trees?

The study underscores that shading alone is not enough. While the canopy of a tree can block direct sunlight, the true game changer is evaporative cooling, which depends on water availability in the soil. Without adequate soil moisture—whether from rainfall or irrigation—trees cannot release enough water vapor to significantly cool the air.

Multi-Scale Modeling to Measure Urban Cooling

To answer their research questions, the team employed a multi-scale modeling approach. They combined the Weather Research and Forecasting (WRF) model, which provides climate forcing data, with ENVI-met, a neighborhood-scale microclimate model. This setup allowed them to simulate multiple scenarios across Zurich’s different Local Climate Zones (LCZ)—ranging from low-density residential areas to compact urban cores.

For each zone, researchers created simplified neighborhood models to test variations in tree cover, urban configuration, and soil moisture levels. They compared scenarios with fully irrigated soils to those with dry conditions. The simulations were validated against local meteorological data to ensure accuracy.

The team measured the impacts on human thermal comfort using the Universal Thermal Climate Index (UTCI). They also separated the cooling effects into two components—shading and evaporative cooling—through controlled sensitivity experiments. This method revealed not only whether trees cool the air, but precisely how much of that cooling comes from soil-driven evaporation.

Key Findings: The Crucial Role of Soil Water

The results reveal a striking pattern: both vegetation density and soil moisture are critical for maximizing thermal comfort, but their combined impact varies across urban settings.

• Low- to medium-density zones benefited the most. When soil moisture was high—thanks to irrigation or naturally wet conditions—tree cover created significant “no thermal stress” areas during typical summer days. Evaporative cooling from both vegetation and soil accounted for more than 50% of the UTCI reduction in these areas.
• High-density urban cores, however, presented a challenge. Even with ample tree cover, dry soil conditions dramatically reduced the cooling effect. During extreme heat events, irrigation improved comfort levels but could not entirely eliminate thermal stress. In these compact zones, shading remained the dominant mechanism of temperature reduction, while evaporative cooling fell sharply when soil moisture was low.

These findings align with previous studies stressing the importance of water availability in green infrastructure performance, but they go further by quantifying how different urban morphologies amplify or dampen these effects.

Practical Implications for Urban Policy and Planning

For city planners and policymakers, the message is clear: planting trees is not enough. Water management is equally critical if urban forests are to deliver their full cooling potential.

• In low-density residential areas, maintaining soil moisture through sustained irrigation, designing water-retentive substrates, and planting species that enhance evaporative cooling can create comfortable microclimates for residents.
• In dense city centers, tree planting should be paired with broader strategies: improving air circulation with ventilation corridors, using low-heat-absorption materials, and even implementing active cooling systems during extreme events.

The researchers recommend integrated water management strategies, such as targeted irrigation in high-impact areas, using treated greywater, and adopting drip irrigation to minimize waste. Local climate modeling can help cities prioritize where such investments will yield the greatest benefit, particularly in communities most vulnerable to heat.

Beyond Trees: A Call for Heat-Resilient Cities

The study warns that during the most extreme heatwaves, even irrigated urban greenery may not fully protect residents. Climate adaptation plans must therefore include complementary measures such as establishing public cooling shelters and emergency heat management systems.

For cities in water-scarce regions, the challenge is even greater. Implementing efficient irrigation technologies and selecting drought-tolerant tree species will be essential. Urban heat mitigation strategies must balance the dual goals of cooling and water conservation.

Conclusion: Rethinking Urban Cooling Strategies

The ETH Zurich team’s work reinforces that urban cooling is as much about water as it is about trees. To create genuinely heat-resilient cities, urban planners must integrate irrigation planning, soil management, and strategic vegetation placement. Only by pairing greenery with smart water use can cities hope to protect human health and comfort in an era of intensifying heat.