Trees are the most effective natural tools for cooling urban environments, combating heatwaves, and creating more livable cities. This cooling power is achieved through two primary mechanisms: providing shade and evapotranspiration. Evapotranspiration refers to the process where water absorbed by tree roots is released into the atmosphere through leaves, cooling the surrounding air. However, this vital ecosystem service depends heavily on soil moisture levels and the water availability for trees.
Suggested citation format: Tušer, M. (2024, August). Cooling effect of trees: Maximizing urban resilience with targeted watering and irrigation. TREEIB Blog. Retrieved from https://www.treeib.com
Environmental Cooling by Tree Evapotranspiration
Through a well-known physical principle, trees use energy from their environment to convert water into vapor. To evaporate just 1 gram of water, approximately 2,45 kJ of energy (1.054 BTU) is required at 20 °C (68 °F). When trees have access to sufficient soil moisture, they can cool their surroundings significantly by releasing large quantities of water as vapor during photosynthesis.
A mature tree with access to adequate soil moisture can easily evapotranspirate up to 500–1 000 liters of water daily (132–265 gallons). This volume can be measured. Research indicates that individual giant sequoias can transpire up to 3,752 liters (approximately 991 gallons) daily! You can easily calculate the transpiration potential of your tree in i-Tree Eco software.
However, during extreme heatwaves, soil moisture is often depleted, and evapotranspiration volume can drop drastically because there is simply not enough water in the soil. This reduction in water availability decreases evapotranspiration and causes significant losses in cooling potential. Tree watering can be a valuable strategy to mitigate the urban heat.
The necessity of securing water availability can be explained in the following example: we can calculate in iTree Eco that a big oak has the potential of evapotranspirating 600 liters of water daily (158 gallons). In drought periods, the water content decreases in the soil, causing the tree to be able to evapotranspiration only 30 liters (8 gallons) per day under extreme drought conditions. This 570-liter (150-gallon) shortfall represents a daily cooling effect loss of approximately 1,4 GJ (1.32 million BTU)—an energy equivalent to continuously running an air conditioning unit in a 20 m² (215 ft²) room for 13 days! Scroll down to see all calculation below as well as explanation, what such energy represents.
Enhancing Urban Cooling with Targeted Watering and Irrigation
The TREEIB® methodology addresses these losses through precision watering. This involves irrigating trees with sufficient water to replenish soil moisture and restore their full cooling potential. TREEIB® watering bags, designed for drip irrigation, deliver water slowly over 8–12 hours, ensuring it penetrates to a depth of 60–90 cm (2–3 feet). This approach effectively restores soil moisture levels and supports maximum utilization of the tree's evapotranspiration potential, thus cooling effect.
To maximize a tree’s cooling effect, TREEIB® recommends providing ten times the tree’s daily transpiration potential in water. For a tree capable of evaporating 600 liters (158 gallons) per day, this equates to 6 000 liters (1,585 gallons) per irrigation session. This volume can be delivered using four TREEIB® 1 500-liter bags (396-gallon bags) simultaneously or over several days if fewer bags are available.
A single TREEIB® irrigation session can sustain a tree’s cooling potential for at least 10 days, even during heatwaves. By restoring vertical water flow in the soil, often interrupted during prolonged dry periods, the effect can last even longer.
Strategic Watering for Resilient Urban Trees
The TREEIB® methodology goes beyond immediate cooling. By providing sufficient water during specific periods, this approach promotes long-term tree growth, increasing canopy size and leaf area, which enhances both shading and evapotranspiration.
For optimal results, TREEIB® recommends seasonal irrigation:
Spring and autumn watering: Supports tree growth when water is typically abundant.
Summer/heatwave watering: Focuses on maximizing cooling effects during extreme conditions, with 1–2 additional irrigation sessions per year.
Even under theoretical extreme heat conditions, an average mature tree would require only six irrigation sessions of 6 000 liters (1,585 gallons) each over the summer to maintain its full cooling capacity.
Quantifying Tree Cooling Losses in Energy Terms
Understanding the significance of irrigation requires contextualizing tree cooling losses in terms of energy:
Household Energy:
A cooling loss of 1,4 GJ (1.32 million BTU) per day equals 7 % of the annual energy consumption of an average Czech household or 1.5 % of a typical US household's annual energy usage.
Automobiles:
This energy is equivalent to 40,8 liters (11.6 gallons) of petrol, enabling a car to travel 544 kilometers (338 miles) in Europe or 290 miles in the US, given differences in average fuel efficiency.
Air Conditioning:
The lost energy could power an air conditioning unit for 323 hours, cooling a 20 m² (215 ft²) room continuously for 13 days.
TREEIB®: A Scalable Solution for Urban Cooling
Implementing the TREEIB® methodology in cities can significantly mitigate urban heat and enhance climate resilience. By prioritizing trees with the highest cooling potential, urban planners, climate officers, and architects can create a strategic framework for sustainable urban cooling.
Targeted watering not only prevents trees from drying out during heatwaves but also maximizes their ecosystem services, including carbon sequestration, air purification, and urban temperature regulation.
Ready to transform your city into a cooler, greener, and more livable space? Explore how TREEIB® solutions can make a difference today!
Localization Insights
Localized tables with detailed calculations for US and European energy comparisons, irrigation metrics, and TREEIB® methodologies will follow below, ensuring accessibility for audiences in Europe as well as the USA.
Author: Ing. Martin Tušer, August 2024.
Image courtesy of: Goran Huljenić, Urbani šumari, Croatia
EUROPE: Comparing Tree Cooling Effect Loss to Other Energy Sources:
What does the loss of 1.4 GJ of cooling energy from one large tree in a day due to insufficient water mean? Just a reminder, this is the amount of energy per ONE DAY!
1. Household Energy Consumption:
- The average Czech household consumes approximately 20 GJ of energy annually.
- 1,3965 GJ would cover about 7% of a household's annual energy consumption.
2. Automobiles:
- Petrol has an energy content of approximately 34,2 MJ/liter.
- 1,3965 GJ is equivalent to the energy contained in about 40,8 liters of petrol (1,3965 GJ × 1000 MJ/GJ ÷ 34,2 MJ/liter).
- This would allow an average car with a consumption of 7,5 liters per 100 km to travel approximately 544 kilometers.
3. Electrical Energy:
- 1 GJ is equivalent to about 278 kWh.
- 1,3965 GJ corresponds to roughly 388 kWh.
- This could cover approximately 1,3 months of electricity consumption for a typical Czech household (with an average monthly consumption of 300 kWh).
4. Air Conditioning:
- The average air conditioning unit consumes approximately 1,2 kW of energy per hour.
- This is equivalent to 1,2 kWh/h.
- 1 GJ is equivalent to about 278 kWh.
- 1,3965 GJ is equivalent to approximately 388 kWh (1,3965 GJ × 278 kWh/GJ).
- This means that 1,3965 GJ can power one air conditioning unit for approximately 323 hours (388 kWh ÷ 1,2 kWh/h).
5. Air-Conditioned Space:
- Considering an average room size of 20 m² and a ceiling height of 2.5 m (total volume 50 m³), and an average air conditioning unit capable of cooling such a space:
- One air conditioning unit consumes 1,2 kWh/h, so 388 kWh would allow cooling this space for 323 hours.
- This means that 1,3965 GJ can air-condition a 20 m² room for approximately 13 days with continuous operation (323 hours ÷ 24 hours/day).
CALCULATION OF ENERGY REQUIRED TO CONVERT 570 LITERS OF WATER TO VAPOR AT 20°C:
Conversion of water volume to mass:
- The density of water is approximately 1 kg/liter.
- 570 liters of water is, therefore, 570 kg of water.
Energy Calculation:
- We know that 1 gram of water requires 2,45 kJ of energy to convert to vapor (2450 kJ/kg ÷ 1000 g/kg).
- 570 kg of water is 570 000 grams of water.
- The energy required to convert 570 000 grams of water to vapor is: 570 000×2,45 kJ = 1 396 500 kJ = 1,3965 GJ, rounded to 1,4 GJ.
The USA: Comparing Tree Cooling Effect Loss to Other Energy Sources:
What does the loss of 1.32 million BTU of cooling energy from one large tree in a day due to insufficient water mean? This is the amount of energy lost in just one day!
1. Household Energy Consumption:
- The average US household consumes approximately 85.4 million BTU of energy annually.
- 1.32 million BTU would cover about 1.5% of a US household's annual energy consumption.
- Compare: 1.32 million BTU covers about 7% of a Czech household's annual energy consumption.
2. Automobiles:
- Gasoline has an energy content of approximately 114,000 BTU per gallon.
- 1.32 million BTU is equivalent to the energy contained in about 11.6 gallons of gasoline.
- This would allow an average U.S. car with a consumption of 25 miles per gallon to travel approximately 290 miles.
Compare: due to lower average consumption of cars in Europe, this amount of energy allows you to travel about 544 kilometers, which is 338 miles.
3. Electrical Energy:
- 1 BTU is equivalent to about 0.000293 kWh.
- 1.32 million BTU corresponds to roughly 387 kWh.
- This could cover approximately 0.43 months of electricity consumption for a typical US household. The average electricity consumption for a typical U.S. household is approximately 10,715 kilowatt-hours (kWh) per year, according to data from the U.S. Energy Information Administration (EIA).
Compare: in Europe, the 387kWh covers the average consumption of a Czech household for about 1.4 months.
4. Air Conditioning:
- The average air conditioning unit consumes approximately 1.2 kW of energy per hour.
- 1.32 million BTU is equivalent to approximately 387 kWh.
- This means that 1.32 million BTU can power one air conditioning unit for approximately 323 hours.
5. Air-Conditioned Space:
- Considering an average room size of 215 square feet and a ceiling height of 8 feet (total volume 1,720 cubic feet), and an average air conditioning unit capable of cooling such a space:
- One air conditioning unit consumes 1.2 kWh/h, so 387 kWh would allow cooling this space for 323 hours.
- This means that 1.32 million BTU can air-condition a 215-square-foot room for approximately 13 days with continuous operation.
Calculation of Energy Required to Convert 150 Gallons of Water to Vapor at 68°F
Conversion of water volume to mass:
- The density of water is approximately 8.34 pounds per gallon.
- 150 gallons of water is, therefore, 1,251 pounds of water.
Energy Calculation:
- We know that 1 gram of water requires 1,054 BTU of energy to convert to vapor.
- 1,251 pounds of water is approximately 568,000 grams of water.
- The energy required to convert 568,000 grams of water to vapor is 568,000 × 1.054 = 1,322,720 BTU (about 1.32 million BTU).
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