The microclimate-moderating effects of light-colored, or cool, roofs have been well documented. These structures use reflective paint or tiles to reflect more sunlight and absorb less heat relative to a standard roof. Typical benefits include reduced air conditioning needs, energy bills and local air temperatures.

In addition to reducing the urban heat island effect, cool roofs can also enhance water conservation. Climate simulations performed by U.S. Lawrence Berkeley National Laboratory (LBNL) researchers for 18 California counties show that widespread deployment of cool roofs could reduce outdoor water consumption by as much as 9 percent. In Los Angeles County alone, total water savings could reach 83 million gallons per day if all buildings had reflective roofs installed.

A high-resolution regional climate model modified to account for irrigation water was validated with data from Northern California’s Contra Costa Water District for customers who were irrigation-only users. Model simulations were run over 15 years in 18 counties in Northern and Southern California, assuming a control scenario that reflects the current status of the urban areas, and a cool roof scenario in which all buildings had commercially available cool roofs installed.

Results indicated countywide irrigation water savings ranged from 4 percent to 9 percent, with per capita savings largest in medium density environments, or those with a mix of buildings and landscaping. Cool roofs can reduce water demand by reducing the ambient air temperature. Urban cooling trends ranging from 1^o to 1.5^o C mean lawns and other landscaping need less water.

The analysis also confirmed a finding that has been emerging: water conservation measures that directly reduce irrigation, such as drought-tolerant landscaping, can have the unintended consequence of increasing temperatures in urban areas. A simulation of the most extreme case – a complete cessation of irrigation – revealed a mean daytime warming of 1^o C averaged over the San Francisco Bay Area. According to the researchers, this suggests that the warming signal from strategies that focus only on outdoor water-use reductions can offset the cooling effects of a major heat mitigation strategy, such as citywide cool roof deployment.

Implementing cool-roofs over Northern (a) and Southern California (b) leads to average urban evaporative water demand reductions of 15% and 18%, respectively. (Source: LBNL)