Quantifying Evapotranspiration from Urban Green Roofs: A Comparison of Chamber Measurements with Commonly Used Predictive Methods

Abstract

Quantifying green roof evapotranspiration (ET) in urban climates is important for assessing environmental benefits, including stormwater runoff attenuation and urban heat island mitigation. In this study, a dynamic chamber method was developed to quantify ET on two extensive green roofs located in New York City, NY. Hourly chamber measurements taken from July 2009 to December 2009 and April 2012 to October 2013 illustrate both diurnal and seasonal variations in ET. Observed monthly total ET depth ranged from 0.22 cm in winter to 15.36 cm in summer. Chamber results were compared to two predictive methods for estimating ET; namely the Penman-based ASCE Standardized Reference Evapotranspiration (ASCE RET) equation, and an energy balance model, both parametrized using on-site environmental conditions. Dynamic chamber ET results were similar to ASCE RET estimates; however, the ASCE RET equation overestimated bottommost ET values during the winter months, and underestimated peak ET values during the summer months. The energy balance method was shown to underestimate ET compared the ASCE RET equation. The work highlights the utility of the chamber method for quantifying green roof evapotranspiration and indicates green roof ET might be better estimated by Penman-based evapotranspiration equations than energy balance methods.

Observation and Estimation of Evapotranspiration from an Irrigated Green Roof in a Rain-Scarce Environment

“…Yet, the observed ET rates during the summer with irrigation applied are not always significantly larger than the reported values in the rain-rich environments. The maximum monthly ET rates of the sedum and grass are lower than the maximum monthly average (7 mm d −1 ) reported by a multi-year study of sedums in Villanova, PA, USA with an annual rainfall of 817-1352 mm [42], but are close to the reported 4.94 mm d −1 in the rain-rich region [86]. There is no consistent relationship between the observed summer ET rates and the reported values.…”

“…For the grass cover, the VIP scores for air temperature, solar radiation, surface moisture, and relative humidity are 1.48, 1.33, 1.07, and 1.01, respectively. dry period measured in a climate-controlled chamber [29] [86]. There is no consistent relationship between the observed summer ET rates and the reported values.…”

“…Compared to the nonvegetated roofs, vegetation could elevate ET rates by over 25% on average. The observed ET rates of sedums and grass tend to be slightly higher than most studies conducted in rain-rich environments [23,29,33,[85][86][87]. For example, Rezaei [85] observed ET rates of 1.68 mm/d for sedums and 1.06 mm/d for the non-vegetated medium with unlimited water supply in an indoor environment.…”

“…For example, Rezaei [85] observed ET rates of 1.68 mm/d for sedums and 1.06 mm/d for the non-vegetated medium with unlimited water supply in an indoor environment. Marasco et al [86] measured via a dynamic chamber 1.71 mm/d on average for sedums in New York City, NY with 1078 mm annual rainfall, while DiGiovanni et al [33] measured via lysimeters 2.27 mm/d for sedums in the Bronx, NY with an average annual rainfall of 1100 mm. In Auckland, New Zealand, Voyde et al [22] reported 2.21 mm/d for sedums with an annual rainfall of 1240 mm via lysimeter measurements.…”

Evaluation of common evapotranspiration models based on measurements from two extensive green roofs in New York City

“…Table 1 summarizes the green roof characteristics. Amore detailed description of the green roof sites, instrumentation, and data collection is provided by Marasco et al(2014).…”