Applicability of Classical Predictive Equations for the Estimation of Evapotranspiration from Urban Green Spaces: Green Roof Results

Green roofs and other urban green spaces can provide a variety of valuable benefits linked to evaporative processes, including storm-water management, reduction of urban heat island, and carbon sequestration. Accurate and representative estimation of urban evapotranspiration (ET) is a necessary tool for predicting such benefits. However, many common ET estimation procedures were developed for agricultural applications, and thus carry inherent assumptions that may not be applicable to urban green spaces, including green roofs. The objective of this paper is to evaluate the performance of two combination methods for the prediction of ET from a green roof. Two ET estimation methodologies were compared, using on-site and regionally available data sets for daily time steps, to weighing lysimeter measurements of actual ET at a green roof site in the Bronx, New York. Regionally available estimates of potential ET did not accurately predict lysimeter measured actual ET on 30 nonconsecutive, non-water-limited days in months from September through December. Over the same period, the ASCE Standardized Reference Evapotranspiration Equation performed well in predicting actual ET with an RMSD of only 0.03 mm d−1. Additionally, the ET equation for short reference types, using on-site climatic data and coupled with a variation of the Thornthwaite-Mather approximation, which accounts for variable media moisture conditions, gave reasonable predictions of actual evapotranspiration for 89 days analyzed (representing months from June through January) with an aggregate underestimation of 10.1%. However, this method was highly sensitive to input parameters, specifically media field capacity. Further on-site data collection is necessary to fully evaluate the performance of the equations over different seasons at this location, and monitoring of supplementary urban green spaces and green infrastructure sites will also lend further insights regarding urban evapotranspiration.