Evapotranspiration in green stormwater infrastructure systems

Evapotranspiration (ET) is a viable runoff reduction mechanism and an important player in the hydrologic cycle of vegetated green stormwater infrastructure (GSI). As a dynamic process, ET is dependent on both meteorological factors (e.g., rainfall characteristics, relative humidity, and air temperature) and GSI properties (e.g., soil media type). This paper investigates the role of ET in runoff volume reduction of green roofs and rain gardens through a comprehensive literature review. Evapotranspiration is mostly unaccounted in the design and crediting of GSI systems because of the complex interaction of soil, plants, and climate that makes its quantification difficult. To improve vegetated GSI design for runoff volume reduction, design methods should consider ET and infiltration processes concurrently. Two methods, complex and simple, are reviewed and discussed herein. The simple method requires minimal input information compared to the more complex continuous simulation method; however continuous simulation yields volume reduction values more similar to field observations. It is demonstrated that modifying the drainage structure and using fine-grained in-situ soils can potentially increase ET in vegetated GSI systems. None of the available ET predictive equations, mostly derived from agricultural sciences, are found to precisely match observed GSI ET data. Until further research is conducted on GSI ET estimation methods, the 1985 Hargreaves method is recommended when performing continuous simulations. The 1985 Hargreaves method is simple, requires limited input data that are readily available, and generates reasonable results. Technical recommendations and directions for future research are provided.