Moisture content behaviour in extensive green roofs during dry periods: The influence of vegetation and substrate characteristics

•New field data on moisture content in four green roof test beds are presented.•Moisture fluctuations during one complete month and five dry periods are detailed.•Moisture loss due to ET is affected by substrate characteristics and vegetation.•Moisture loss due to ET is significantly affected by substrate moisture content.•A model to predict moisture loss from PET is presented.

SummaryEvapotranspiration (ET) is a key parameter that influences the stormwater retention capacity, and thus the hydrological performance, of green roofs. This paper investigates how the moisture content in extensive green roofs varies during dry periods due to evapotranspiration. The study is supported by 29 months continuous field monitoring of the moisture content within four green roof test beds. The beds incorporated three different substrates, with three being vegetated with sedum and one left unvegetated. Water content reflectometers were located at three different soil depths to measure the soil moisture profile and to record temporal changes in moisture content at a five-minute resolution. The moisture content vertical profiles varied consistently, with slightly elevated moisture content levels being recorded at the deepest substrate layer in the vegetated systems. Daily moisture loss rates were influenced by both temperature and moisture content, with reduced moisture loss/evapotranspiration when the soil moisture was restricted. The presence of vegetation resulted in higher daily moisture loss. Finally, it is demonstrated that the observed moisture content data can be accurately simulated using a hydrologic model based on water balance and two conventional Potential ET models (Hargreaves and FAO56 Penman-Monteith) combined with a soil moisture extraction function. Configuration-specific correction factors have been proposed to account for differences between green roof systems and standard reference crops.