Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Evapotranspiration from green roofs during the drying cycle Simon Poë Alumasc & University of Sheffield – poes@alumasc-exteriors.co.uk Introduction The hydrologic cycle is impelled by gravitational forces and solar energy; inducing moisture vapour transfer from the earth’s surface to the atmosphere via evapotranspiration (ET). The rate at which this transfer takes place is important to a green roof’s response to a subsequent storm event. A green roof has a finite hydrological capacity, which is seldom fully available, due to the presence of antecedent moisture within the configuration. Through ET, the green roof’s capacity available for the retention of stormwater is regenerated; depleting this residual moisture. Not only must the climatological factors be considered in the modelling approach, but so too must the physical characteristics of the green roof that influence matric (and osmotic) forces that affect retention and release of moisture. A continuous modelling approach, featuring a water budget at its core, would account for the influences of the hydrologic cycle and the physical characteristics of the green roof configuration upon the capacity available for retention of a storm event.
Overview of Methodology Experimental studies were conducted to identify the drying cycle behaviour of 9 different green roof configurations when exposed to two climatic regimes. By using a climatecontrolled chamber at the University of Sheffield, air temperature, relative humidity and lighting could be set to replicate typical UK spring and summer conditions. Each of the nine microcosms was saturated and drained to field capacity, before being placed on to load cells for the continuous recording of mass at hourly intervals. Changes in mass were inferred to be changes in moisture content (with moisture loss equal to ET).
Figure 1 – Climate-Controlled Environmental Facility, University of Sheffield
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Key Findings There is a clear trend for mean daily ET losses to decay with respect to time and, specifically, ADWP (see Figure 2). In summer, a distinctive pattern of exponential decay in ET rates is evident. In spring conditions, this trend is quasi-linear. Daily ET rates recorded at shorter ADWPs are greater in summer than in spring. However, at longer ADWPs, summer ET rates fall below spring ET rates. This trend can be attributed to the low availability of moisture residing in the substrate after the fast initial rates of ET during summer conditions. 5
ET Rates [mm/day]
4 3
2 1 0 -1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
-2 ADWP (Days) Mean Summer
Mean Spring
Figure 2 – Mean Daily ET Rates
The importance of the relationship between moisture and ET losses is demonstrated in the strong correlation observed between mean daily ET rates and available water capacity [AWC] (see Figure 3). Here, there is a relatively linear correlation between daily ET rates and AWC in both climatic regimes - with R2 of 0.922 in spring and 0.966 in summer. With such strong correlations apparent here, any method of forecasting ET rates must surely consider AWC as a key modelling parameter; reflecting the difference between ET and PET.
Figure 3 – Correlation of ET to Available Moisture Capacity
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Further Reading Rezaei, F., Jarrett, A.R., 2006, Measure and Predict Evapotranspiration Rate from Green Roof Plant Species, Penn State College of Engineering Research Symposium, Penn State University. Stovin, V., Vesuviano, G. and Kasmin, H., 2012, The hydrological performance of a green roof test bed under UK climatic conditions, Journal of Hydrology, Vol. 414-415, 148-161. ISSN 0022-1694. DOI: 10.1016/j.jhydrol.2011.10.022 Voyde, E., Fassman, E., Simcock, R., 2010, Hydrology of an extensive living roof under sub-tropical climate conditions in Auckland, New Zealand. Journal of Hydrology, Vol. 394, 384-395. https://sites.google.com/a/sheffield.ac.uk/green-roof-research/ http://greenroof.shef.ac.uk/