GreenRoof software 1

GreenRoof Water balance model

The GreenRoof model is public domain software and hence freely available. The user-friendly software is menu driven and requires no specific computer knowledge. Reference Raes, D., Timmerman, A., Hermy, M., and Mentens, J. 2006. GreenRoof water balance model. K.U.Leuven University, Faculty of Bioscience Engineering, Division of Soil and Water Management, Leuven, Belgium. Installation procedure and structure Download the installation disk from the web site www.iupware.be (select downloads and subsequently software and manuals) and install the GreenRoof software by running the SETUP.EXE (application) file, which you will guide through the installation procedure. If the GreenRoof software is correctly installed, the main directory (default C:\Program Files\IUPWARE\GreenRoof ) should contain: (i) the following files: - GreenRoof.EXE (the executable file ); - DEFAULT.PAR (a file with the default program parameters); (ii) and three subdirectories: - DATA: which contains files with roof characteristics (*.TOP), and files with climatic

data (evaporation demand of the atmosphere (*.ETo), daily rainfall data (*. PLU), and type files (*.TYP) with reference to an ETo and a PLU file);

- HELP: which contains the help file GREENROOF.HLP; - OUTP: where output of the program will be stored, i.e. a file with the characteristics

of the analyzed roofs (*.CHA) and a file with the values of parameters of the water balance of the roofs (*.OUT).

GreenRoof software 2

Concept The GreenRoof model weighs the water balance of two roofs against each other (roof A versus B). The amount of water (i) retained on the roofs and (ii) evacuated as run-off from the roofs are computed on a daily basis for a simulation period to be specified by the user. Rain water retained on the roof is removed by evapotranspiration when the weather conditions are favorable. The input consists of files containing daily climatic data for the location and characteristics for the studied roof. Gravel roofs, tile and slate roofs, bitumen roofs and extensive green-roofs with various types of vegetation and degree of vegetation cover can be selected or created and saved in the data bank for later use. The roofs are characterized by their surface area, position, orientation, and slope and for green-roofs by the type and extension of the vegetation cover, depth of the substrate layer and the presence of a drainage/reservoir layer. The climatic data consists of daily rainfall observed in a representative weather station and grass reference evapotranspiration (ETo). The grass reference evapotranspiration characterizes the evaporative demand of the atmosphere for the location and is derived from meteorological data. Calculation procedure The water balance is computed with a daily time step by keeping track of the daily incoming (WIN) and outgoing (WRO and WET) water fluxes: Amount of water received by rainfall (WIN): Since roofs oriented to a dominant wind direction will receive more rainfall than flat roofs or oriented to another direction, the model estimate the incoming water WIN (liter.day-1) by considering not only rainfall but also the cosine of the slope and the orientation of the roof

)1(180

XaXCosRW RainobsIN +

= (Eq. 1) where Robs is the observed rainfall (mm.day-1), the surface area (m) of the roof, aRAIN a dimensionless program parameter and X the slope of the roof (degree). The values for aRAIN depend on the orientation of the roof and local wind and rainfall characteristics. The program parameter aRAIN can be adjusted by the user to consider local conditions.

Amount of water lost by run-off (WRO): If all incoming water WIN can be stored on the roof, WRO is zero. If WIN exceeds the actual storage capacity of the roof, the excess incoming rainfall is lost by surface run-off: ( )SactSINRO WWWW = max (Eq. 2) where WSact is the actual and WSmax the maximum amount of water (liter) that can be retained on the roof. At each time step WSact is updated by considering the incoming rainfall and the outgoing evapotranspiration. WSact ranges between zero when the roof is

GreenRoof software 3

completely dry and WSmax when the roof is soaked by rainfall. WSmax is a program parameter which value depends on the surface area of the roof and the roof type. The program parameter WSmax can be adjusted by the user to consider local conditions. Amount of water lost by evapotranspiration (WET): Evapotranspiration (ET) refers to the evaporation (E) of water on the roof or vegetation surfaces and the transpiration (T) of water by vegetation stored in the substrate of green-roofs. The amount of water lost by ET is determined by (i) the given evaporative demand of the atmosphere (ETo), (ii) the slope and orientation of the roof, (iii) the position of the roof, (iv) the characteristics of the roof surface, (v) the amount of water retained (WSact) on the roof and (vi) the surface area: (i) ETo (mm.day-1) is given as input; (ii) The slope and orientation of the roof will increase or decrease the amount of

radiation received and hence ETo. The adjusted ETo (mm.day-1) is given by

)1( XaEToETo RADadjusted += (Eq. 3)

where aRAD is a program parameter and X the slope of the roof (degree). The program parameter aRAD can be adjusted by the user to consider local conditions.

(iii) Although ETo is not very sensitive to wind speed, a correction is required if the wind speed observed on the roof strongly deviates from the observations in the meteorological station. The required correction is negative when the roof is shut in by other buildings and positive for tall, fully exposed buildings. The required correction is negative when the roof is shut in by other buildings and positive for tall, fully exposed buildings. Four exponential equations are used to adjust the evaporation rate. The coefficients of the equations are program parameters and can be adjusted by the user to consider local conditions;

(iv) For identical environmental conditions, the ET from a wet (green-)roof differs

from ETo due to differences in the characteristics of the roof surface when compared with the reference grass surface. By integrating the effect of evaporation and transpiration into a single crop coefficient (Kc), the ET from a wet roof is given by the Kc approach (Allen et al., 1998):

*EToKcET roofwet = (Eq. 4)

where ETo* (mm.day-1) is the reference evapotranspiration corrected and adjusted for slope, orientation and position if required. By selecting a roof type, the program considers the appropriate Kc coefficient. The Kc of green-roofs depends on the type of vegetation and degree of soil cover. The Kc coefficient can be adjusted by the user to consider local conditions;

GreenRoof software 4

(v) When water is lost by ET from the roof, the actual water content WSact decreases and water becomes more strongly bound to the matrix and is more difficult to extract. This is simulated by considering a water stress coefficient Ks

*EToKcKsETroof = (Eq. 5)

Ks is 1 when the water content is above a threshold value. When the water content drops below the threshold, Ks decreases linearly from 1 to 0. Ks is 0 when WSact is zero. The threshold value is specified by the fraction p of the maximum of water that can be retained (WSmax). In the model it is assumed that water retained on the roof or vegetation surface can fully evaporate without any restriction, i.e. the evaporation rate is only dictated by the climatic conditions and p is 1. The default values for p for the evaporation from the substrate layer and transpiration by the vegetation are given as program parameters. The p coefficient can be adjusted by the user to consider local conditions;

(vi) The amount of water lost by evapotranspiration, WET (liter.day-1), is given by

*EToKcKsWET = (Eq. 6)