Planning considerations - Living Roofs in Integrated Urban Water Systems - page 78

Table 3.3 Runoff curve number according to Köppen Geiger climate zone

Köppen Geiger climate zone a Data origins (# of living

roofs monitored)

% of data at each site with
P/S > 0.46

Average CN
(Std. Deviation)

Cfa New York City, NY (2)

Villanova, PA (1)

52, 45
45

92 (2)

Cfb Raleigh, NC (1)

Kinston, NC (1)
Goldsboro, NC (1)
Athens, GA (1) b
Shefield, UK (1)
Auckland, NZ (4)

52
72
64
35
71
16, 28, 52, 18

90 (3)

Csa Genoa, Italy (1) 100 93

Csb Portland, OR (2) 37, 14 79 (13)

Dfa Chicago, IL (1)

State College, PA (1)
Southield, MI (1)
Brownstown, MI (1)
Toronto, Ontario, CA (1)

9
27
52
70
57

90 (6)

Dfb East Lansing, MI (4) n/a c 88 (3)

Dfa/Cfa d Pittsburgh, PA (1) 92 96

Notes
a First letter (capital) indicates main climate (C = warm temperate, D = snow), second letter indicates precipitation (f = fully humid, s =
summer dry), third letter indicates temperature (a = hot summer; b = warm summer; c = cool summer).
b For consistent comparison, CN for the Athens, GA site was herein calculated using the Hawkins et al. (2009) methodology. Carter and
Rasmussen (2006) used a different methodology for this site, resulting in CN = 86.
c CN was determined directly by Getter et al. (2007) using the methodology provided by Carter and Rasmussen (2006). Original data
was not available for recalculation herein.
d Pittsburgh is characterized by multiple climate zones. Only 12 storm events contributed to the CN for this site.

by relatively short monitoring programs (most are a few months, perhaps within
a single season, or up to about one year), a consequence most often constrained
by terms of grant funding. Considering the available data sets, the CN s in Table
3.3 were determined using storms as small as 2 mm (with the exception of
Genoa, which was 8 mm) and the majority of all events were less than 12.5 mm;
therefore the average CN s are likely overly conservative.
A further potential limitation arises from the assumption of I a . Equations 3.1
and 3.2 depend, in part, on an assumed relationship between the initial abstrac-
tion ( I a ) and the total maximum catchment retention (i.e., storage, S ) after runoff
begins. The National Engineering Handbook states that an empirical relationship
of I a = 0.2 S was found to provide the best it for 50 percent of the data. 9 This
relationship became a procedural step and underlying assumption in TR-55, and
remains consistent instruction in applying CN s for agricultural or urban areas in
NRCS (2004a). NRCS indicates that alternative relationships (versus Equation 3.1)
should be derived for areas where the relationship I a = 0.2 S does not apply.
Hawkins et al. (2009) provide compelling evidence that I a = 0.05 S , and go on to
calculate new CN s for all land uses. In either case, I a from a living roof may be