Page 135 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED W ater Ef ficiency 115

Credit Determinations Now that the total water needs for the landscaping have been
determined, the amount of potable water (or other special waters that are recommended
not to be used for irrigation for environmental considerations) that will be applied in
the design case (TPWA) can be determined from the total water applied for the design
case (TWAD) less the reuse water (RW). The author has interpreted reuse water to be any
waters from rainwater harvesting, reused graywaters, and municipal supplied “reuse”
waters applied for irrigation that are not for potable uses. Other waters from the site
such as pond water or nonpotable groundwater may not be subtracted out, except as
noted for groundwater seepage pumped from slab and foundation areas. These other
waters are considered to be important for other environmental reasons on the site.
However, even with this clarification there are many definitions of graywaters. The
WEc2 subcategory section on Innovative Wastewater Technologies gives some further
insight into the definition of graywaters.
TPWA = TWAD − RW (3.1.7)

Note that the units must all cancel to give a unit for the TWAB and TWAD in volume
per month or hour. Typically, the ET rates are given in inches per hour or month, the
areas are given in square feet, and the other factors are dimensionless. In these cases, the
2
conversion factor to obtain TWAB or TWAD in gallons per unit time is 0.6233 gal/(ft · in),
or inversely, 1.604 ft · in/gal.
2
The values for TWAB, TWAD, and TPWA can be used for the credit verifications.
Two points for LEED 2009 WEc1 may be earned if the following inequality is true:
TPWA ≤ 050 (3.1.8)
.
TWAB
Two additional points can be given for LEED 2009 WEc1 if the following two equations are
both true:
TWAD
.
≤ 050 (3.1.9)
TWAB
and
TPWA = 0 (3.1.10)

Special Circumstances and Exemplary Performance (WE Credit 1)
As mentioned previously, if there is no irrigation, then there are no calculations necessary
to obtain both credits. If there is any irrigation, even if all is from reuse water, then
calculations usually must be performed. There are some circumstances where it may be
unclear whether the water used for watering is “irrigation” water per the intent of the
credit or “process” water. One example is for the main field in a baseball stadium, where
watering the main playing field may be considered irrigation or may be considered as
a special water application (process water). Typical playing fields are irrigated, but
main sports venue fields are sometimes looked at differently and have different
requirements. These special exceptions should be reviewed through the CIR (Credit
Interpretation Ruling) procedure.
There are many projects which do not provide any landscaping on-site and the
LEED 2009 WEc1 credit points are not available for use on these projects, unless there