Water Efficiency: 5 possible points
Overview
GOALS
A long term goal of Stanford University is to reduce its reliance on municipal water
supplies in two ways: Reduce campus water use, and reuse water from non-potable
sources on or near campus. To do its own part, and meet its own water reduction goals,
the new GSB campus will strive to use water responsibly. In the process, it will earn all
five LEED-NC Water Efficiency credits, and possibly one or more additional Innovation
Credits. The new campus will benefit by using water reducing resources and methods
already in place on campus, and by pushing the design to use water extremely efficiently.
These methods -- some well understood, and others more modern -- are discussed in this
report.
METHODS
Native Vegetation -- Stanford has a
strong tradition of using native, droughttolerant plants that beautify the campus
while requiring minimal or no watering.
The new GSB campus can use species
successfully grown at Stanford on its
new campus to help reduce water use.
Stanford’s Grounds Services publishes
recommended trees, plants, and shrubs
that can be used at the GSB.
Stanford Campus photo by John Millea
Non Potable Water Sources -- Water is not unlimited at Stanford, and the GSB must
help to reduce strains on the municipal water supply by using non potable water
whenever appropriate. Stanford already derives irrigation water from its central energy
plant, and can use non potable water from Searsville Lake, 4.5 miles from the GSB.
Coupled with stormwater and graywater recovery, the new campus can fulfill all
irrigation needs without using potable water.
Responsible Sewage Conveyance -- By using low flow toilets fixtures (especially
waterless urinals), the GSB can reduce or eliminate the need to use potable water for
sewage conveyance. Couple these low flow fixtures with additional captured rainwater
or recycled graywater, and the need for potable water for this use can be reduced or even
eliminated.
Potable Water Use Reduction -- Despite all these water saving strategies, the new GSB
will still need potable water. By using low flow fixtures in bathroom and kitchen sinks,
showers, and drinking fountain, the amount of potable water used can be greatly reduced.
Also by using captured graywater for tasks normally completed with potable water, the
reliance on this scarce resource can be drastically reduced.
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 1
Water Efficiency: 5 possible points
LEED-NC Scorecard
LEED for New Construction v2.2
Registered Project Checklist
Project Name: Stanford GSB: Knight Management Center
Project Address: Stanford, CA
Yes
5
1
1
1
1
1
?
No
Water Efficiency
Credit 1.1
Credit 1.2
Credit 2
Credit 3.1
Credit 3.2
5 Points
Water Efficient Landscaping, Reduce by 50%
Water Efficient Landscaping, No Potable Use or No Irrigation
Innovative Wastewater Technologies
Water Use Reduction, 20% Reduction
Water Use Reduction, 30% Reduction
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 2
1
1
1
1
1
WE Credit 1.1: Water Efficient Landscaping, Reduce by 50%
LEED-NC Credit: YES | UNSURE | NO
Credit Intent
Reduce or try to eliminate strains on the municipal water supply by using non-potable
water for 50% or more of total landscape irrigation.
Feasibility
Stanford has a strong culture in place to limit potable water use, a valuable resource that
will only become scarcer in the future. This credit should be pursued not only to achieve
the LEED-NC credit, but more importantly to help meet the long term goals of the
Stanford community. As ARUP notes:
“The new Stanford Graduate School of Business aspires to elevate the level of
attention paid to water as a resource, in part as a response to ever increasing
demands for the limited supply and in part as a natural progression of what has
long been a University wide desire to sustain itself within the natural limits of its
environment.”1
To achieve this 50% reduction in potable water use, two steps need to be carried out,
most of which luckily already fit within Stanford’s typical practices: (1)Trees, plants, and
grass should be native to the region and should not require excessive watering to stay
healthy -- this will help reduce irrigation demand immediately. (2) Strategies should be
implemented to use non-potable water (rainwater, graywater, recycled wastewater, etc.)
sources before tapping into potable water supplies. By applying these two strategies, this
credit can be achieved.
Analysis
(1) Use landscaping that requires minimal watering
This is one of Stanford’s strengths; many native plants
exist throughout campus that require very little or no
irrigation. While it appears that no firm landscaping
plans have been set for the new GSB, some simple
guidelines can help ensure that the installed landscaping
and vegetation requires minimal watering.
Use native plants -- Stanford Facilities’ Landscaping
Guidelines recommend that the architect consider
“landscape design concepts that incorporate water
Figure 1: Western Rosebud4
and energy conservation methods, including appropriate
irrigation methods…”2 Luckily for Stanford, this can mean many different things. Here
is a brief list of native California drought-tolerant plants, grown successfully on the
Stanford, and recommended by Stanford Grounds Services on their website:3
1
ARUP, Pre-SD Plumbing Narrative, 4/30/07
Stanford University, Landscaping Guidelines,
http://facilities.stanford.edu/fdg/docs/Landscaping%20Guidelines.pdf.
3
Stanford University Grounds Services, California Native Plants at Stanford,
http://grounds.stanford.edu/topics/calnativeplantchart.htm#
2
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 3
WE Credit 1.1: Water Efficient Landscaping, Reduce by 50%
LEED-NC Credit: YES | UNSURE | NO
Trees
● California Buckeye -- Drought tolerant, can have moderate watering in summer
● Valley Oak -- Best without any watering
● Western Redbud (see Figure 1 above) -- Can tolerate moderate summer
watering, but this water isn’t required.
Shrubs
● Dark Star California Lilac -- Requires no
watering
● Julia Phelps CA Lilac -- Requires no watering;
needs good drainage (see Figure 2, to right)
● Flannel Bush -- No watering needed
Grasses
● Deer Grass -- Moderate watering OK in summer
Figure 2: Julia Phelps CA Lilac4
While designing the landscaping for the new GSB campus, special consideration should
be given to these and other plant species that thrive on and beautify the Stanford campus
while requiring zero or minimal watering. Such an approach to landscaping conforms to
LEED-NC Credit 1 recommendations of choosing plants that will easily adapt to the site.
The LEED-NC guide also notes that in many projects, non-potable water use has been
reduced by 50% simply by selecting drought-resistant or low maintenance vegetation.
(2) Use non-potable water sources before tapping potable resources
After reducing watering demand by using campus-friendly plants, the design team should
make use of non-potable water for irrigation before using any potable water. In ARUP’s
plumbing narrative, it notes preliminary intentions to reclaim water: “The schematic
intent is to reclaim process water sourced from the Central Energy Facility (CEF) cooling
tower blowdown and boiler condensate bleed-off,” which is strategy Stanford has used on
other campus projects.5 Immediately, this campus-originated water will help to reduce
potable water demand, and in many cases may meet the landscaping demands of the new
GSB site. This strategy is also beneficial because the CEF is on campus, and will require
minimal infrastructure improvements to tie to the new GSB campus.
Also, Stanford’s landscape design guidelines note that much of Stanford’s irrigation
system is connected to the Searsville water system, a non-potable source.6
However, it appears that allowances may be made to use potable water if needed. In its
plumbing narrative, ARUP also notes that a potable water backup supply will be kept to
supply the campus if non-potable sources are temporarily exhausted. This may endanger
the achievement of Credit 1.2, which is discussed following Credit 1.1.
4
Stanford University Grounds Services, California Native Plants at Stanford,
http://grounds.stanford.edu/topics/calnativeplantchart.htm#
5
ARUP, Pre-SD Civil Narrative, 4/30/07
6
Stanford University, Landscaping Guidelines,
http://facilities.stanford.edu/fdg/docs/Landscaping%20Guidelines.pdf.
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 4
WE Credit 1.1: Water Efficient Landscaping, Reduce by 50%
LEED-NC Credit: YES | UNSURE | NO
While at this preliminary stage it is difficult to arrive at hard numbers proving a 50%
potable water reduction, the LEED-NC WE Credit 1 process is followed below to
illustrate the calculation. Assumptions are made where appropriate.
Calculation of Potable Water Use -- Design Case
For simplicity, we follow LEED recommendations, breaking landscaping vegetation into
4 components: Trees, shrubs, groundcovers, and turfgrass. We neglect ‘mixed use’ to
keep the analysis straightforward, but it can certainly be included. Stanford also
recommends drip irrigation wherever appropriate, but to
be conservative in our design estimation we assume only
75% drip irrigation on the GSB site.
Also, in this analysis, we assume 378,000 square feet of
open space on the GSB site, estimated previously in SS
Credit 5.2 (568,000 site area - 190,000 building footprint
area).
Ks (species factor) is assumed to be low for shrubs and
groundcover, but slightly higher for turfgrass planned in
the main GSB courtyard areas. Plants for the new GSB
also call for many palm trees along the Academic Walk
(pictured at right), which can require greater watering
than other tree species and are not native to Stanford.7
Kd (density factor) is generally assumed to be low, because there will be minimal shading
on the GSB site from relatively low building heights and low numbers of trees. We
assume that turfgrass and groundcover get slightly more shade, increasing their density
factors slightly.
Kmc (microclimate factor) is assumed to take an average
value of 1.0 across the entire site. More detailed heat
island and shading effects will need to be analyzed to
give more exact values to different types of vegetation.
This calculation will be accurate after the majority of
design decisions are determined, especially given the size
of the site.
Evapotranspiration (ETo) is calculated in July for the
Stanford campus to be 5.58 inches (Zone 3), as shown to
the right in Figure 3.8
Figure 3: ETo map9
7
Stanford University Grounds Services, California Native Plants at Stanford,
http://grounds.stanford.edu/topics/calnativeplantchart.htm#
8
CA Irrigation Management Information Systems, ETo zones map,
http://www.cimis.water.ca.gov/cimis/info.jsp
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 5
WE Credit 1.1: Water Efficient Landscaping, Reduce by 50%
LEED-NC Credit: YES | UNSURE | NO
Irrigation Efficiency (IE) is calculated by assuming 75% drip irrigation and 25%
sprinkler irrigation. (0.75*0.9) + (0.25*0.625) = 0.831
Controller Efficiency (CE) is also assumed to be 0.75, because we simply have no way
of calculating this number at present.
Gallons reuse is assumed to be zero in design and baseline cases, to provide an objective
comparison. Also any reuse value calculated now would be preliminary and not
necessarily accurate.
Design Case Summary
Total Open Space
ETo
IE
CE
Vegetation Type
% of Open Space
Area (sf)
378000
5.58
0.831
0.75
Trees
(75% drip)
Shrubs
Groundcover
Turfgrass
10
37800
15
56700
15
56700
10
37800
Ks, Species Factor
Kd, Density Factor
Kmc, Microclimate
Factor
KL (equation 1)
ETL (equation 2)
TWA (equation 3)
0.7
0.5
0.2
0.5
0.2
0.8
0.7
0.8
1
0.35
1.953
41529
1
0.1
0.558
17798
1
0.16
0.8928
28477
1
0.56
3.1248
66446
TOTAL TWA, gals
Gallons reuse
TOTA TPWA, gals
154251
0
154,251
Equation 1: KL = Ks x Kd x Kmc
Equation 2: ETL = ETo x KL
Equation 3: Design TWA = (Area x (ETL/IE)) x CE x 0.6233
Calculation of Potable Water Use -- Baseline Case
For the baseline case, Ks, Kd and IE are assumed to all take on average values, except for
higher allowances for new palm trees at the GSB site. Kmc and ETo are the same as the
design case, while CE is neglected (taken as 1).
Irrigation Efficiency (IE) is calculated by assuming 50% drip and 50% sprinkler
irrigation. (0.9+0.625)/2 = 0.7625
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 6
WE Credit 1.1: Water Efficient Landscaping, Reduce by 50%
LEED-NC Credit: YES | UNSURE | NO
Baseline Case Summary
Total Open Space
Eto
IE
CE
Vegetation Type
% of Open Space
Area (sf)
378000
5.58
0.7625
1
Trees
(50% drip)
Shrubs
Groundcover
Turfgrass
10
37800
15
56700
15
56700
10
37800
Ks, Species Factor
Kd, Density Factor
Kmc, Microclimate
Factor
KL (equation 1)
ETL (equation 2)
TWA (equation 5)
0.7
1
0.5
1
0.5
1
0.7
1
1
0.7
3.906
120693
1
0.5
2.79
129314
1
0.5
2.79
129314
1
0.7
3.906
120693
TOTAL TWA, gals
Gallons reuse
TOTA TPWA, gals
500013
0
500,013
Equation 5: Baseline TWA = (Area x (ETL/IE)) x 0.6233
Percent Potable Water Reduction
Percent Reduction = (1-154,251/500,013) x 100 = 69% reduction
So we see that in this simplified analysis, we achieve a 69% reduction in the use of
potable water, assuming that no water is reused. This meets the requirements of LEEDNC credit 1.1. It may be possible that the irrigation needs of the campus can be met
entirely with non-potable sources, which is the subject of Credit 1.2. For instance, the
prepared Outline Specifications note that “stormwater will be captured for irrigation or
other purposes in a stormwater collection tank.”9
9
Schematic Narrative and Outline Specifications, BCJ Architects, 4/30/07, pg. 1.7.
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 7
WE Credit 1.2: Water Efficient Landscaping, No Potable Water Use or No Irrigation
LEED-NC Credit: YES | UNSURE | NO
Credit Intent
Completely eliminate reliance on potable water sources for irrigation by using only nonpotable water for landscape watering.
Feasibility
This credit should certainly be pursued. In his presentation, Cole Roberts of ARUP noted
that the water strategy for the GSB includes “no potable water for irrigation”. In fact, the
only two uses for potable water at the new campus will be for faucets and processes.
Roberts also notes that the non-potable water will come from three sources: rainwater,
lake water, and graywater. Non-potable water not used for irrigation will be used for
flushing toilets in GSB buildings.10
Analysis
Achieving this credit is fairly straightforward: Use no potable for landscaping and prove
it. We know that initial plans call for the achievement of this credit, but there are several
things the design team can do to ensure that adequate non-potable water is supplied for
all landscaping needs.
Figure 4: Non potable water use in the GSB10
Stormwater should be retained, especially in the winter when it rains quite frequently at
Stanford. The outline specifications note that a stormwater collection tank will be used at
the GSB site for this purpose.11
Use water from the Searsville water system, as noted in Stanford’s landscaping
guidelines.12 This water, used for Stanford irrigation, comes from Searsville Lake, which
is roughly 4.5 miles from the new GSB campus (see Figure 5 below). This is positive
because the link between Searsville Lake and campus had already been established.
However, other non-potable sources should be used before tapping into the Searsville
system, so the new GSB does not strain existing campus landscaping needs.
10
Roberts, Cole. ARUP, Presentation: GSB Engineering, 4/30/07.
Schematic Narrative and Outline Specifications, BCJ Architects, 4/30/07, pg. 1.7.
12
Stanford University, Landscaping Guidelines,
http://facilities.stanford.edu/fdg/docs/Landscaping%20Guidelines.pdf.
11
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 8
WE Credit 1.2: Water Efficient Landscaping, No Potable Water Use or No Irrigation
LEED-NC Credit: YES | UNSURE | NO
Searsville Lake and Stanford
4.5 miles
Figure 5: Map showing distance between Searsville Lake, a non-potable irrigation
water source, and the new Stanford GSB campus. From Google Maps.
As Cole Roberts showed in Figure 4 above, the majority of irrigation water will most
likely come from graywater, which is defined as any wastewater that hasn’t been in
contact with toilet waste or kitchen sinks. In this case, graywater will come from sinks in
restrooms, water fountains, showers, and laundry facilities on the GSB campus. Another
source of graywater, as mentioned in credit 1.1 above, will come from the Central Energy
Facility (CEF) on campus. This graywater, supplemented with captured rainwater and
water from Searsville lake, can be expected to meet the irrigation needs of the new GSB
campus.
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 9
WE Credit 2: Innovative Wastewater Technologies
LEED-NC Credit: YES | UNSURE | NO
Credit Intent
Decrease potable water demand while reducing wastewater discharge and recharging
local aquifers. This will help Stanford meet its water reduction goals.
Feasibility
Again, this credit should be pursued. It is probably more likely that this credit can be
achieved through Option 1 (because detailed on-site wastewater treatment guidelines
have not yet been established), which requires that potable water used for sewage
conveyance be reduced by 50%. The preliminary thrust of this project seems to
encourage water-saving strategies that will help to achieve this credit.
Analysis
To achieve this credit, we must show how potable water used for sewage conveyance will
be reduced by at least 50% when compared to a baseline case. Several assumptions are
made in this process, because finalized plumbing plans have not been published.
From ARUP’s pre-SD plumbing narrative, we know the following:13
● Waterless urinals will be used in Men’s restrooms (see Figure 6 below)
● Dual flush toilets will be used in all Women’s restrooms -- allows user to
choose between low and regular flow.
● A non-potable system for water closest flushing is planned
Design Case Assumptions
● From SS Credit 4.2, we assume the GSB will have 1,400 Full Time Equivalent
(FTE) occupants -- the FTE must remain consistent for all LEED credits.
● Per LEED-NC guidelines, it is assumed that the GSB will house half males and
half females (700 each)
● In a typical day a male will use a urinal twice and the water closet once, while a
woman will use the water closet three times.
● Assume that water closets are low-flow (1.1 gallons per flush).
● Because graywater or stormwater amount used for sewage conveyance are not
known, in the design cases we assume the amount to be equal to the amount of
water closest sewage generation. This is a
strategy, as noted above, that is planned by
ARUP.
● Assume number yearly workdays to be 300
-- many GSB users will probably be around
on weekends.
Figure 6: Typical Waterless Urinal14
13
ARUP, Pre-SD Plumbing Narrative, 4/30/07
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 10
WE Credit 2: Innovative Wastewater Technologies
LEED-NC Credit: YES | UNSURE | NO
Design Case
Fixture Type
Daily Uses
Waterless Urinal (Male)
Waterless Urinal (Female)
Watercloset (Male)
Watercloset (Female)
Flowrate, gpf
2
0
1
3
Occupants
0
0
1.1
1.1
Sewage
Generation, gal
700
700
700
700
0
0
770
2310
Totally Daily Volume, gal
Annual Work Days
Annual Volume, gal
Rain/Graywater volume, gal
3080
300
924000
924000
TOTAL ANNUAL VOLUME
0
Baseline Case Assumptions
Same number of daily uses and occupants as design case assumed. Flow rates of water
closets increased to 1.6 gallons per flush, and urinal flow rates increased to 1.0 gallons
per flush. We also assume that no rain or graywater is used for sewage conveyance.
Baseline Case
Fixture Type
Daily Uses
Conventional Urinal (Male)
Conventional Urinal (Female)
Watercloset (Male)
Watercloset (Female)
Flowrate, gpf
2
0
1
3
700
700
700
700
Sewage
Generation, gal
1400
0
1120
3360
Totally Daily Volume, gal
Annual Work Days
Annual Volume, gal
Rain/Graywater volume, gal
5880
300
1764000
0
TOTAL ANNUAL VOLUME
1764000
1
1
1.6
1.6
Occupants
So we see that we definitely have reduced potable water used for sewage conveyance, in
this case by using no potable water at all. It should be noted that if the calculations for
the design case hold in the actual GSB (i.e., all waterless urinals, and no potable water
used in water closets), then an Innovation Credit would be earned.
Also, if we disregard the planned use rainwater or graywater for sewage conveyance, we
still achieve a 47% decrease in potable water use. With some additional adjustments (i.e.,
using even a minimal amount -- 28,000 gallons -- of graywater) this credit could still be
achieved. [(1-(924000/1764000))*100 = 47.6%]
14
Oikos Green Building Source: Waterless Urinal, http://oikos.com/products/mechanical/waterless/
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 11
WE Credits 3.1 & 3.2: Water Use Reduction, 20% & 30% Reduction
LEED-NC Credit: YES | UNSURE | NO
Credit Intent
Reduce municipal water supply and wastewater system burden by increasing the water
efficiency of interior building functions such as kitchens, showers, and restrooms.
Feasibility
This credit, as well as Credit 3.2 (30% reduction) should be pursued, and their attainment
can be proven in the same analysis by showing that overall interior water use is reduced
by 30 percent, which will earn two points (achieving 40% reduction also earns an
Innovation Credit). Again, the thrust of this credit fits with the spirit of Stanford’s own
water reduction goals, and the desire of the GSB to be an environmental leader.
Analysis
Like previous WE credits, we compare an assumed GSB design scenario with a baseline
case, and show how much water use has been (or is estimated to be) reduced. For
consistency, fixture values identical to those used in WE Credit 2 are used in this
analysis. Also, although we have no quantitative evidence, we assume that the GSB will
be used low flow fixtures wherever possible. ARUP notes that “low flow
faucets/showerheads will be provided for all lavatories and showering facilities”.15 Also,
given the emphasis on environmental leadership, as well as LEED Platinum certification
goal, we feel this is a valid assumption for the analysis below.
Design Case Assumptions
● Flush fixtures (urinal and water closet) assumed to identical WE Credit 2 values
● Low flow lavatory sinks assumed (1.8 gpm)
● Low flow kitchen sinks assumed (1.8 gpm)
● Low flow showers assumed (1.8 gpm)
● Rain or graywater used for all water closet conveyance
Design Case
Fixture Type
Waterless Urinal (Male)
Waterless Urinal (Female)
Watercloset (Male)
Watercloset (Female)
Fixture Type
Low-flow Lavatory Sink
Low-flow Kitchen Sink
Low-flow Shower
Daily Uses
2
0
1
3
Daily Uses
3
1
0.1
Flowrate
(gpf)
0
0
1.1
1.1
Duration
(flush)
1
1
1
1
Occupants
Flowrate
(gpf)
1.8
1.8
1.8
Duration
(sec)
15
15
300
Occupants
700
700
700
700
1400
1400
1400
Totally Daily Volume, gal
Annual Work Days
Annual Volume, gal
Rain/Graywater volume, gal
TOTAL YEARLY VOL
15
Water Use
(gal)
0
0
770
2310
Water Use
(gal)
1890
630
1260
6860
300
2058000
924000
1134000
ARUP, Pre-SD Plumbing Narrative, 4/30/07
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 12
WE Credits 3.1 & 3.2: Water Use Reduction, 20% & 30% Reduction
LEED-NC Credit: YES | UNSURE | NO
Baseline Case Assumptions
● Flush fixtures (urinal and water closet) assumed standard
● Conventional lavatory sinks assumed (2.5 gpm)
● Conventional kitchen sinks assumed (2.5gpm)
● Conventional showers assumed (2.5 gpm)
● No rain or graywater used
Baseline Case
Fixture Type
Urinal (Male)
Urinal (Female)
Conv. Watercloset (Male)
Conv. Watercloset (Female)
Fixture Type
Conventional Lavatory Sink
Kitchen Sink
Shower
Daily Uses
2
0
1
3
Daily Uses
3
1
0.1
Flowrate
(gpf)
1
1
1.6
1.6
Duration
(flush)
1
1
1
1
Occupants
Flowrate
(gpf)
2.5
2.5
2.5
Duration
(sec)
15
15
300
Occupants
700
700
700
700
1400
1400
1400
Totally Daily Volume, gal
Annual Work Days
Annual Volume, gal
Rain/Graywater volume, gal
TOTAL YEARLY VOL
Water Use
(gal)
1400
0
1120
3360
Water Use
(gal)
2625
875
1750
11130
300
3339000
0
3339000
Here we see that water use has been greatly reduced:
(1-(1134000/3339000)*100 = 66% Water Use Reduction
We also see that this reduction also results in an Innovation Credit, because we have
exceeded a 40% reduction. However, given the assumptions in this analysis, it is prudent
to assume that only credits 3.1 and 3.2 are attained at this time. When more exact flow
values become available (from the plumbing supplier or sub-contractors dealing with the
plumbing system), these analysis should be conducted again to see precisely how much
water use is reduced. Also, after the GSB is occupied, water use should be monitored to
ensure that the campus buildings are performing as they were intended.
Billings, Millea, Victorsson | LEED-NC Analysis: New GSB Campus | WE Page 13