Want to strike up a lively conversation with an urban water efficiency professional?
Try this: “So, what’s your GPCD?”
Gallons per capita per day” (GPCD)
is the traditional measurement for projecting future water demand and evaluating
community-wide water efficiency. Most agencies use this to project the amount of
water and infrastructure the agency will need to sustain future populations.
Likewise, it can serve as relative measure of a community’s water efficiency. By
achieving lower GPCD, agencies can reduce future water and infrastructure
needs.
On the surface GPCD seems to be a
simple creature with only two inputs: the total gallons of water produced by the
agency and the number of permanent residents the utility served. Divide the
gallons by the people, then again by the 365 days in a year. A simple third
grade math problem, right?
If that’s the case, why is it
contentious? And, why will a literature search turn up differing GPCD results
for the same city? Perhaps it is because there are no authoritative,
nationally-accepted standards for how it must be calculated.
GPCD makes for great conversation,
because the answer almost always comes with a story. Few water pros can simply
offer the number without diving into a story about the agency’s past, or its
plans for the future. Asking someone’s GPCD is a perfectly legitimate technical
question, but some water professionals might rather divulge age or weight.
Sometimes an agency will provide a
flattering subsegment of its GPCD; “Our potable GPCD is 188.”
“Potable?” I ask.
“Well, yes, that’s the number we
give out, but if you include reclaimed water that’s another fifty, and raw water
is another six.”
Hey, that’s great. By the way, I’m
6’10” (with my arms outstretched above my head); I can do 200 pushups (from my
knees) and run a mile in under six (rest stops).
While working on a survey for my
agency, we called a number of peer cities to ask the seemingly simple question,
“What's your GPCD?” Every agency provided a number, but we discovered some
agencies required prompting to include all sources of water or all customer
sectors. Some didn’t include reclaimed water on the first pass. Others offered
only water accounted for through meters, ignoring system losses (which can be
more than 20%). Most stunning was the agency that matter-of-factly admitted that
it included transient workers, who commuted daily from another jurisdiction, in
its “customer” population.
It should be getting clearer as to
why you might see vastly different numbers for the same community. To get an
accurate answer, you can’t ask just the one question—you really need to ask
many.
These challenges are amplified
when “watchdog” organizations attempt to compare peer agencies. Too often, the
data are simply collected by telephone and recorded without any substantiation.
Even worse, GPCD is sometimes quoted from other outdated or erroneous reports,
leading readers to lend it even greater credibility.
In many respects, confusion is
excusable. In fact, here in Las Vegas I’ve had to explain to people that we
can’t consider the 330,000 tourists in our city on any given day to be “resident
equivalents.” Sure, that’s like having the entire population of Wichita, KS in
your guest room, but no matter how long they stay, or how long they shower, they
simply cannot be added into the GPCD denominator.
If we are to truly measure our
progress, it is important we agree upon the markings of the yardstick.
Collection of standardized, reliable information is a step in the right
direction.
Here are some of the challenges we
must overcome to improve the credibility of GPCD as a planning and communication
tool:
Resident Population
Sometimes agencies include the
entire population of a region instead of just the portion served by the agency.
Population estimation is, in itself, a challenging task. An agency needs to
accurately derive its population using credible techniques. Since populations
are not static, you cannot simply use year-end water production and population.
Furthermore, if GPCD is to be used for planning purposes, it may need to be
normalized to account for weather influence.
Variations of GPCD
Is there a role for variants of
GPCD? I think there is, provided we establish standard procedures and
designations. These variants could be categorized by such things as the source
or quality of water (potable GPCD), point of measurement (metered GPCD), or even
the fate of the water after use (consumptive GPCD). Though they seem clearly
defined subsets, there is still great room for discrepancies, such as using
metered GPCD in a community that has some unmetered connections. Residential
GPCD is often cited as a metric for comparing communities, but there is no clear
definition of how it is derived. Should it be the total water use of all
residential sectors divided by total population, or just the water use of
single-family homes divided by that portion of the service population in
single-family homes? Perhaps each is worthy of its own designation and
protocol?
Community Comparisons
Every city is unique in
demographics, housing mix, industries, economy, and climate. A bedroom community
may have a low GPCD, because it doesn’t include much commercial or industrial
water use, while the nearby industrial community has a seemingly excessive GPCD,
because it has few residents to divide water use among. Even if you can find two
apples to compare, you’ll likely wind up with a Winesap and a Fuji. Agencies
should maintain general awareness of the GPCD of peer communities, without
feeling as though they are competing with them. Instead, use your agency’s GPCD
history and goals to measure your community’s water efficiency progress.
The water industry is a champion
of standards. Perhaps because of its seemingly simple nature, GPCD has largely
been overlooked. Although some states and cities have administrative policies
for calculating it, even those standards are not in alignment.
We cannot truly manage water if we
don’t exercise full accountability for every drop we use. Calculating GPCD is an
arena where we need industry consensus.