WATER HARVESTING BASICS
Introduction
Because Santa Fe is located in a semi arid area,
more water evaporates than we receive from rainfall, i. e.,
more water goes up than comes down. This makes every
drop precious. Barring a cataclysm, the factor that will
most affect the community in the next 20 years will be
water availability. Water harvesting and stormwater
management are part of an integrated strategy for dealing
with water issues. Water harvesting can provide a variable
supply which can be employed for landscape irrigation,
clothes washing, swamp coolers and toilet flushing. These
utilizations relieve the demands on the potable water
system. Where the water is sent to the sewer system, it
creates return flows that were not diverted from local
ground or surface water resources initially.

Quite a bit of precipitation falls on landscaped or
unpaved areas and contributes to the greenery of the
community. There is also a significant amount that falls on
roofs, roads, parking lots, and other impervious surfaces.
Much of this rainfall becomes runoff surge which causes
damaging erosion, or it evaporates from retention ponds,
contributing no benefit. This water could be harvested
and used in place of potable water, to preserve the supply.
Water is similar to money in that it is accumulated
and spent, and it can be budgeted in a similar way. A
water budget accounts for the supply of both ground and
surface water, and it considers water expenditures both
inside and outside of the built environment. It includes
contingencies for interruption of the supply, and priorities
for the usage of potable reserve, landscape, fire protection,
etc. These priorities, along with financial incentives and
restrictive covenants, determine how a “water savings
account.” is managed.

The basic water budget:
Income = Ground or Surface Water Withdrawal +
Precipitation + Recycled Effluent
Expense = Indoor Use (Potable such as drinking water +
Nonpotable such as toilet flushing) + Outdoor Use
(Infiltration into the ground + Plant uptake and
evapotranspiration + Runoff) Note that most indoor use
produces a return flow of effluent that becomes a
secondary source when used correctly.

Where income and expense are relatively equal, the water
budget is balanced, and the system is sustainable. As with
finances, managing demand is the most effective method
for achieving a balanced budget. Rainwater harvesting is
the equivalent of working an extra job or cashing in
investments for more income. The value of rainwater
harvesting can be seen by looking at the water use and
precipitation curves for Santa Fe. The rainfall curve closely
follows the demand curve, demonstrating that municipal
water harvesting has the potential to contribute significantly
to supply.

Water harvesting has only two basic principles:

Principle #1: Keep it as close to where it falls as is
practical. This is implemented one of two ways.
The first method is passive water harvesting, or
“Spread and Sink.” In this scenario, water is spread out
and infiltrated into the ground in places where it will be
useful. This can be as simple as a curb cut to drain a
parking lot into a planted island. This is the least
expensive method and could be implemented throughout
the community.

The second, employs active systems that store and
release rainwater when it is needed. In this method, the
water is harvested, usually off of impermeable surfaces like
roofs and pavement, then flushed, filtered and stored in
cisterns or other structures. This water is usually used for
nonpotable applications but it can be used for drinking
water with correct design. Harvested water can be saved
and “spent” at the most advantageous time.

Principle #2: Put it to the best use where it is. This
means that the water budget has been carefully
calculated and we know what the best use is. This may
change with the season and the amount of precipitation.
For example, if a greywater system was used when the
WWTP is below capacity, the draw would be a
consumptive use and detract from WWTP revenues. This
would be a time to use harvested rainwater which would
reduce potable draw while maintaining return flow credits
and WWTP revenues. Conversely, if the WWTP is over
capacity, then greywater would be the better choice. This
type of design requires either a high level of citizen
interest and participation, or a remotely controlled system
from a centralized location.
Putting both of these principles into practice
ensures that rainwater will be used in the most efficient
and cost effective way. If they are applied properly, a
cost benefit analysis will show a long- term savings of
money as well as water.

The Water Systems Design Process
System design follows a process. Each step follows
in logical order.

The first step is a site analysis. Water is the key
element to any landscape, so the entire landscape
must be considered. This includes all of the normal
components, geology, topography, solar aspect, pre-
vailing winds, micro climates, surrounding land, in-
tended uses, legal restrictions, habitat, etc. The
property must also be considered in its geographical
biological, and human context. This gives direction
to the use of the water.

The second step is the water budget. The budget
considers all sources and all demands. The budget
must also look at reserves and surpluses. Other
elements of the water budget are peak flows and
scenario modeling.

The third step is system design in the form of a sche-
matics. Schematics should show all elements of the
liquid flow. This includes all active and passive ele-
ments. A plan for electric power and controls is usu-
ally required as well .

The final step in the process is to generate parts
lists, construction details, site plans, and budgets.
All of the design steps are processes and not events.
Feedback loops are essential for complicated de-
signs and collaboration with all stakeholders will re-
sult in the design that is most likely to succeed over
time.

Semi Arid Water Harvesting
Semi arid areas receive 10 to 20 inches (250- 500
mm) of precipitation per year. Evapotranspiration
exceeds precipitation. Rainfall events are infrequent
and can be locally intense.

The nature of our storms makes certain require-
ments for equipment. Conveyance and filtration
must be able to handle high flows. This is especially
challenging for filtration where level of filtration and
flow capacity are inversely related. In humid areas
filter design is often only 90% efficient. The lost 10%
is used to continuously clean the filter and to remove
the filtrate. Water is too precious in semi arid areas
to use this type of system. Instead we look to coarse
filter at inlets to the collection system, then foul flush,
then use high flow filtration. The end use of the har-
vested water determines the minimum level of filtra-
tion that is acceptable. Overflow filtration is also
used where small events receive one level of filtra-
tion and large events receive a cruder level that al-
lows higher flows.

Tankage is also different in semi arid areas. It must
be able to handle large flows and then deliver water
over long dry periods. An accurate water budget is
required to determine tank sizing. However, Robin
Elkin, Santa Fe City Planner summed it up well, “The
only tank that is too large is the one that you cannot
afford”.

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