A Water Bill Is Not a Water Balance
Why water management should begin where rain first meets the roof and land
By Antonina Lourdes “Queenie” E. Cunanan
Water & Land Strategist | Co-Founder and President, Ecosensya Solutions
My background in industrial engineering taught me that we cannot understand—or improve—a system by measuring only one part of it.
Yet this is often how we manage water.
We look at the water bill and assume we understand our water use.
The bill tells us how much treated water we purchased from a utility. It can help us monitor consumption, detect unusual increases and manage costs.
But it does not tell us the whole water story.
It does not show:
• How much rain fell on the property
• How much groundwater was withdrawn
• How much rainwater ran from roofs and paved surfaces into the street
• How much was retained in tanks, ponds, soil or vegetation
• How much returned to the atmosphere through evaporation and plants
• How much infiltrated into the ground
• Whether some of that infiltration eventually contributed to groundwater recharge
• How much water left through drainage or wastewater
A water bill is an important record—but it is not a water balance.
What is a water balance?
In its simplest form, a water balance accounts for:
Water entering a system = Water leaving the system + Change in storage
For a property, water may enter through rainfall, utility supply, groundwater extraction or water delivered from another source.
It may leave through household or industrial use, irrigation, evaporation, plant transpiration, runoff, drainage and wastewater discharge.
Some water may remain temporarily stored in tanks, ponds, soil or the landscape.
The total volume of water moving through a property is therefore much larger and more complex than the amount appearing on its monthly bill.
Rain is part of the property’s water account
The Philippines is a rain-rich country, although rainfall varies considerably by location and season.
For current planning, PAGASA publishes station-specific climatological normals based principally on the 1991–2020 reference period.
These normals represent long-term averages rather than rainfall for a single year. Some stations use shorter periods where complete records are unavailable.
This distinction matters.
A national average is not sufficient for designing a property-level water system. Rainfall information should come from the nearest representative PAGASA station and, where possible, be supported by local observations and site-specific analysis.
One millimeter of rain falling on one square meter produces approximately one liter of water.
Therefore, a 500-square-meter property receiving 2,000 millimeters of rain theoretically receives about one million liters of rainfall across its total area in a year.
That does not mean all one million liters can—or should—be captured.
Some water will evaporate, support vegetation, infiltrate the soil or continue toward waterways and downstream communities. Rain does not fall evenly throughout the year, and intense storms may exceed the capacity of a property’s storage, detention and infiltration systems.
The calculation nevertheless reveals something important:
A property receives a significant volume of water that never appears on its water bill.
The question is:
What happens to it?
From rainfall to runoff
When rain falls on roofs, roads, parking areas and other impervious surfaces, it cannot easily enter the soil. It becomes runoff and moves toward gutters, canals and drainage systems.
At one property, this may appear insignificant.
Across hundreds or thousands of properties, it becomes a large volume of water reaching public drainage systems—often at nearly the same time.
Every roof, driveway, parking area, farm and open space influences how the wider watershed behaves.
A property can rapidly discharge rain or help slow, store, use, safely infiltrate and gradually release part of it.
Can managing rain on individual properties help reduce flooding?
Yes—but the benefit should be understood as cumulative rather than absolute.
Every liter of rainwater retained, stored, safely infiltrated or temporarily detained during a storm is one less liter entering the canal or drainage system at that moment.
One household will not solve a city’s flooding problem. But when many homes, buildings, farms, subdivisions and commercial properties release less runoff—or release it more slowly—the combined effect can reduce the volume and peak flow reaching shared drainage infrastructure.
Timing is as important as volume.
A tank that stores water for later use removes part of the rainfall from immediate runoff. A detention system holds water temporarily and releases it after the storm peak. Healthy soil, vegetation and suitable infiltration areas can slow water and allow part of it to enter the landscape.
These measures cannot replace properly designed canals, drainage networks, waterways and flood-control infrastructure.
They can, however, reduce the pressure placed on those systems.
Flood management should not begin only when rain reaches the street or canal.
It should begin where rain first meets the roof and land.
Infiltration is not automatically groundwater recharge
It is tempting to assume that any water entering the soil immediately replenishes an aquifer.
The reality is more complex.
Some infiltrated water remains in the upper soil and is later used by plants. Some returns to the atmosphere through evapotranspiration.
Some moves laterally. Only a portion may travel deeply enough to reach the saturated zone and contribute to groundwater recharge.
The result depends on soil, geology, slope, vegetation, groundwater conditions, rainfall intensity, contamination risks and other site-specific factors.
Clay soil may absorb water slowly. A shallow water table may limit additional storage. Polluted runoff should not simply be sent underground. Unstable slopes and areas near buildings also require careful assessment.
This is why a recharge intervention should not be copied from another site without investigation.
Nature-based does not mean assumption-based.
A tank is useful—but it is not the whole answer
Rainwater tanks can reduce dependence on treated water and provide water for appropriate uses such as cleaning, irrigation and toilet flushing, subject to proper design, treatment and applicable standards.
But a tank has limited capacity.
If it remains full during several consecutive rainy days, the next rainfall becomes overflow. If there is little demand during the wet season, a large and expensive tank may be underused.
The better question is not simply:
“How large should the tank be?”
It is:
“How should water move through the entire property?”
The answer may involve a connected system of storage, controlled release, reuse, soil improvement, vegetation, detention, filtration and—where site conditions are suitable—managed infiltration or groundwater recharge.
Water management should begin at the design table
Water management should be considered when designing every new home, building, subdivision, commercial property and industrial estate.
It is easier and generally more efficient to provide space for rainwater storage, detention, reuse, safe overflow and landscape infiltration before construction begins than to add them after the property has been completed.
A water-sensitive design can consider:
• Roof areas and downspout locations
• Expected rainfall and runoff volumes
• Tank or cistern location• Appropriate non-potable uses
• Separate plumbing for rainwater reuse
• First-flush diversion and filtration
• Safe overflow routes
• Temporary detention capacity
• Soil, slope and geological conditions
• Permeable areas, vegetation and trees
• Maintenance and cleaning access
• Protection of buildings, neighboring properties and downstream communities
Existing properties should not be excluded.
Renovation provides an opportunity to redirect downspouts, install modular storage, create controlled overflow, replace selected paved areas with permeable surfaces, introduce planted detention areas and improve how water moves through the site.
Not every property will use the same solution.
A small urban home, a school, a farm and an industrial estate have different constraints and opportunities.
The goal is not to force one technology into every property.
The goal is to make water management a normal part of architectural, engineering, landscape and renovation decisions.
Five questions every property should ask
Before selecting a technology, decision-makers should understand:
1. How much water does the property receive?
Consider rainfall, utility supply, groundwater and other sources.
2. How much does it consume or withdraw?
Examine domestic, agricultural, commercial, industrial and landscape demand.
3. Where does the rain go?
Map roofs, paved surfaces, drains, low points, erosion paths and discharge locations.
4. How much can be responsibly retained, reused, detained or infiltrated?
Consider demand, available space, soil, geology, water quality, environmental flows and extreme rainfall.
5. What happens beyond the property boundary?
A responsible design should not merely transfer water, pollution or flood risk to neighboring properties and downstream communities.
From water consumer to water steward
A water consumer asks:
How much water did we use?
A water steward also asks:
How much water did we receive?
How much did we allow to run away?
How much did we retain or reuse?
How much did we help return to the landscape?
What effect did our decisions have downstream?
This change in perspective can influence how we plan homes, farms, schools, subdivisions, industrial estates, commercial properties and cities.
It can also change how we value land.
Perhaps property performance should not be measured only by what can be built or produced on it.
Perhaps we should also ask:
How well does this land manage water?
Understand the system before designing the solution
At Ecosensya, our approach begins with research.
We seek to understand rainfall, water demand, runoff pathways, soil, geology, topography, land use and stakeholder needs before recommending an intervention.
We then connect that research to education and site-responsive Nature-based design.
For organizations that want to begin, a preliminary Water Opportunity Assessment can help identify:
• How water enters and leaves the property
• Where rainfall currently flows
• Where water may be unnecessarily lost
• What opportunities may exist for retention and reuse
• Where detention or infiltration may—or may not—be appropriate
• What technical studies are required before design
A Water Opportunity Assessment is not yet a final engineering design. It is the evidence-based starting point that helps prevent premature, unsuitable or unnecessarily expensive solutions.
Imagine if every new home and building were designed to manage part of the rain falling on it—and if every major renovation improved the property’s water balance.
The contribution of one property may appear small.
Across a neighborhood, subdivision, commercial district or city, those contributions can accumulate.
Public drainage infrastructure remains essential. But it should not be expected to carry every drop of rain away as quickly as possible.
Part of the responsibility—and opportunity—exists within each property.
Before asking what tank, pond, rain garden or recharge structure to install, let us first understand the site’s water balance.
Then let us design buildings and landscapes that receive rain, retain what they responsibly can, reuse what is suitable, safely manage overflow and help reduce pressure on shared drainage systems.
Measure the water.
Manage rain where it falls.
Bank the rain.
Bank with the whole system in mind.
Sources and further reading:
PAGASA, Climatological Normals:https://www.pagasa.dost.gov.ph/climate/climatological-normals
PAGASA, Climate Data:https://www.pagasa.dost.gov.ph/climate/climate-data
US EPA, Green Infrastructure and Groundwater Protection:https://www.epa.gov/green-infrastructure/green-infrastructure-and-groundwater-protection
US EPA, Assessing Impacts of Green Infrastructure on Groundwater Quality:https://www.epa.gov/sciencematters/assessing-impacts-green-infrastructure-groundwater-quality
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