Water Balance Assessments Explained
Where the rain goes before and after development, and how the deficit is worked out.
A water balance assessment answers a simple question with careful arithmetic: when rain and snow fall on a site, where does the water go, and how does development change that? This guide explains the parts of the equation, how the before-and-after comparison works and why so many Ontario policies now ask for one.
The equation in plain terms
The provincial Oak Ridges Moraine Conservation Plan (section 25) lists the components of the water balance equation: precipitation, evapotranspiration, groundwater inflow and outflow, surface water outflow, change in storage, withdrawals and returns. For a single development site the calculation is usually reduced to four terms:
- Precipitation: rain and snow, from long-term climate records.
- Evapotranspiration: water returned to the air from soil, plants and surfaces.
- Runoff: water that flows off the surface to ditches, sewers and streams.
- Infiltration: water that soaks into the ground, part of which recharges groundwater.
Over a year, what falls has to equal what evaporates, runs off and soaks in. Soil type, slope and land cover decide the split, which is why the assessment starts with a description of the site as it is.
Before and after development
The assessment calculates the budget twice, monthly and annually: once for the site today and once for the proposed roofs, pavement and landscaping. Hard surfaces turn water that used to soak in into runoff, so infiltration usually drops. The difference between the two infiltration totals, in cubic metres per year, is the infiltration deficit, and the rest of the report is about making it up with low impact development (LID) measures such as infiltration trenches, soakaway pits and permeable paving.
An illustrative calculation
Illustrative figures, not from a real site
Take a 2 ha site (20,000 m²) where, after the soils and climate work, about 200 mm of water a year is assumed to infiltrate on pervious land. Today the whole site is pervious. After development, half of it is roofs and pavement.
| Step | Annual infiltration |
|---|---|
| Before: 20,000 m² × 0.2 m | 4,000 m³ |
| After: 10,000 m² pervious × 0.2 m | 2,000 m³ |
| Infiltration deficit | 2,000 m³ per year |
The mitigation design then has to put about 2,000 m³ a year back into the ground, for example by directing roof runoff to infiltration galleries sized from measured infiltration rates. A real assessment works month by month and uses the site's own soils, climate station and land cover, so the numbers will differ.
Who asks for one, and why
| Where | What drives it |
|---|---|
| Lake Simcoe watershed | Lake Simcoe Protection Plan policies reviewed by LSRCA: a water balance for major development, and recharge protection in significant recharge areas. Unmitigated deficits can carry an offsetting fee under the 2023 policy, set at $100 per m³ plus a 15% administrative fee when approved and adjusted for inflation each year. |
| CTC source protection region | Policy REC-1: a water balance for site plans and subdivisions for major development in wellhead protection areas for quantity with significant threats, such as Acton (CTC Source Protection Region water balance guidance) |
| Peterborough | A hydrogeological assessment for all major development applications, including maintaining the pre- to post-development water balance (Peterborough Official Plan s. 6.1.4 f) |
| Toronto and Credit Valley watersheds | TRCA and CVC water balance criteria for natural features, to keep water supply to wetlands, streams and woodlands stable |
What makes a water balance hold up in review
- Inputs and assumptions shown in full: LSRCA asks that another qualified person could recreate the work from the report alone.
- Monthly as well as annual results, so seasonal effects on wetlands and streams can be seen.
- Measured infiltration rates and the seasonal high water table wherever LID is relied on, so the measures can actually work.
- Mitigation sized against the deficit, with its location shown on the site plan.
The field data is what takes time, so it is worth starting early. The water balance assessment page covers scope, inputs and cost.
Related questions
Is a water balance the same as a stormwater management report?
No, although they use some of the same inputs. A stormwater report is mostly about peak flows and water quality leaving the site in storms; a water balance is about annual volumes, and in particular how much water still soaks into the ground after development.
What is the difference between a water balance and a water budget?
The difference is mostly one of scale and policy wording. The provincial Oak Ridges Moraine Conservation Plan uses "water budget" for the quantified water balance equation, and requires one with a water conservation plan for certain major development. A site water balance applies the same idea to one property, before and after development.
Can the deficit always be made up on site?
Not always: high groundwater and clay soils limit infiltration. In the Lake Simcoe watershed, LSRCA's recharge offsetting policy expects the deficit to be mitigated on site first, and only then allows offsite compensation or a fee; deficits of 100 m³ a year or less are excluded.
Where to go next
- Water Balance Assessment: scope, cost and what the reviewer checks
- Hydrogeological study East Gwillimbury: local rules, data and costs
- Hydrogeological study Halton Hills: local rules, data and costs
- Hydrogeological study Peterborough: local rules, data and costs
- Does my severance need a study?
- D-5-4 nitrate impact assessment explained
Sources
- O. Reg. 140/02: Oak Ridges Moraine Conservation Plan (opens in a new tab), Government of Ontario (e-Laws).
- Hydrogeology (planning application reviews) (opens in a new tab), Lake Simcoe Region Conservation Authority.
- Water Balance Recharge Offsetting Policy (May 1, 2023) (opens in a new tab), Lake Simcoe Region Conservation Authority.
- Guidance: Water Balance Assessment, version 2.1 (October 21, 2019), for CTC Source Protection Plan policy REC-1 (opens in a new tab), CTC Source Protection Region (published by Credit Valley Conservation, a CTC source protection authority).
- City of Peterborough Official Plan (consolidated June 2024), sections 6.1.4 and 7.21.2 (opens in a new tab), City of Peterborough.
- Water balance for the protection of natural features (opens in a new tab), Toronto and Region Conservation Authority / Credit Valley Conservation.