A water balance assessment compares how precipitation on a site divides between evapotranspiration, surface runoff and infiltration before and after development, then shows how the plan will keep groundwater recharge close to existing conditions. In Ontario it is required by conservation authorities and provincial plan policies for many major developments, especially in recharge areas. It is prepared by a licensed hydrogeologist (P.Geo.) or professional engineer (P.Eng.), often alongside the stormwater engineer.
Where the requirement comes from
The clearest published example is the Lake Simcoe Region Conservation Authority, which reviews planning applications for its member municipalities against several policies:
| Policy | What it asks for |
|---|---|
| Lake Simcoe Protection Plan, stormwater policy 4.8-DP | All major development in the watershed: a water balance showing how runoff is reduced and infiltration increased |
| Lake Simcoe Protection Plan, recharge policy 6.40-DP | Major development in significant or ecologically significant groundwater recharge areas: hydrogeological assessment and water balance showing recharge function is protected, improved or restored |
| Oak Ridges Moraine Conservation Plan | Major development: show how groundwater quantity and quality will be maintained, through a hydrogeological assessment and water balance (LSRCA's summary; see the plan's own wording below) |
| South Georgian Bay Lake Simcoe source protection policy LUP-12 | Major development in a WHPA-Q2: show the existing water balance can be maintained using best management practices such as LID |
LSRCA notes that the definition of major development differs between these plans.
The Oak Ridges Moraine plan (O. Reg. 140/02) itself defines a water budget as quantifying the components of the water balance equation: precipitation, evapotranspiration, groundwater inflow and outflow, surface water outflow, change in storage, withdrawals and returns (section 25). Section 24 requires a water budget and water conservation plan for certain major development applications, and section 26 requires a hydrological evaluation for development within the minimum area of influence of a key hydrologic feature.
Outside the Lake Simcoe watershed, Toronto and Region Conservation Authority and Credit Valley Conservation apply water balance criteria for natural features, aimed at keeping the water supply to wetlands, watercourses and woodlands stable after development.
How the assessment is built
- Existing conditions. Land cover, soils and surficial geology, slopes and drainage areas, with long-term climate data for precipitation and temperature.
- Pre-development budget. Monthly and annual evapotranspiration, runoff and infiltration for the site as it is today.
- Post-development budget. The same calculation for the proposed roofs, pavement and landscaping.
- Deficit and mitigation. The infiltration shortfall in cubic metres per year, and the LID measures sized to make it up.
- Feasibility check. Seasonal high groundwater and measured infiltration rates where LID is relied on, so the measures can actually work.
For the equation in plain terms and a worked illustration of the deficit, see water balance assessments explained.
How long it takes
A balance built from existing soils, land cover and climate data is mostly desk work. The schedule stretches when the plan relies on LID: the field data behind it, seasonal high groundwater and measured infiltration rates, has to be collected first, and agencies set monitoring minimums. LSRCA, for example, generally expects at least 12 continuous months of groundwater monitoring to support a planning application. Starting monitoring early, while the site plan is still being drawn, is the usual way to keep it off the critical path.
What you get
A signed report with the pre- and post-development tables, the inputs and assumptions behind them, the LID sizing, and the field data that supports it. LSRCA asks for assessments that stand alone: another qualified person must be able to recreate the work from the report alone.
Water balance assessment cost
| Answer | Indicative range |
|---|---|
| Starting point (Under 1 ha, No) | $3,000 – $5,000 |
| Site area | |
| Under 1 ha (starting point) | $3,000 – $5,000 |
| 1 to 5 ha | $4,000 – $7,000 |
| 5 ha or more | $6,000 – $10,000 |
| LID or infiltration design input | |
| No (starting point) | $3,000 – $5,000 |
| Yes | $4,000 – $7,000 |
| Full span across all answers | $3,000 – $14,000 |
Each row changes one answer and leaves the others at the starting point. Indicative ranges, rounded to the nearest $500. They exclude well drilling, government application fees and HST. Final scope is confirmed by the hydrogeologist after reviewing your site.
A desktop balance for a small site with no LID design input sits at the low end. Larger sites, design input for infiltration features, and feature-based balances for nearby wetlands cost more. If infiltration rates have to be measured, see infiltration testing.
Inputs the hydrogeologist will need
- Draft plan or site plan with impervious areas
- The stormwater management concept, if started
- Soils and geotechnical information
- Any natural heritage features on or next to the site
Common pitfalls
- LID sited over a high water table. A trench that sits in groundwater for part of the year will not infiltrate.
- Infiltration rates assumed, not measured, where the reviewing agency expects in-situ testing.
- Wrong climate data. Inputs that do not follow what the reviewing agency specifies for the watershed.
- Feature water balance missed when a wetland next door depends on the site's runoff or groundwater.
Worked cost examples
Each example runs the estimator's own calculation on a realistic set of answers.
-
Small site under one hectare, no LID design input$3,000 – $5,000
Site area: Under 1 ha · LID or infiltration design input: No
A desktop balance from soils, land cover and climate data, enough where the agency only needs the pre- and post-development comparison.
-
Subdivision of 1 to 5 hectares with LID design input$6,000 – $10,000
Site area: 1 to 5 ha · LID or infiltration design input: Yes
Both answers multiply the base. Sizing infiltration features to close the deficit needs more drainage areas, iteration with the stormwater design and checks against field data.
-
Site over 5 hectares with LID design input$8,500 – $14,000
Site area: 5 ha or more · LID or infiltration design input: Yes
The largest case in the estimator. Infiltration testing, if needed, is a separate scope.
What the reviewer checks
- Climate data source and period, and the method used (for example Thornthwaite-Mather)
- Soils, surficial geology and land cover mapped for each drainage area
- Monthly and annual pre- and post-development tables
- The infiltration deficit in cubic metres per year
- LID measures sized to the deficit, with their locations
- Seasonal high groundwater and measured infiltration rates under each LID feature
- A feature-based balance where a wetland or watercourse depends on the site
- Inputs and assumptions shown in full, so the work can be recreated
Water balance or stormwater management report?
| Aspect | Water balance assessment | Stormwater management report |
|---|---|---|
| Time frame | Monthly and annual totals | Design storms and peak events |
| Main question | Is recharge kept up? | Are flooding, erosion and water quality controlled? |
| Prepared by | Hydrogeologist, often with the engineer | Water resources or civil engineer |
| Shared element | LID features, which both reports rely on and must describe consistently | |
Where this study is commonly required
Location pages with local requirements, groundwater data and the agencies involved, by region:
Greater Toronto and Hamilton Area
- Hydrogeological study Richmond Hill
- Hydrogeological study Brampton
- Hydrogeological study Caledon
- Hydrogeological study Milton
- Hydrogeological study Clarington
- Hydrogeological study East Gwillimbury
- Hydrogeological study King Township
- Hydrogeological study Whitchurch-Stouffville
- Hydrogeological study Halton Hills
Water balance questions
Is a water balance the same as a stormwater management report?
No: they overlap but answer different questions. The stormwater report deals with peak flows, quality and erosion. The water balance tracks the annual split of precipitation into evapotranspiration, runoff and infiltration, and shows whether infiltration and recharge are kept up after development. The two are usually coordinated, and LSRCA's guidelines treat water balance as one of the stormwater submission topics.
What method is used?
A monthly soil-moisture budget is common. LSRCA's review, for example, evaluates the Thornthwaite-Mather water balance analysis, according to its hydrogeology page. Feature-based balances for wetlands may need a calibrated hydrologic model and monitoring of the feature itself.
What if the post-development site cannot infiltrate as much as before?
The usual answer is low impact development: infiltration trenches, soakaway pits, permeable pavement or bioretention sized to make up the deficit. Where on-site measures cannot close the gap, some conservation authorities have offsetting policies; LSRCA has a water budget offsetting approach for its watershed.
Do I need infiltration testing as well?
If LID features are part of the plan, generally yes. LSRCA states that where LID features are proposed to mitigate the water balance, a scoped hydrogeological investigation of seasonal high groundwater levels and in-situ infiltration rates is required.
Does a small infill lot need a water balance?
Usually only if the site falls in a designated recharge area, a wellhead protection area with a quantity policy, or a provincial plan area where the development counts as major development. The definitions differ between plans, so check with the municipality or conservation authority.
Related studies
- Infiltration Testing (LID)
In-situ infiltration testing used to size low impact development stormwater features.
- Development Hydrogeological Study
Groundwater assessments that support subdivision, site plan and zoning applications.
- Source Water Protection Risk Assessment
Assessments for activities proposed inside wellhead protection areas and other vulnerable areas.
- Hydrogeological study cost guide
Ranges for every study type, side by side.
Sources
- Hydrogeology (planning application reviews) (opens in a new tab), Lake Simcoe Region Conservation Authority.
- O. Reg. 140/02: Oak Ridges Moraine Conservation Plan (opens in a new tab), Government of Ontario (e-Laws).
- Water balance for the protection of natural features (opens in a new tab), Toronto and Region Conservation Authority / Credit Valley Conservation.
- Technical guidelines for stormwater management submissions (January 2026) (opens in a new tab), Lake Simcoe Region Conservation Authority.