What is the best free software for calculating site runoff?

QGIS is the best free option for calculating site runoff. It's an open-source GIS platform that, when paired with hydrology plugins like GRASS GIS, can delineate watersheds, calculate flow accumulation, and model surface runoff from digital elevation models. It requires some technical knowledge but produces professional-grade results at no cost.

How do I calculate stormwater runoff for a residential property?

The most common method for residential properties is the Rational Method, which uses the formula Q = C × i × A (runoff coefficient × rainfall intensity × drainage area). You need to identify your drainage areas, assign runoff coefficients based on surface types (0.90 for roofs, 0.25 for lawns on clay soil, etc.), and look up your local rainfall intensity for the design storm. Landscape estimating software automates this process.

What is a runoff coefficient and how does it affect drainage design?

A runoff coefficient (C value) is a number between 0 and 1 that represents the fraction of rainfall that becomes surface runoff. Impervious surfaces like concrete have high coefficients (0.90–0.95), while vegetated areas on sandy soil have low coefficients (0.10–0.20). Higher coefficients mean more runoff, requiring larger drainage infrastructure.

Do landscape contractors need hydrology software or can they use general estimating tools?

For standard residential drainage work like French drains and yard regrading, general landscape estimating tools with material takeoff capabilities are usually sufficient when combined with basic Rational Method calculations. However, for projects requiring stormwater management plans, permitting, or complex multi-area drainage designs, dedicated hydrology software like HydroCAD provides the engineering rigor needed.

How accurate are software-generated runoff calculations compared to field measurements?

Software calculations are typically within 10–25% of actual field measurements when accurate input data is used — including verified soil types, correct impervious area measurements, and appropriate rainfall data. The biggest sources of error are inaccurate terrain models, use of default soil values without field verification, and failure to account for offsite contributing drainage areas.

What rainfall data should I use for residential drainage design?

Most residential drainage designs use NOAA Atlas 14 precipitation frequency estimates, targeting a 10-year return period storm. This means the drainage system is designed to handle a storm that has a 10% chance of occurring in any given year. Check your local stormwater ordinance, as some jurisdictions require design for 25-year or even 100-year storms in certain situations.

Best Landscape Estimating Software for Site Runoff Calculations

Discover the best landscape estimating software for calculating complex site runoff. Compare tools, methods, and features to manage stormwater on any property.

By Low Point Labs

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