Rainwater Runoff Calculator
Understanding Rainwater Runoff
Rainwater runoff is the water from rain, snowmelt, or irrigation that flows over the land surface. When rain falls on a roof, it doesn't all soak into the ground. Instead, a significant portion flows off the surface, especially from impervious materials like asphalt shingles, metal, or tile roofs. This runoff can be a valuable resource, particularly for rainwater harvesting systems, or it can be a source of environmental concern, contributing to stormwater pollution and erosion.
The amount of water that runs off from a surface is influenced by several factors, including:
- Surface Area: Larger surfaces collect more rain, thus generating more runoff. In the context of a building, the roof area is the primary determinant.
- Rainfall Intensity and Duration: A heavier or longer rainfall event will produce more runoff than a light shower.
- Surface Permeability (Runoff Coefficient): This is a crucial factor. Impervious surfaces (like concrete or metal roofs) have high runoff coefficients (close to 1), meaning most of the water that hits them runs off. Pervious surfaces (like vegetated areas) have low runoff coefficients, allowing water to infiltrate. Different roofing materials have varying coefficients. For example, asphalt shingles typically have a coefficient around 0.8-0.9, while metal roofs can be as high as 0.95.
- Slope: Steeper slopes can increase runoff velocity and volume.
- Antecedent Moisture: If the surface is already saturated from previous rainfall, less new water will infiltrate, leading to higher runoff.
How the Calculator Works
This calculator estimates the volume of rainwater runoff you can expect from your roof during a specific rainfall event. It uses a simplified formula based on the principles of hydrology:
Runoff Volume (gallons) = Roof Area (sq ft) × Rainfall Amount (inches) × Runoff Coefficient × 0.623
- Roof Area (sq ft): This is the total surface area of your roof that collects rain.
- Rainfall Amount (inches): This is the depth of rain that falls during a given period.
- Runoff Coefficient: This dimensionless number (between 0 and 1) represents the fraction of rainfall that becomes runoff. It accounts for the surface's ability to absorb or retain water. A value of 1 means all rain becomes runoff, while a value of 0 means no rain becomes runoff (e.g., highly permeable soil).
- 0.623: This is a conversion factor. Multiplying square feet by inches gives cubic feet. There are approximately 7.48 gallons in a cubic foot, so 1 / 7.48 ≈ 0.1337 cubic feet per gallon. However, the standard conversion used in many hydrological contexts for runoff volume from area and depth (in feet) to gallons is often cited with a factor of 0.623, derived from (1 ft/12 in) * 7.48 gal/ft³ ≈ 0.623 gal/in. For practical purposes in this context, using 0.623 directly with inches of rainfall and square feet of area yields gallons.
Example Calculation
Let's consider a house with a roof area of 1,600 square feet. A storm brings 2 inches of rain. The roof is covered with standard asphalt shingles, which have a runoff coefficient of approximately 0.85.
- Roof Area: 1,600 sq ft
- Rainfall Amount: 2 inches
- Runoff Coefficient: 0.85
Using the calculator:
Runoff Volume = 1,600 sq ft × 2 inches × 0.85 × 0.623 ≈ 1,695.47 gallons
This means that approximately 1,695.47 gallons of water could potentially be collected or flow off this roof during this storm event. This calculation is an estimate and can be useful for sizing rainwater harvesting systems, understanding potential stormwater impacts, or planning drainage solutions.