How to Calculate Surface Runoff

How to Calculate Surface Runoff
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Water can take many routes as it falls from the sky in the form of rain and other precipitation, and finally seeps into the ground. You can figure out how much water can direct itself through these paths of sinking through soil or other material into the earth after heavy amounts of rain. The surface runoff of water is one way of determining how much water an event of precipitation produces.

Direct Runoff Formula

Simple, straightforward methods of calculating runoff can tell you the amount of water that storms bring to the earth. For a given surface area such as a roof or yard, multiply the area by the inches of rainfall and divide by 231 to obtain the runoff in gallons. The factor 231 comes from the fact that the volume of 1 gallon equals 231 cubic inches. When calculating roof runoff volume, you can use a ​direct runoff formula​ (in in3) which calls for multiplying the area that covers the roof by the inches of rainfall.

More nuanced, complicated equations take into account factors such as variations in how much rain a storm creates over time. One method, known as the ​Rational Method​ uses the ​Rational Equation​:


for runoff coefficient ​C​, peak runoff rate ​Q​, rainfall intensity ​i​ (in in/hour) and size of the area ​A​ (usually in acres).

Other runoff coefficients use different units of measurement for the other variables such as area in m2 and intensity in mm/hr. Several runoff coefficient tables exist for calculating stormwater runoff, such the Runoff Coefficient (C) Fact Sheet by the California State Water Resources Control Board. Online calculators exist for the formula itself, too, like the one by LMNO Engineering, Research, and Software.

Peak Runoff Rate

You can measure the peak runoff rate ​Q​ using a storm's ​Unit Hydrograph​, the runoff of a storm over time for a location where rainfall collects in land, to the unit input of rainfall. This graph depends on the individual storm itself. Scientists and engineers create hydrographs from the measurements of rainfall during storms themselves.

They do so while addressing issues such as differences in area or time over which measurements are made. These calculations also give scientists and engineers a way of modeling storms using computational techniques.

Using the data they gain from these measurements, researchers can then use probability and statistics to determine the likelihood it will rain in the future and what type of precipitation may occur. They do this by using characteristics for various types of weather such as high-intensity, short-duration rainfall that may occur in regions in many parts of the world. This lets them search for patterns and trends from which they can form predictions about the future.

Research has shown that about 50 percent of all rain happens at an intensity greater than 20 mm/hour while around 20 to 30 percent happens at 40 mm/hour or greater, and these likelihoods occur independently of the long-term average rainfall for locations.

Properties of Runoff

Scientists and engineers define runoff as the part of precipitation, snow melt or irrigation water that gathers when the land cannot absorb it. From these observations, researchers can them account for factors like how quickly it emerges after rainfall or whether it can be called surface runoff, interflow or ground runoff.

Surface runoff​ is from the land surface directly. ​Interflow​ is the phenomena of flow that occurs when a material layer such as soil causes rainfall to collect on the surface. ​Ground runoff,​ by its nature, can accumulate soil contaminants like pesticides.

The instruments used in determining runoff affect the precision of the data. You should take into account the precision of the how you measured the amount of rainfall, the duration of the rainfall, how the precipitation distributes itself (including whether it has components of sleet or snow), the direction the storm travels and whatever other causes may affect the climate. This could range from temperature to wind, humidity and variations in the season.

Other features more unique to the areas of rainfall itself include elevation, topography, basin shape, drain area, soil type and the proximity of ponds, lakes, reservoirs, sinks and other components of the basin that may affect runoff.

As researchers study the nature of these phenomena with respect to geology, they can use the data and information they obtain to study phenomena in the atmosphere in other areas. The effects due to surface and runoff between storms in the United States and those in the Amazon may differ greatly from one another.

Studies have shown that about one third of precipitation over land ends up as runoff in streams and rivers which eventually lead towards the ocean. The other amount of precipitation is lost either to evaporation, transpiration and infiltration (soaking into groundwater). By studying these patterns among runoff phenomena, researchers gain a greater understanding of how humans are affecting the environment and what the phenomena of the Earth themselves produce.

The Human Effect on Runoff

The human impact on the Earth has brought roads, buildings and other man-made structures that have reduced runoff's ability to infiltrate into the ground or reach rivers and streams. Other actions by humans like removing vegetation and soil and creating surfaces that water cannot penetrate increase runoff. They've caused the volume and frequency of floods from streams to increase. Raising public awareness and creating discussions on how these can hurt the planet can address these issues.

Urbanization in cities across the world have affected runoff patterns on surfaces. Comparing the behavior of runoff and flow of water in natural areas such as rainforests to man-made ones like roads and cities in general can give you an idea of how easy it is for water to naturally flow to its streams and rivers in the former while struggling to do so in the latter. Urban floods occur, and hydrographs take more irregular forms in measuring how much rain is falling to show this danger.

There are many ways humans can address these environmental issues. Individuals working on farms and gardens can limit how much fertilizer they use and urban areas can use fewer impenetrable surfaces as basic steps. Planting can help, too. Some plants have natural ways of preventing erosion from occurring, and this can limit the amount of harmful runoff into waterways.

Water Pollution and Runoff

Studying how soil particles can be picked up by runoff can show you how runoff processes can affect the pollution of water. Nonpoint source pollution refers to human-caused soil erosion and the chemical applications of those effects.

These processes cause chemicals in the soil to stick to water or dissolve in them in a way that pollutes the environment. The water itself can spread litter, petroleum, chemicals and fertilizers that carry nitrogen and phosphorous to reduce water quality.

The characteristics of soil itself can affect the process by which water pollution happens as a result of runoff. It can depend upon the porosity, the amount of open space between soil grains, of soil that can adversely affect storage and movement of water.

It also depends on the roughness of the soil's surface which can capture pollutants more easily. Studying the chemical and physical nature of water in the presence of soil can give researchers better ideas of how to address the issues of water pollution as as they relate to runoff.

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