What happens when it rains?


With recent summer storms along the Wasatch Front, it is a good time to discuss what happens when it rains, especially after water falls from the sky.

This is one of the central questions of hydrology, with practical implications for managing stormwater, protecting property, controlling floods, and preserving water quality in natural and engineered environments.

There are four basic processes that follow precipitation: abstraction, evaporation, infiltration, and runoff. The behavior and magnitude of each component depend on many factors.


Initial abstraction is the portion of rain that is intercepted by trees and vegetation before it reaches the ground. Full, leafy vegetation can capture significant amounts of water.


Some rain evaporates back into the atmosphere, a process governed by temperature, wind, humidity, and other conditions.


Rainwater infiltrates into the ground at various rates depending on soil characteristics and the amount of water. Infiltration continues until the soil surface is saturated.


For the purposes of stormwater management, runoff—the portion that flows over land—is the most important component. In urban areas runoff is collected and conveyed in storm drain systems with gutters, inlets, and pipes. This water ultimately infiltrates in retention or detention ponds or is discharged to natural water bodies. Impervious surfaces such as roads, driveways, and buildings significantly increase runoff. Soil type, vegetative cover, and land slope also affect runoff behavior. Runoff from urban areas can quickly accumulate and overwhelm storm drains if systems are not properly designed for peak flows. Runoff can also cause erosion, sediment pollution, nutrient pollution, and other environmental problems.

How much?

For engineers, city planners, developers, utility crews, and the general public, the question “What happens when it rains?” is an important one. Determining the magnitude and timing of each component, particularly the peak runoff flow, is critical to effective stormwater management.

Robert B. Sowby

Robert B. Sowby is a water resources engineer at Hansen, Allen & Luce. A graduate of BYU, MIT, and the U, he has contributed to over 100 civil, water, and environmental projects throughout North America.

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