What is the hydrologic cycle?

The hydrologic (water) cycle is the continuous movement of water through the environment: evaporation from oceans and surfaces, transpiration from plants, condensation into clouds, precipitation as rain or snow, infiltration into the ground, runoff over the surface to streams and rivers, and return to the oceans. Engineering hydrology quantifies each of these fluxes for a watershed.

What does return period mean?

The return period (or recurrence interval) is the average time between events of a given magnitude — a 100-year flood is one with a 1% chance of being equaled or exceeded in any single year. It does not mean such a flood happens exactly once a century; two 100-year floods can occur in consecutive years. It is a statistical, probability-based concept used to set design levels.

What is an IDF curve?

An Intensity-Duration-Frequency (IDF) curve is a graph showing, for a given location, how rainfall intensity relates to storm duration for various return periods. Shorter storms have higher intensities. Engineers read the design rainfall intensity off the IDF curve for a chosen duration (often the time of concentration) and frequency to drive runoff calculations.

What is a unit hydrograph?

A unit hydrograph is the runoff hydrograph at a watershed outlet resulting from one unit (e.g., one inch) of excess rainfall applied uniformly over the watershed during a specified duration. Because runoff response is treated as linear, the unit hydrograph can be scaled and superimposed to predict the hydrograph from any storm, making it a powerful tool for flood prediction.

Engineering·12 min read·April 4, 2025

💧 Hydrology: Rainfall, Runoff & the Hydrologic Cycle

An engineering hydrology primer — the hydrologic cycle, precipitation and infiltration, IDF curves and return period, peak-flow estimation methods, and the unit hydrograph.

📖Environmental & Water Resources Engineering Handbook (Full Access)

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What Engineering Hydrology Does

Hydrology is the science of water's movement, distribution, and quality across the Earth. For engineers, it provides the input to nearly every water-resources design: how much rain falls, how much runs off, how big a flood to expect, and how often. Get the hydrology wrong and even a perfectly engineered culvert, dam, or storm sewer will be undersized or wastefully overbuilt.

Topics covered

hydrologyhydrologic cyclerainfallrunoffinfiltrationprecipitationIDF curvereturn periodpeak flowunit hydrographwatershedSCS curve numberrecurrence intervalevapotranspiration