Hydrology Cheat Sheet

The core ideas of Hydrology distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Hydrologic Cycle

The continuous movement of water through the Earth system via evaporation, condensation, precipitation, infiltration, runoff, and transpiration. It is driven primarily by solar energy and gravity.

Watershed (Drainage Basin)

An area of land where all precipitation and surface water drains to a common outlet point, such as a river mouth, lake, or ocean. Watersheds are the fundamental spatial unit for hydrologic analysis.

Runoff

The portion of precipitation that flows over the land surface or through shallow subsurface pathways to reach streams and rivers, rather than infiltrating deeply into the ground or evaporating.

Infiltration

The process by which water on the ground surface enters the soil. Infiltration rate depends on soil type, moisture content, vegetation cover, and rainfall intensity.

Evapotranspiration

The combined process of evaporation from soil and water surfaces plus transpiration from plants. It represents the return of water from the land surface to the atmosphere.

Groundwater Recharge

The process by which water moves downward from the surface through the unsaturated zone to replenish an aquifer. Recharge rates depend on geology, land use, climate, and soil properties.

Hydrograph

A graph showing the rate of flow (discharge) versus time at a specific point in a river or channel. The shape of a hydrograph reveals how a watershed responds to a precipitation event.

Flood Frequency Analysis

A statistical method used to estimate the probability of floods of various magnitudes occurring at a given location, often expressed as return periods (e.g., the 100-year flood).

Water Balance Equation

A fundamental equation in hydrology stating that inputs (precipitation) equal outputs (runoff, evapotranspiration, deep percolation) plus or minus changes in storage within a defined area over a given time period.

Darcy's Law

A foundational equation in groundwater hydrology that describes the flow of fluid through a porous medium, stating that discharge is proportional to the hydraulic gradient and hydraulic conductivity.

Key Terms at a Glance

Aquifer:A body of permeable rock or sediment that stores and transmits groundwater in usable quantities.

Baseflow:The component of streamflow derived from groundwater discharge, sustaining flow during dry periods.

Capillary Fringe:The zone immediately above the water table where water is drawn upward by capillary forces in soil pores.

Confined Aquifer:An aquifer bounded above and below by impermeable layers (aquitards), in which water is under pressure greater than atmospheric.

Darcy's Law:A fundamental equation relating groundwater flow rate to hydraulic conductivity, cross-sectional area, and hydraulic gradient.

Discharge:The volume of water flowing past a point in a stream per unit time, typically expressed in cubic meters per second (m³/s) or cubic feet per second (cfs).

Drainage Divide:The topographic boundary separating adjacent drainage basins; precipitation falling on either side flows to different outlets.

Evapotranspiration:The sum of evaporation from land and water surfaces plus transpiration from vegetation, returning water to the atmosphere.

Flood Frequency:The statistical characterization of how often floods of a given magnitude are expected to occur at a particular location.

Groundwater:Water found beneath the Earth's surface in the pores and fractures of rock and sediment, below the water table.

Hydraulic Conductivity:A measure of a porous medium's ability to transmit water, depending on both the medium properties and the fluid properties.

Hydraulic Gradient:The change in hydraulic head per unit distance in the direction of groundwater flow; it drives the movement of groundwater.

Hydrograph:A graph showing discharge versus time at a particular location in a stream or river, used to analyze watershed response to precipitation.

Hydrologic Cycle:The continuous natural cycle of water moving through evaporation, condensation, precipitation, runoff, infiltration, and storage.

Infiltration:The entry of water from the surface into the soil, governed by soil type, moisture content, surface conditions, and rainfall intensity.

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