---
id:"kb-2026-00371"
title:"Water Cycle"
schema_type:"TechArticle"
category:"science"
language:"en"
confidence:"high"
last_verified:"2026-05-22"
generation_method:"ai_assisted"
ai_models:["claude-opus"]
derived_from_human_seed:true
primary_sources:
  - title:"NASA Earth Observatory — Water Cycle"
    type:"documentation"
    year:2026
    url:"https://earthobservatory.nasa.gov/features/Water"
    institution:"NASA"
secondary_sources:
  - title: "MDN Web Docs — HTTP"
    type: "documentation"
    year: 2026
    url: "https://developer.mozilla.org/en-US/docs/Web/HTTP"
    institution: "Mozilla"
completeness: 0.88
ai_citations:
  last_citation_check:"2026-05-22"
---

## TL;DR

The water (hydrologic) cycle describes continuous movement of water on, above, and below Earth's surface. Processes: evaporation (liquid→vapor), transpiration (from plants), condensation (vapor→liquid, forms clouds), precipitation (rain, snow), infiltration (into soil), runoff (to rivers/oceans). Earth's total water volume: ~1.386 billion km³.

## Core Explanation

Oceans: 96.5% of all water. Freshwater: 2.5% — mostly locked in glaciers/ice caps (68.7%) and groundwater (30.1%). Surface freshwater (lakes, rivers): only 0.3% of freshwater. Average residence time: oceans ~3,200 years, atmosphere ~9 days. Groundwater: crucial for drinking/irrigation, threatened by over-extraction and pollution. Climate change intensifies water cycle — more floods and droughts.

## Further Reading

- [NASA Earth Observatory — Water Cycle](https://earthobservatory.nasa.gov/features/Water)