# Every Drop Counts

Earth is called the "Blue Planet" because water covers about 71% of our surface. From space, our world looks like a giant water world. But here's a troubling paradox: despite all that water, freshwater shortages threaten billions of people.

How can a planet covered in water have water shortages?

The answer lies in understanding WHERE Earth's water is, what STATE it's in, and how it MOVES through our planet's systems.

Essential Question: Why is freshwater considered a limited resource when so much of Earth is covered by water?

# Where is Earth's Water?

## Earth's Water Budget

Earth contains about 1.386 billion cubic kilometers of water. That sounds like a lot—but look at how it's distributed:

| Reservoir | Percentage | Notes | |-----------|------------|-------| | Oceans (saltwater) | 96.5% | Too salty to drink or use for agriculture | | Ice caps, glaciers, permanent snow | 1.74% | Frozen; not readily accessible | | Fresh groundwater | 1.05% | Underground; accessible with wells | | Saline groundwater | 0.94% | Too salty for most uses | | Freshwater lakes | 0.007% | Accessible surface water | | Soil moisture | 0.001% | In soil; supports plant growth | | Atmosphere | 0.001% | Water vapor, clouds | | Rivers | 0.0002% | Tiny fraction but crucial for humans | | Swamps/marshes | 0.0008% | Wetland ecosystems | | Living organisms | 0.0001% | Water in plants and animals |

## The Critical Math

Total freshwater: Only about 2.5% of Earth's water is fresh (not salty)

But of that freshwater: - ~69% is locked in ice caps and glaciers - ~30% is groundwater (much of it deep and hard to access) - ~1% is surface freshwater (lakes, rivers, swamps)

Bottom line: Only about 0.5-1% of Earth's total water is readily accessible freshwater for human use!

# The Water Cycle

## Powered by the Sun

The water cycle (also called the hydrologic cycle) is the continuous movement of water through Earth's systems. The Sun's energy powers this cycle by: 1. Heating water to cause evaporation 2. Creating temperature differences that drive winds 3. Warming air so it can hold more water vapor

## Major Processes

Evaporation - Water changes from liquid to gas (water vapor) - Requires energy (540 calories per gram of water) - Occurs mainly from ocean surfaces (about 86% of all evaporation) - Rate increases with temperature, wind, and surface area

Transpiration - Water released by plants through their leaves - Plants "breathe out" water as part of photosynthesis - A large tree can release 400+ liters of water per day! - "Evapotranspiration" = evaporation + transpiration combined

Condensation - Water vapor changes back to liquid droplets - Releases energy (540 calories per gram—same amount absorbed during evaporation) - Forms clouds when water vapor condenses around tiny particles (dust, pollen) - This released energy helps power weather systems

Precipitation - Water falls from atmosphere as rain, snow, sleet, or hail - Occurs when water droplets/ice crystals become too heavy to stay suspended - Globally, precipitation equals evaporation (balanced system)

Infiltration - Water soaks into the ground - Becomes soil moisture or groundwater - Rate depends on soil type, slope, vegetation, and saturation

Runoff - Water flows over land surface to streams, rivers, and eventually oceans - Carries sediments and dissolved materials - Increases when ground is saturated or impermeable

# Residence Time: How Long Water Stays

## What is Residence Time?

Residence time is the average amount of time a water molecule spends in a particular reservoir before moving to another part of the water cycle.

| Reservoir | Typical Residence Time | Implication | |-----------|------------------------|-------------| | Atmosphere | 9 days | Weather changes rapidly | | Rivers | 2-6 months | Quick renewal | | Soil moisture | 1-2 months | Seasonal variation | | Large lakes | 50-100 years | Slower renewal | | Shallow groundwater | 100-200 years | Pollution persists for generations | | Deep groundwater | 10,000+ years | Essentially non-renewable | | Oceans | 3,000-4,000 years | Very slow cycling | | Antarctic ice sheet | ~900,000 years | Climate archive |

## Why Residence Time Matters

For Pollution: - Atmosphere: Air pollution can be cleaned up relatively quickly - Groundwater: Pollution persists for centuries—prevention is critical - Oceans: Plastic and pollutants accumulate over millennia

For Resources: - Short residence time = renewable (rain refills rivers) - Long residence time = essentially non-renewable (deep aquifers)

# Summary

## Key Concepts Review

Water Distribution: - 96.5% of Earth's water is salty ocean water - Only ~2.5% is freshwater (most frozen in ice caps) - Less than 1% is accessible freshwater for human use - Rivers hold only 0.0002% of Earth's water!

Water Cycle Processes: - Evaporation: Liquid to gas (absorbs energy) - Transpiration: Plants release water vapor - Condensation: Gas to liquid (releases energy, forms clouds) - Precipitation: Water falls as rain, snow, sleet, or hail - Infiltration: Water soaks into ground - Runoff: Water flows over surface to streams/oceans

Energy in the Water Cycle: - Sun powers the entire cycle - Latent heat transfer drives weather and storms - Evaporation cools, condensation warms

Residence Time: - Atmosphere: 9 days (rapid cycling) - Groundwater: 100-10,000+ years (slow cycling) - Long residence time makes pollution cleanup nearly impossible