Water Cycle

NODE 29.01

Hydrological Cycle

• Continuous planetary water movement
• Above, on, and below surface
• Links all biogeochemical cycles
• Perfect gaseous-type cycle

NODE 29.02

Solar-Driven Engine

• Sun provides thermal energy
• Drives phase changes (liquid to gas)
• Gravity pulls precipitation down
• Thermodynamic mass-transfer system

NODE 29.03

Global Water Distribution

97%

• 97% Saline (Oceans)
• 3% Freshwater total
• ~68% of fresh is ice/glaciers
• ~30% of fresh is groundwater

NODE 29.04

Evaporation

• Liquid to vapor phase change
• Requires latent heat of vaporization
• Primary source: Oceans (86%)
• Purifies water (leaves salts behind)

NODE 29.05

Transpiration

• Biological water release
• Via plant leaf stomata
• Driven by root-pressure & capillary action
• Cools leaves, transports nutrients

NODE 29.06

Evapotranspiration (ET)

• Combined land-to-air flux
• Evaporation (soil/water) + Transpiration (plants)
• Dominant water loss in terrestrial biomes
• Drives the terrestrial water budget

NODE 29.07

Potential vs Actual ET

PET

vs

AET

• PET: Max possible if water was unlimited
• AET: Realized ET based on actual water present
• Desert: PET is huge, AET is tiny
• Rainforest: PET ≈ AET

NODE 29.08

Condensation

• Vapor to liquid phase change
• Occurs when air cools to dew point
• Releases latent heat to atmosphere
• Forms clouds, fog, and dew

NODE 29.09

Cloud Condensation Nuclei (CCN)

• Micro-particles required for droplets
• Dust, sea salt, smoke, aerosols
• Without CCN, vapor cannot easily condense
• Pollution can alter cloud reflectivity

NODE 29.10

Precipitation

• Droplets grow too heavy for updrafts
• Gravity pulls water to surface
• Main input to terrestrial ecosystems
• Measured in mm/year

NODE 29.11

Precipitation Types

RAIN

SNOW

HAIL

• Liquid: Rain, drizzle
• Solid: Snow, hail, ice pellets
• Depends on atmospheric temperature profile
• Snowpack stores water for spring release

NODE 29.12

Canopy Interception

• Rain caught by leaves and branches
• Evaporates directly back to atmosphere
• Never reaches the soil
• Reduces runoff and soil erosion

NODE 29.13

Stemflow & Throughfall

• Throughfall: drips through gaps in leaves
• Stemflow: trickles down the trunk
• Concentrates water and nutrients at roots
• Bypasses interception loss

NODE 29.14

Infiltration

• Water entering the soil surface
• Depends on soil porosity & vegetation
• Fills soil moisture (vadose zone)
• Available for plant root uptake

NODE 29.15

Percolation

• Downward movement past root zone
• Moves through soil profile and rock pores
• Driven strictly by gravity
• Filters and purifies the water

NODE 29.16

Groundwater Recharge

AQUIFER

• Percolating water reaching saturated zone
• Replenishes deep aquifers
• Very slow process (years to millennia)
• Critical for dry-season baseflow

NODE 29.17

Aquifers & Water Table

• Aquifer: Permeable rock holding water
• Water Table: Upper boundary of saturated zone
• Unconfined (shallow) vs Confined (deep/artesian)
• Massive hidden slow-moving reservoir

NODE 29.18

Surface Runoff

• Precipitation rate > Infiltration rate
• Flows overland to streams and rivers
• Causes soil erosion and nutrient wash
• Increases drastically on paved/deforested land

NODE 29.19

Catchment (Watershed)

• Topographic basin collecting water
• Defined by ridgelines (divides)
• Funnels all runoff to a single exit point
• The fundamental unit of hydrological management

NODE 29.20

River Discharge

Q = V × A

• Volume of water flowing per unit time (m³/s)
• Returns terrestrial water to oceans
• Baseflow (from groundwater) + Stormflow (runoff)
• Closes the physical loop

NODE 29.21

Sublimation & Deposition

SUB →← DEP

• Sublimation: Solid ice directly to vapor
• Skips the liquid phase (dry cold winds)
• Deposition: Vapor directly to solid frost
• Key processes in polar/alpine ecosystems

NODE 29.22

Snowmelt & Glacial Runoff

• Delayed release of winter precipitation
• Sustains rivers during dry summer months
• Himalayan rivers heavily dependent
• Highly vulnerable to climate warming

NODE 29.23

Ocean Reservoir

97%

• The ultimate sink and source
• Stores 97% of Earth's water
• Drives global climate via thermal mass
• Net evaporation exceeds precipitation over oceans

NODE 29.24

Atmospheric Water Vapor

ATMOS 0.001%

• Tiny reservoir fraction (0.001%)
• Most potent natural greenhouse gas
• Transports massive latent heat globally
• Extremely rapid turnover (9-10 days)

NODE 29.25

Residence Time

ATMOS: DAYS

OCEAN: 3000 YR

• Average time a molecule spends in a reservoir
• Atmosphere: ~9 days (Fastest)
• Rivers: ~2 weeks
• Deep Groundwater/Oceans: Thousands of years

NODE 29.26

Human Cycle Disruption

X

• Urban paving stops infiltration, spikes runoff (floods)
• Dams alter river discharge timing and sediment
• Deforestation stops transpiration, causing local drought

NODE 29.27

Groundwater Depletion

• Extraction far exceeds slow natural recharge
• Primarily for agricultural irrigation
• Causes land subsidence and saltwater intrusion
• Mining fossil water

NODE 29.28

Climate Intensification

HEAT↑→

FASTER

• Warmer air holds more moisture (Clausius-Clapeyron)
• Accelerates evaporation and transpiration
• Leads to heavier, flashier precipitation events
• Dry gets drier, wet gets wetter

NODE 29.29

Blue vs Green Water

BLUE: RIVERSGREEN: SOIL/PLANT

• Blue Water: Liquid water in rivers, lakes, aquifers
• Green Water: Soil moisture used by plants (ET)
• Grey Water: Water required to dilute pollution
• Virtual Water: Water hidden in traded goods

NODE 29.30

Water Cycle Synthesis

• The master transport system of the Biosphere
• Drives chemical weathering (P Cycle)
• Transports dissolved nutrients (N, C Cycles)
• Bridge to Realm 30 (Carbon Cycle)