Topic 1.7 Notes – The Hydrologic (Water) Cycle

What the Hydrologic Cycle Is

The hydrologic cycle is water moving through Earth in solid, liquid, and gas form. The water itself is still $H_2O$. It usually does not change chemical identity the way carbon or nitrogen does.

A few core ideas hold the whole topic together:

• The sun powers the cycle by driving evaporation and sublimation.
• Gravity brings water back to Earth as precipitation and moves it downhill as runoff and groundwater flow.
• Water is transferred and stored, not created or destroyed. That is conservation of matter.
• AP questions usually want reservoir interactions. Don’t just say “evaporation, condensation, precipitation.” Say from where to where.

This diagram is a good way to picture those reservoir-to-reservoir transfers before you learn the major storage locations in more detail.

Hydrologic cycle reservoirs and flows

Major Reservoirs and Reservoir Roles

A reservoir is a place where water is stored. Major reservoirs are:

• Oceans which hold most of Earth’s water
• Atmosphere which stores water vapor and clouds
• Glaciers and ice caps which store most freshwater
• Groundwater in saturated soil and rock underground
• Lakes, rivers, wetlands
• Soil moisture
• Living organisms

A few numbers matter:

• About 97% of Earth’s water is saline
• About 3% is freshwater
• Most freshwater is in glaciers/ice caps or groundwater

Source and sink

A source releases water to another part of the system. A sink receives and stores it for a time. The same reservoir can be either one.

• Ocean as source → evaporation
• Ocean as sink → precipitation or river discharge into it

Flux and residence time

• Flux means the rate of water movement between reservoirs
• Residence time means the average time water stays in one reservoir

The atmosphere has a short residence time. Oceans, glaciers, and deep groundwater can hold water much longer.

Main Steps and Transfers in the Water Cycle

Water can enter the atmosphere by:

1. Evaporation
   Liquid water becomes water vapor from oceans, lakes, rivers, wetlands, and moist soil.
2. Transpiration
   Plants release water vapor through stomata.
3. Evapotranspiration
   The combined total of evaporation and transpiration.
4. Sublimation
   Solid water changes directly to vapor from snow or ice.

In the atmosphere, water vapor condenses into droplets or ice crystals, forming clouds. Winds then move that water, often from ocean to land.

Water returns as precipitation:

• rain
• snow
• sleet
• hail

Once on land, water can take several paths:

• Interception by leaves and surfaces
• Infiltration into soil
• Percolation downward through soil and rock
• Groundwater recharge into saturated underground storage
• Plant uptake by roots
• Surface runoff into streams, rivers, lakes, wetlands, and oceans

Groundwater and surface water connect in both directions. Groundwater can discharge into streams, and surface water can seep into the ground.

What Controls the Path Water Takes

The path depends on land conditions.

More infiltration happens with:

• permeable soil
• vegetation
• dry ground

More runoff happens with:

• steep slopes
• saturated soil
• frozen ground
• compacted soil
• intense precipitation
• impermeable surfaces like pavement

Vegetation matters a lot. It increases interception, infiltration, uptake, and transpiration. If infiltration drops, runoff usually rises. Snowpack, glaciers, and ice delay water movement by storing it.

How Humans Alter the Water Cycle

Human activity changes where water goes and how fast.

• Deforestation lowers interception, root uptake, and transpiration. It often lowers infiltration and raises runoff, erosion, and flood risk.
• Urbanization adds pavement and roads, so infiltration and groundwater recharge fall. Runoff becomes faster, flood risk rises, and later stream baseflow can drop.
• Irrigation and groundwater pumping move water from rivers, reservoirs, or aquifers onto fields. If pumping exceeds recharge, groundwater storage declines.
• Dams and reservoirs change the timing and location of flow and can increase evaporation from stored surface water.
• Climate change can raise evaporation, shift precipitation, reduce snowpack, and change melt timing.
• Pollution usually changes water quality, not whether water cycles. Moving water carries nutrients, sediment, and pollutants between reservoirs.