IB Environmental systems and societies 4.1A Hydrological Cycle Notes

Hydrological cycle: a closed system of water stores and flows powered by solar energy and gravity

1. Solar energy drives the upward movements of water: evaporation and transpiration.
   1. Heat gives water molecules enough energy to change from liquid to vapour and enter the atmosphere.
2. Gravity drives the downward movements: precipitation falls, and water flows downhill as runoff, infiltration and streamflow.
   1. This downhill flow returns water from land to the oceans, completing the cycle.

System behaviour: water is conserved globally, closed for matter but open for energy

1. The global hydrological cycle is a closed system for matter: water is neither added to nor lost from Earth, so the total volume stays constant.
2. It is an open system for energy: solar radiation enters and heat is lost, and this energy drives every flow.
3. The system is built from stores (reservoirs of water) linked by flows (movements between them).
4. Flows are either transfers that change a store's location or transformations that change water's state.

Stores: oceans hold almost all of Earth's water, while accessible freshwater is a tiny fraction

1. Earth's water sits in six main stores, which differ in volume, state and residence time.
2. Oceans: ~96.5% of all water, saline, the largest reservoir.
3. Ice caps and glaciers: ~1.7%, frozen freshwater with a long residence time.
4. Groundwater: ~1.7%, the largest accessible store of liquid freshwater, held in aquifers.
5. Surface freshwater (lakes and rivers): <0.02%, tiny but the most accessible and fastest-responding store.
6. Atmosphere: ~0.001%, vapour and clouds with a residence time of only 8 to 10 days.
7. Biosphere (living organisms): a minute fraction, held briefly before being returned through respiration and transpiration.

Only about 2.5 to 3% of Earth's water is freshwater, and most of that is locked in ice, so less than 1% is readily available for human use.

Flows: transfers change water's location, transformations change its state

1. Transfers move water between stores without changing its state.
2. Precipitation: rain, snow, sleet or hail falling from the atmosphere to the surface.
3. Surface runoff and streamflow: overland flow and channel flow that carry water to the oceans.
4. Infiltration: water soaking downward from the surface into the soil.
5. Percolation: the deeper movement of that water through rock down to the water table.
6. Throughflow and groundwater flow: lateral movement through soil and slow flow through aquifers, the slowest transfer of all.
7. Advection: horizontal transport of vapour or clouds by wind, redistributing moisture across the globe.

Transformations: changes of state that move and store energy

1. Evaporation: liquid water becomes vapour using solar energy, the main link from hydrosphere to atmosphere.
2. Condensation: vapour cools into droplets to form clouds, releasing latent heat that powers storms.
3. Freezing and melting: changes between liquid and solid that store or release latent heat.
4. Sublimation and deposition: direct change between ice and vapour without a liquid stage, common in polar regions.
5. Transpiration and evapotranspiration: water lost as vapour from plant leaves, which combined with evaporation supplies most atmospheric moisture.

Systems diagram: stores are boxes, flows are labelled arrows

1. Draw stores (oceans, glaciers, groundwater) as boxes.
2. Draw flows (evaporation, infiltration, runoff) as labelled arrows showing direction, with transformations marked where water changes state.

Human impact: land-use change perturbs the system's equilibrium by cutting infiltration and boosting runoff

1. Deforestation removes interception and roots, so infiltration and transpiration fall while surface runoff and erosion rise.
2. Urbanisation seals the ground with impermeable surfaces, cutting groundwater recharge and speeding runoff, which raises flood risk.
3. Agriculture and irrigation withdraw large volumes of groundwater and surface water, depleting aquifers and lowering the store.
4. Together these shift flows and stores away from their natural balance, and a warming climate intensifies the effect by increasing evaporation and precipitation extremes.

Steady-state equilibrium: inputs balance outputs over time, held by negative feedback

1. The cycle is in steady-state (dynamic) equilibrium: long-term inputs and outputs stay balanced even though local and seasonal fluctuations occur.
2. A single water body is in steady state when its inputs equal its outputs.
   1. Inputs: precipitation, surface inflow, groundwater inflow and runoff from surrounding land.
   2. Outputs: evapotranspiration, river outflow, groundwater outflow and human extraction.
3. Negative feedback restores balance, for example when more evaporation leads to more rainfall that replenishes the stores.
4. Positive feedback instead amplifies change, for example ice melt lowering albedo and causing further warming and melting.