Hydrologic Cycle and Human Impacts

Water Systems

4.1.1 Hydrologic Cycle

The hydrologic cycle describes the movements of water in the hydrosphere, driven by solar radiation and gravity.

• Solar Radiation: Provides the energy for evaporation.

• Heat: Required for evaporation and released during condensation.

• Gravity: Causes water to drain through soil (infiltration and percolation) and in rivers to the sea. Excess water from precipitation results in surface run-off.

Hydrologic Cycle: A Step-by-Step Flow

The hydrologic cycle, also known as the water cycle, is a continuous movement of water on, above, and below the Earth's surface, driven by solar radiation and gravity. It involves several key processes:

1. Evaporation:

   • Solar radiation transforms water from a liquid to a gaseous state (water vapor).

   • Occurs primarily from oceans, lakes, and rivers.

2. Transpiration:

   • Plants absorb water through roots and release it into the atmosphere as water vapor through leaves.

   • The combined effect of evaporation and transpiration is called evapotranspiration.

3. Condensation:

   • Water vapor rises into the atmosphere and cools.

   • Cooling causes water vapor to change back into a liquid state, forming clouds. This process releases heat.

4. Advection:

   • Winds transport clouds (stores of water vapor) through the atmosphere.

   • This movement of water vapor is known as advection.

5. Precipitation:

   • Water droplets in clouds become too heavy and fall back to Earth.

   • Forms include rain, snow, sleet, and hail.

6. Infiltration:

   • Water that reaches the Earth's surface seeps into the soil.

   • This process of water entering the soil is called infiltration.

7. Percolation:

   • Water moves further down through the soil layers after infiltration.

   • This downward movement of water within the soil is called percolation.

8. Groundwater Flow:

   • Water that has percolated deep into the ground becomes groundwater.

   • Groundwater flows slowly underground, eventually reaching bodies of water like rivers, lakes, and oceans.

9. Surface Runoff:

   • When the soil is saturated, compacted, or precipitation is heavy, water flows over the land as surface runoff.

   • Surface runoff flows into streams, rivers, and lakes, and ultimately back to the oceans.

10. Streamflow:

    • Any movement of water.

The hydrologic cycle is a continuous loop where water evaporates, transpires, condenses, is advected, precipitates, infiltrates, percolates, flows as groundwater, and runs off the surface, eventually returning to its sources.

4.1.2 Global Hydrological Cycle

The global hydrological cycle operates as a closed system, with stores and flows, describing the continuous movement of water within Earth’s atmosphere, surface, and below the ground. It is a closed system, meaning no new water is added to or leaves the system, but it continuously moves between stores and flows. In water cycle diagrams, stores should be shown as boxes and flows as arrows.

4.1.3 Main Stores in the Hydrological Cycle

Main stores in the hydrological cycle:

• Oceans (96.5%)

• Glaciers and ice caps (1.7%)

• Groundwater (1.7%)

• Surface freshwater (0.02%)

• Atmosphere (0.001%)

• Organisms (0.0001%)

Understanding the relative proportions of water distribution is key. Oceans hold the majority of Earth's water, while freshwater readily usable by humans is a tiny fraction.

4.1.4 Hydrological Cycle Details

The hydrological cycle shows how water moves between different stores. This movement [the flows] are driven by energy from solar radiation and gravity. Energy from the sun heats up water[storage] so that it moves from being a liquid to a gas[flow]. This transformation (a change in state) is called evaporation[process flow].

• Evaporation (process flow): Transformation from liquid to gas.

• Transpiration (flow): Plants move water through their systems; evaporation from plants' leaves. Evaporation of water from plants' leaves. Sometimes referred to as evapotranspiration (process flow).

  Evaporating water cools spaces because water requires an unusually high amount of energy to heat up. This is why evaporating water can be used to cool down spaces (or our bodies as sweating) and is used as a nature-based solution to cooling buildings. It is also why having water bodies next to cities helps to make their climates milder.

• Condensation (process flow): Transformation when water turns back to a liquid state, releasing heat. Has to happen before water can fall from the sky (flow) as precipitation

• Precipitation: Water falling from the sky.

• Advection: Clouds (stores of water vapor) of condensed or frozen water droplets moved through the atmosphere by winds.

• Freezing: Transformation when water turns to a solid state.

• Melting: Reverse of freezing.

• Sublimation: Water molecules moving directly from the solid state to the gaseous state.

Gravity means that water moves down from high to low, so watersheds (the location where rivers start) are at higher altitudes than seas and oceans [flow]. First it infiltrates the soil[process flow] then percolates[process flow] through the soil to enter an aquifer (an underground store of water) [flow]. Underground movement of water is called groundwater flow.

• Infiltration: Water entering the soil.

• Percolation: Water movement in the soil.

• Groundwater flow: Underground movement of water.

• Streamflow: Any movement of water.

• Surface runoff: Water flowing over the surface because the soil is waterlogged, compacted, or precipitation is too heavy.

• Turquoise - flows

• Purple - transfer

• Turquoise box - stores

• Orange - transformation

4.1.5 Human Activities Altering Flows and Stores

Human activities can alter these flows and stores:

Impervious surfaces, such as pavements (roads, sidewalks, driveways, and parking lots), significantly alter water distribution by increasing runoff and reducing infiltration.

4.1.6 Steady State of a Water Body

The steady-state of any water body can be demonstrated through flow diagrams of inputs and outputs. These can be used to calculate sustainable rates of harvesting from, for example, lakes and aquifers. If the total rate of inflow is greater than the total rate of outflow, the water body will increase in size. For the water body to remain constant, the inflows must be balanced by the outflows (equilibrium/stable state/sustainable). The volume of water in a lake will only remain constant if the combination of all the inflows is equal to the combination of all outflows.