Solar radiation and gravity are the two main forces driving water through the hydrological cycle. Solar radiation supplies energy for water to enter the atmosphere, while gravity moves water back toward Earth’s surface and from higher to lower elevations.
This is SL and HL content from IB ESS subtopic 4.1 Water systems.
How the Drivers Work
The hydrological cycle is a system containing water storages connected by flows. Its largest storage is the ocean, while other storages include glaciers, groundwater, surface freshwater, the atmosphere, and organisms.
Solar radiation heats oceans, lakes, soils, and vegetation. This causes evaporation, in which liquid water becomes water vapour, and supports transpiration, in which plants release water vapour. It can also cause sublimation, where ice changes directly into water vapour.
As moist air rises and cools, water vapour undergoes condensation to form clouds. Water is transported horizontally through the atmosphere by advection, driven by winds produced partly by unequal solar heating.
Gravity then pulls water toward Earth as precipitation. It also drives water downslope as surface run-off, streamflow, and groundwater flow, and helps water move downward through soil by infiltration and percolation.
| Driver | Hydrological flows primarily affected |
|---|---|
| Solar radiation | Evaporation, transpiration, sublimation, and atmospheric circulation |
| Gravity | Precipitation, surface run-off, streamflow, infiltration, percolation, and groundwater flow |
A common misconception is that human activity drives the cycle. Humans modify its storages and flows, but do not provide its fundamental energy. For example, water diversion for irrigation contributed to the shrinking of the Aral Sea by reducing river inputs, while deforestation can reduce transpiration and increase surface run-off.