5. Water on Land: Watersheds, Groundwater & Aquifers

Once precipitation reaches the ground, it goes two main ways: it runs off the surface, or it soaks in.

Water that runs off flows downhill into streams and rivers. All the land that drains into a particular river or lake is its watershed (or drainage basin). Everything that happens on that land — including pollution — can end up in the shared water downstream.

Water that soaks in becomes groundwater. Underground it fills the tiny spaces between soil and rock:

• Near the surface is the unsaturated zone, where the gaps hold both air and water.
• Deeper down, the gaps are completely full of water — the saturated zone.
• The boundary between them is the water table (it rises in wet seasons and falls in droughts).
• A layer of saturated rock or sediment that can store and supply usable water is an aquifer. We reach it by drilling a well below the water table; where it meets the surface naturally, a spring flows.

⚠️ Common misconception: groundwater is not a hidden underground 'lake' or river (except in rare caves). It mostly fills the pore spaces between grains of rock and soil, like water in a sponge.

💡 Tip: runoff fills rivers (and defines a watershed); infiltration fills the spaces underground, and the top of the fully wet zone is the water table.