1.7 The Hydrologic (Water) Cycle

Energy from the sun heats water and evaporates it, lifting it into the air. Gravity then pulls water back down as precipitation and moves it downhill over and through the ground. Together, the sun and gravity keep water moving between reservoirs.

• Evaporation: liquid water turns into water vapor, mostly from the ocean surface.
• Transpiration: plants release water vapor through tiny pores (stomata) in their leaves. Evaporation plus transpiration is called evapotranspiration.
• Condensation: as water vapor rises and cools, it turns back into tiny liquid droplets, forming clouds.
• Precipitation: water falls back to Earth as rain, snow, sleet or hail.
• Infiltration: water soaks into the ground. Some refills aquifers, which are underground layers of rock or sediment that hold groundwater. Refilling an aquifer is called recharge.
• Runoff: water that doesn't soak in flows over the surface into streams, lakes and the ocean.
• Sublimation: ice turns directly into water vapor without melting first, as on cold, dry, windy glaciers.

Because most fresh water is frozen or underground, the easy-to-reach fresh water in lakes and rivers is a tiny sliver of the total. That's why water scarcity is a real problem even on a planet covered in water.

Below the surface, the soil and rock are first only partly filled with water. Deeper down, every pore and crack is filled; this is the saturated zone, and its top surface is the water table. An aquifer is a layer of porous rock, sand or gravel in the saturated zone that holds enough water to pump from a well.

Water can stay underground for a very long time. Shallow groundwater may be replaced in years, but deep aquifers can hold water that soaked in thousands of years ago. When people pump an aquifer faster than precipitation recharges it, the water is effectively being mined like a nonrenewable resource. Water also stays a long time in the ocean and in ice, while water vapor in the air is replaced in about a week or two.

Paving land with roads and parking lots blocks infiltration, so more water runs off. That raises flood risk and reduces aquifer recharge. Cutting down forests reduces transpiration, which can lower local rainfall, since in places like the Amazon a big share of rain comes from water released by the forest itself.

Pumping groundwater faster than it recharges lowers the water table, and near coasts it can pull salt water into wells. Warming temperatures increase evaporation, which tends to make heavy rainstorms more intense and droughts more severe in some places.