What you will learn in The Hydrosphere and the Water Cycle

1. 1. The inventory

   All the water on, in and above the Earth, gathered into one ball and set down beside the planet at true scale. Then the fresh part, and then the lakes and rivers — a ball 56 km across.

2. 2. How long water stays

   Divide what a reservoir holds by what flows through it each year and you have how long a molecule stays — and how long the reservoir takes to forget a disturbance. Nine days for the air, three thousand years for the sea.

3. 3. The cycle running

   The whole cycle as boxes and arrows that must balance. The school circle of evaporation, cloud and rain does not — it leaves out the arrows that carry water through plants and through the ground.

4. 4. Underground

   Groundwater is not an underground river. It fills the spaces between grains and creeps through them, and how much ground holds is a different question from how fast it lets go. Pump a well and watch the neighbours, then move to a coast.

5. 5. Basins and hydrographs

   A river drains a basin bounded by a divide, and the pattern of its streams reads the rock beneath. In a storm, the river's rise and fall is a hydrograph — and turning forest into city makes it rise sooner and higher with not a drop more rain.