Raindrop A ... Raindrop B...

... infiltrates the soil, reaches the water-table and becomes groundwater.

  • After years in underground it is pumped from a water-well and used for potable supply.
  • It is then discharge as sewage effluent to a river, becoming surface water perched above the local water-table, which seeps through its bed to recharge the underlying aquifer.
  • The raindrop then joins the groundwater flow in aquifers and discharges directly to the sea.

... falls directly into an upland lake, becoming surface water.

  • After some days it evaporates back to the local atmosphere and falls again as rain, but this time e.g. on permeable ground where it infiltrates to become groundwater.
  • If flows underground in an aquifer for more for several years and discharge eventually as a lowland spring.
  • It thus becomes surface water again, part of a stream and river system which some days or month later reaches the sea.
  • If not infiltrating it becomes directly surface water, being part of a lake, stream or river system and reaches some days or month later the ocean.

The table above demonstrate possibilities of a water drop changing from surface water to ground water and vice versa, or evaporating into the atmosphere.

An entirely distinction between surface water and ground water is not possible considering the processes of the hydrologic cycle, but depending on the time considered, clear separation is possible.