Fill the sieve half full with sand or soil. Pour on all of your water on the top without spilling it over the sides. This is your rainfall. Immediately start the stopwatch. Place a measuring cylinder under the sieve to catch the drips, and record how much water has been collected each minute.

You can see that it takes time for water to get through the soil (and seep out to the river/measuring cylinder) and that it carries on dripping for a long time after rainfall. This is why rivers keep flowing between rainstorms.

Advanced

If you want to find out how the river flow changes, plot the amount of water against time as a column chart. The top of the columns show a gently declining curve. It may be hours before even your sieve of soil stops dripping. Think what must be stored in a whole hillside!