Explore in 3D · Land and water model

Rain on a little island

Build hills and dig basins. Make it rain and watch the water run downhill, soak in and carry soil. Then cover some soil with plants and see what stays in place.

Goes with

The island model

A model, not to scale

A little island sits on the page. Pick a picture button, then tap the land.

Try this

  1. Tap the land. Press Raise here to build a hill. Then press Rain. Where does the water go? Where does it make a pool?

  2. Press Fair test, then Rain. Both slopes get the same rain. Which pond gets more soil? Try Heavy rain too.

  3. Press Map view during a storm. Watch the height lines. What does the map show? What does it leave out?

What did you notice?

  • Rain water always runs …
  • Where did the water stop and make a pool?
  • Which soil stayed in place? What held it there?
  • What happened when the rain was heavier?

Tell a grown-up, or draw it in your workbook.

For grown-ups: what this model shows, and what it changes
  • Not to scale. The island is tiny, and rain and soil movement are sped up: one storm takes about 20 model seconds. Some real changes are fast, like a flood. Others take many years.
  • How the water moves. The land is a grid of small squares. Each square passes water to its four neighbors when its water surface is higher, through "virtual pipes" (a simple shallow-water method used in computer graphics). Every bit of water is counted: it flows on, soaks in, sits in a pool, or reaches the sea or pond. Puddles left from one storm on the island are counted again in the next storm.
  • How the soil moves. Faster, deeper water on a steeper slope can carry more soil. Where it can carry more than it holds, it picks soil up; the soil moves with the water; where the water slows, soil settles out. That is why muddy water shows up in the pond or the sea and new soil builds up at the shore.
  • Covered soil. Plants and mulch slow the water, let more of it soak in, and roots hold soil in place. The model lowers soil pick-up under cover by 90 percent, and makes water soak in about twice as fast. Those numbers are chosen to show the idea; real soils, covers and storms can give different results.
  • A fair test. The two trays start as exact mirror images, with the same soil, the same slope and the same rain. Only the cover differs. The ponds catch runoff and are covered from the rain, like the cup at the bottom of a real tray test. Start again always restores the same two slopes.
  • Units. Soil amounts are "soil units" and water amounts are percent of all the water: the rain that fell, plus any water left on the island from an earlier storm (the meter says when there is some). Soaked in, still on the land and reached the sea always add up to 100 percent. They are model units, not grams, liters or millimeters, so compare them with each other, not with real measurements.
  • Model and map. The map view uses contour lines that update as the land changes: each line joins places at the same height, and lines close together mean a steep slope. A model and a map each show some things and miss some (week 31).
  • Real test first. Build the workbook's landscape and runoff tests with real soil and water. Use this model afterward to talk about what happened.