Water Cycle 3D Sandbox · WaterCycle

Water on Earth

Most of Earth's water is ocean water, about 96.5% of the total. Glaciers, groundwater, lakes and rivers on land, vapour in the air and the water inside living things together make up only about 3.5%.

Fresh water is only about 2.5%, and nearly 70% of it is locked in polar and mountain glaciers. River water, freshwater lakes and shallow groundwater, which people can use easily, are less than 0.3% of all fresh water.

Click a water body on the right to mark it on the sandbox. Compare their renewal periods: atmospheric water renews in about 8 days, groundwater takes over a thousand years.

Three states of water

Water keeps changing between solid (ice, snow), liquid (water) and gas (water vapour). These changes of state drive the water cycle.

Water at the sea and lake surface absorbs heat from sunlight and evaporates into vapour. Rising vapour cools and condenses into tiny droplets in clouds; where it is colder higher up, it turns straight into ice crystals by deposition. In winter snow falls and builds up in the mountains; in spring and summer snow and glaciers melt and the meltwater flows into rivers.

Switch between summer / winter to see the snow line rise and fall; drag “Global warming” to watch the glaciers and snow change.

Sea–land cycle

The sea–land cycle runs between the ocean and the land. It is the most important type of water cycle: it keeps refilling the water on land, so water resources are renewed.

① Evaporation → ② Vapour transport → ③ Precipitation → ④ Surface runoff / ⑤ Infiltration → ⑥ Groundwater runoff → ocean

Click the steps below one by one, or press Step-by-step to play them automatically. Infiltration and groundwater runoff happen underground — look at the cut-away section at the front of the sandbox: rainwater soaks through the soil to the water table, then flows slowly through the permeable layer to the sea.

Three types of water cycle

By where they take place, water cycles fall into three types, shown in different colours on the sandbox:

Ocean cycle: seawater evaporates, condenses over the ocean and falls back into it. It moves the largest amount of water.

Sea–land cycle: the cycle between ocean and land, which keeps refilling and renewing water on land.

Land cycle: water on land enters the air by evaporation and plant transpiration, then falls on land again. The basin behind the mountains is an endorheic (interior drainage) region: inland rivers end in an inland lake, and the lake water can only leave by evaporation. This cycle moves the smallest amount of water.

Infiltration vs surface runoff

Rain that reaches the ground has three ways to go: it is intercepted by plants and evaporates, it infiltrates into the soil, or it becomes surface runoff. The more that infiltrates, the less runs off and the more groundwater is recharged.

Main factors that affect infiltration:

· Rainfall intensity: the heavier the rain, the more water cannot soak in fast enough· Slope: the steeper the slope, the faster the water flows and the less soaks in· Vegetation: roots and leaf litter loosen the soil, so more water soaks in· Soil texture: sand has large pores and absorbs fast; clay absorbs slowly· Paved surfaces: concrete and asphalt let almost no water through

Adjust the parameters, then press Bring a rainstorm and compare the split and the hydrograph on the right.

Why the water cycle matters

The water cycle links the atmosphere, hydrosphere, lithosphere and biosphere:

① It keeps the global amount of water in dynamic balance: water evaporated from the ocean returns to it as precipitation and runoff.

② It moves matter and exchanges energy: rivers carry sediment and dissolved substances from land to sea; vapour absorbs heat when it evaporates and releases it when it condenses, carrying heat from low to high latitudes and easing the heat imbalance between latitudes.

③ It shapes the land surface: running water erodes V-shaped valleys upstream and deposits alluvial plains and deltas downstream and at the river mouth.

④ It keeps renewing water resources: fresh water that has been used is replenished by the water cycle.

Urbanisation and sponge cities

As a city grows, farmland and green space turn into buildings, concrete and asphalt. Less rain infiltrates, surface runoff increases, and drains carry it quickly into the river:

· flood peaks get higher and arrive earlier, so urban flooding becomes more likely· groundwater is not recharged and rivers carry less water in the dry season· less evaporation and transpiration make the city hotter and drier

A sponge city uses permeable paving, green roofs, sunken green space and rain gardens so that it can “soak up, store, filter and purify” water like a sponge, and “release” the stored water when it is needed.

Drag the paved share, switch on the sponge city, then bring a rainstorm and compare the hydrographs.

Human impacts

Human activity affects surface runoff more than any other process, and can also change infiltration, evaporation and precipitation:

· Building reservoirs: store floodwater and release it in the dry season — cutting flood peaks and topping up low flows; the larger water surface also evaporates more· Inter-basin water transfer (e.g. China's South-to-North Water Diversion): changes the spatial distribution of surface runoff· Afforestation: more infiltration and transpiration, less surface runoff and soil erosion· Over-pumping groundwater: creates a cone of depression in the water table, causes land subsidence and, near the coast, seawater intrusion· Cloud seeding: spreading catalysts in clouds helps vapour condense and fall as rain

Water balance

Averaged over many years, Earth's water budget is balanced:

Ocean: precipitation + runoff into the sea = evaporationLand: precipitation = evaporation + runoff

More water evaporates from the ocean than falls back on it. The surplus is carried to the land as vapour, and it equals the runoff that flows from the land into the sea.

For a drainage basin over a period of time:

P = E + R + ΔS

P is precipitation, E is evaporation (including transpiration), R is runoff and ΔS is the change in water stored in the basin. Over many years ΔS ≈ 0. An endorheic region has no runoff leaving it, so all the precipitation finally evaporates: P ≈ E.

In-class testing

The arrows on the sandbox now show letters. Read the diagram and answer; an explanation appears after each answer.

Free exploration

All parameters are open. Combine different conditions and see how each part of the water cycle changes:

· strong onshore wind + summer: heavy rain on the windward slope, high river flow· winter: snow builds up in the mountains, rivers are fed by groundwater· deforestation + heavy paving + rainstorm: the flood peak shoots up· global warming: the snow line rises and glaciers retreat