Water Cycle & Watershed Simulator
Set rainfall, evapotranspiration and land cover and watch a real mass-balance model split water between runoff, infiltration and storage, with a live streamflow hydrograph.
How to use: set precipitation, evapotranspiration, ground permeability and surface imperviousness, then watch how a watershed splits rainfall between runoff, infiltration and storage - and see the streamflow hydrograph respond to a storm.
Challenge 1: balance the budget
Tune the sliders so precipitation ≈ evapotranspiration + runoff + infiltration (mass balance within 5%), keeping storage roughly steady.
Challenge 2: trigger a flash flood
Raise the impervious surface fraction and rainfall so peak streamflow exceeds 80 m³/s - simulating urban flash flooding.
About the Water Cycle & Watershed Simulator
Free water cycle & watershed simulator. Set rainfall, evapotranspiration and land cover and watch a real mass-balance model split water between runoff, infiltration and storage, with a live streamflow hydrograph. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for environmental science, the water cycle & watershed simulator runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Set rainfall, evapotranspiration and land cover and watch a real mass-balance model split water between runoff, infiltration and storage, with a live streamflow hydrograph. Use it to explore environmental science ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Water Cycle & Watershed Simulator
- Use the controls to change Precipitation (mm/hr), Evapotranspiration (mm/hr), Soil permeability (0-10), Impervious surface fraction (%), Watershed area (km²). The simulation reacts instantly.
- Press "▶ Run", "Reset run", "⛈ Trigger a storm burst", "Reset to defaults" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Set impervious surface to 80% and rainfall to 40 mm/hr, then watch the hydrograph spike.
- Compare the same rainfall on high permeability, 0% impervious land versus low permeability, 60% impervious land.
- Try to balance precipitation against ET+runoff+infiltration exactly.
- Shrink the watershed area and see how much streamflow drops for the same runoff depth.
Key ideas you can learn
- The full water cycle mass balance is: Precipitation = Evapotranspiration + Runoff + Infiltration ± change in storage.
- How much rain becomes runoff versus infiltration depends heavily on land cover: permeable soil and vegetation soak up much more water, while paved or compacted surfaces send nearly all of it straight to runoff.
- Streamflow discharge equals the runoff depth multiplied by the watershed's drainage area, so the same storm produces far more streamflow over a large watershed than a small one.
- A hydrograph - streamflow plotted over time - rises quickly and peaks higher when a watershed is more paved (impervious), which is the physical basis of urban flash flooding.
Where this is used in the real world
Civil engineers size storm drains and floodplain maps using exactly this kind of runoff-versus-infiltration and hydrograph modeling, especially when urban development changes a watershed's impervious surface fraction.
Who is this simulation for?
Students, teachers and curious learners of all ages.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why do cities flood more easily than forests in the same rainstorm?
Paved, impervious surfaces let almost all rainfall run off immediately instead of infiltrating into soil, so a city's watershed converts far more of a storm into fast runoff, producing a higher, faster streamflow peak than a forested watershed with the same rainfall.
If precipitation exceeds evapotranspiration, runoff and infiltration added together, where does the extra water go?
By mass balance it must be building up somewhere in the system as increased storage - as soil moisture, groundwater or a rising water table - which is exactly the ΔStorage term in the water balance equation.
Is the Water Cycle & Watershed Simulator free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Water Cycle & Watershed Simulator work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.