Water Cycle & Watershed Simulator Community
Questions, answers and tips from people who use the 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..
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This page shows only real posts from real people, so it starts empty. The guide below has the questions people most often have about the Water Cycle & Watershed Simulator. Pick one to start a thread, or write your own.
From the tool guide: common questions about Water Cycle & Watershed Simulator
The items below are not posts by community members. They come from the written guide for this tool and are here to get a conversation started. Use “Ask a follow-up” to open a thread about any of them.
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.
From the tool guide Ask a follow-upIf 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.
From the tool guide Ask a follow-upKey ideas
- 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.
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.
Where this is used
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.
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