This Runoff Simulation https://runoff.modelmywatershed.org/ allows users to learn how land use and soil together determine whether rainfall infiltrates into the soil, runs off into streams, or is evaporated and transpired by plants.
Using the slider you can adjust the level of rainfall within 24 hours from a small rainstorm to a hurricane. As you adjust the slider from left to right, the rainfall increases. The amount of rainfall is shown on the box on the upper left.
Selecting Land Cover Type
Below the rainfall, you can select the land cover type. These land cover types and the general color scheme are taken directly from the National Land Cover Database made by the U.S. Geological Survey. A help bubble describing each land cover type will appear as you hover over that land type.
Selecting Soil Group
On the lower right, you can also select the hydrologic soil group. These four hydrologic soil groups were defined by the U.S. Department of Agriculture in 1955. Soil scientists group all soils in the U.S. into one of these four groups based on how the soils absorb and transmit water. There are also help bubbles here describing the soil types.
Reading the Results
As the parameters are selected on the right part of the screen, the infographic on the left changes. It will always show a cube of land with the selected land cover on top and the soil type on the sides.
In the upper left corner of the infographic is a box with the amount of rainfall listed. The boxes at the other corners of the infographic show the amount of rainfall that will end up as evapotranspiration (ET, water that immediately evaporates or is taken up by plants), runoff (R, water that flows across the land surface), and infiltration (I, water that seeps through the soil).
Arrows coming out of the block in the infographic change size as the amount going into each component increases. The size of the arrows is proportional to the amount of rainfall; larger rainfall amounts will always lead to larger amounts of water coming out and thus bigger arrows.
To the right of the cube of land is a bar graph showing the distribution of water. This bar graph shows the percent of water going into each category (ET, I, or R). The size of the bar graph is the same for every storm size.
If more than 5 cm of rainfall will become runoff, an exclamation point will appear at the top of the bar graph. The colors of the bar graph match the colors of the arrows and the colors of the numbers in the text boxes.
A few notes about using this model:
• It is possible that the sum of the amount of infiltration, runoff, and evapotranspiration will not add up to exactly the total rainfall. This is due to rounding in the model.
• This model does not predict flooding; it only predicts the distribution of water between infiltration, runoff, and evapotranspiration. A true flooding model is much more complicated. To predict flooding you need a great deal of detail about the whole landscape and years of historical weather and flooding data.
• This runoff simulation is only a simplified way of demonstrating the effects of different land cover and soil types on water distribution.
This tool was built by Stroud Water Research Center. They have many more resources available at wikiwatershed.org. Their complementary Model My Watershed (MMW) Site Storm Model performs the same model calculations for a selected land area within the continental United States by using actual land cover and soil data for the selected land area.