Weather experiment: Flash flooding

With thunderstorms plaguing much of the southwestern U.S. thanks to the monsoon, these areas are highly susceptible to flash flooding, as depicted in the video experiment above.

Flash flooding is flooding that takes place within a short period of time, following intense downpours. Several factors can influence where and whether flash flooding will occur, such as the rainfall intensity and distribution, but also land use, topography, and soil type.

Given the drought and lack of rainfall the Southwest usually has, the soil has become extremely dry, meaning that water is not easily absorbed into the ground. Consequently, when rainfall, particularly heavy rainfall occurs during monsoon season, the water just pools on the surface and leads to flooding - yes, even in the desert!

From the experiment above, you can see how this can occur. On the left side you have the parched soil, and on the right, the moist soil. The parched, non-porous soil on the left loses its ability to absorb water, so instead of soaking into the ground initially, it stays on top of the soil for longer.