Runoff defined: An important aspect of the hydrologic cycle

Weather Glossary

ROCHESTER, NY (WROC) – The hydrologic cycle is the basis of how water moves from area to area changing phases and having different impacts on our ground and air. Runoff is an important part of this cycle that includes in no particular order:

  1. Evaporation/Transpiration
  2. Condensation
  3. Precipitation
  4. Runoff

Runoff occurs when the ground is completely saturated. Water flows from those areas into creeks, streams, and rivers that eventually flow into lakes and oceans. The image above shows how runoff can pool into a stream and quickly flow into another area that may be bone dry. Other factors that determine runoff are:

  • Type of precipitation
  • Rainfall intensity and amount
  • Soil moisture content before the rain
  • Slope of the ground

Flooding is the number one killer when it comes to weather disasters and storm runoff is directly related to most flooding deaths. As rain falls and the ground becomes completely saturated, rainwater takes the path of least resistance. That can be a dry creek bed that has not been touched for a long period of time. Something that can surge to over two feet in depth, which is plenty high to sweep away a car.

The other silent runoff is from snowpack. Every spring there is a long period where runoff becomes the dominant source of ground water and river water. This can also lead to flooding if there is a day where warming temperatures and windy conditions lead to snow melt.

Climate change is leading to significant runoff all across the world. Glaciers are melting along with the ice on Antarctica and the Greenland Ice Sheet. This runoff is leading to sea-level rise and is also putting fresh water into the salty ocean, disrupting the salinity and ecosystem where the runoff is occurring.

Development also contributes to runoff, often negatively as parking lots and buildings force rainwater and meltwater to flow into a single point that may not be able to handle increased runoff. There may also be pollutants in this runoff from vehicles and other waste products.

Only a few inches of water can sweep away a vehicle.

Here are some tips from the NWS about what do to when flooding may be possible:

  • Get out of areas subject to flooding. This includes dips, low spots, canyons, washes, etc.
  • Avoid already flooded and high velocity flow areas. Do not attempt to cross flowing streams.
  • If driving, be aware that the road bed may not be intact under flood waters. Turn around and go another way. NEVER drive through flooded roadways!
  • If the vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and its occupants and sweep them away. Remember, it’s better to be wet than dead!
  • Be especially cautious at night when it is harder to recognize flood dangers.
  • Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.

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