Knowing how a watershed works will help the waterwise gardener understand rainwater harvesting.
Anyone who’s ever seen rainwater flowing down a hillside has seen a watershed.
But many watersheds are much more subtle and harder to define.
Essentially, a watershed is an area that directs rainfall downhill. When rainfall hits the surface, it sinks into the ground. When the ground becomes saturated, excess water runs downhill.
Slowing or Quickening Watersheds
In many areas, water runoff is slowed by vegetation, natural obstacles and even manmade structures. Flash floods are created by a watershed directing too much water too quickly into a narrow defile.
Stormwater
In the case of cities, water either comes down on, or is directed to, parking lots, streets, sidewalks and other impervious areas. As cities and suburban areas grow, natural watersheds are replaced by these hard surfaces. The water is channeled into low lying areas, causing flooding and most probably pollution from garbage, overrunning sewers and toxic substances.
Thermal Shock
- Another effect of runoff from impervious surfaces is thermal shock. Here’s how thermal shock works: Cool rainwater falls onto warm surfaces (like streets, parking lots and roofs).
- These surfaces are almost always warmer than the rain falling on it.
- The warm surfaces heat the rainwater. The heated rainwater eventually reaches streams and waterways.
- The warmer water changes the ecosystems of the waterways and increases bacterial activity.
- This in turn diminishes the amount of oxygen in the water, and causes changes in the ecology of the water (this can kill of damage fish and other wildlife).
Another Effect of Warmer Water
The warmer water itself can also damage certain wildlife, such as trout, which require water to be below a certain temperature to survive.
Pollution
As water runs across streets and parking lots and yards, it picks up a number of pollutants. Roads and parking lots accumulate oil, heavy metals, grease and other substances. Many yards contain animal wastes, fertilizer, grass clipping, leaves and other materials. Large amounts of rain also carries these materials directly into streams and waterways. This organic material also helps decrease the amount of oxygen in waterways.
Hydrologic Process
Rainwater is part of the planet’s hydrologic process. The process begins when water vapor in the air condenses to form clouds. When the condensation becomes great enough, the clouds release moisture (in the form of water, snow or ice). This moisture then comes down and either percolates through the soil into groundwater or, when the ground becomes saturated, it runs off into streams and waterways. Evaporation brings the water back into the atmosphere as water vapor. Then the cycle begins all over again.
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