forest & flood

[batranac]

I know that Flood is a nature disaster. To help to lessen its impact one should protect the forest. Is it correct? Why? Thank for help.
Ba Tran

[Ron Wenrich]

Generally speaking, a forest will help prevent some flooding.  The forest floor acts like a sponge, and can hold large amounts of water.  However, after this is saturated, it will run off.

Saturation levels vary with the type of terrain, and the type of ground cover.  Concrete and buildings have a very low saturation level.  Lawns and field have a higher saturation level, but not as high as a forest.

Logging roads should be retired to lessen the effects of rain and soil erosion.  

Protection does not mean preservation.  It is possible to manage a forest for lumber production, wildlife, recreation, and a lot of other products without harming the beneficial effects of the forest.

[PeterRennie]

A forest generally does not lessen the total amount of water entering a stream after a rainfall. However, it lengthens the amount of time it takes for the water to reach the stream, so if you graph the stream level after a rainstorm, in a forested area the graph will a sharp rise, then roll over the top and a sloped tail (think cross section of an airplane wing only taller and shorter). If that area was then logged, the stream level graph would be taller and tail would be shorter.

[Forester Frank]

Pete: You want to try explaining that again.

I bet you are a forest hydrologist?

[PRennie]

Nope, but I have taken some hydrology courses and I need to know a certain amount just to do the job.

When rain falls on a hillslope, some is lost to evaporation, some is lost to traspiration and the rest makes its way to the stream.

To complicate things a bit, the amount that is lost to evaporation or transpiration varies according to whether the soil is already saturated and the weather conditions at the time. In other words, if the soil is already wet, the trees are transpiring all the water they can, and adding more water to the soil is not going to make them transpire more. Also if the rain is a quick shower after and before a dry spell, then the evaporation rate is going to be higher than if there is constant fog.

When I made my post before, I made the assumption, but didn't state it, that we are in a constant wet period and this rainfall isn't some freak event. Rather it is an exceptionally hard rain during the fall rainy period (or whenever the rainy period is in your locale).

If you measure and chart the height of a stream during this period, after a rainfall, you will find the graph looks a bit like an airplane wing (x section). It rises steeply, rolls over the top and then tapers back to the normal stream level. However, if you log a portion of the hillslope, what happens is the curve on the graph is a little higher and the tail is a little shorter, but the total amount of water is not that much different. So this says the water after the rain runs into the stream faster, but the hillslope also drains faster. Because you are putting the same or slightly more water in the stream in a shorter time the stream level will rise higher. How significant this is depends on the stream and the climate.

For west coast douglas fir forests, a new crop of trees becomes the equivelant to a stand of mature trees hydrologically, when they reach approximately 9 m in height.

This a pretty simple explanation, this topic is the subject of hundreds of Phd theses and research papers, so there are many variations to how water flows into a stream depending on soils, slope, climate, elevation or bedrock.

[Ron Scott]

In simple terms, forest lands act as nature's blotter. They absorb water and slow down excessive run off that may be damaging to other forest resources and structures. Different forested ecosystems respond in various ways as to their moisure holding capabilities. They are also important to maintaining water quality. Professional forest management considers flood plains, hydric soils, and forested ecosystems.