•The hydrograph

How would you expect deforestation to influence this response?---Consider the terms of the hydrological balance equation

Short-term response to a precipitation event

Human activities can markedly influence the shape of the hydrograph

Deforestation •reduces the lag time •raises the peak• reduces the base flow

Fig 5-2 in your text

How does deforestation affect the hydrological balance[P-E]DA – S/t

We can explain the effects of deforestation on the hydrograph in terms of the hydrological balance, mainly in terms of changes in Evaporation and storage.

Evaporation directly from the soil surface can increase when plant cover is reduced, but usually transpiration from plant leaves is decreased more than surface evaporation increases.

•This leads to increased runoff—more area under the hydrograph curve

Storage is reduced—water stored in plant tissues is removed, and the water holding capacity of soils is reduced by the death of roots.

•Thus less of the water from the precipation event goes into recharging storage pools (S is less), and thus more water enters the stream directly, reducing the lag time on the rising limb, and giving a higher and sharper peak in the deforested watershed.

•Recession will also be faster and base flow lower in the deforested watershed since there will be less storage to sustain the stream flow after the rain stops.

The hysteresis curve can be used to estimate the storage capacity

Discharge

m3/sec

Precipation

For a system with no storage capacity like a storm sewer, the hydrograph will tend to stick close to the center line

Patterns in stream flow and how they generate habitat diversity in streams

•Annual discharge patterns—floodplain habitats

•Stream meander—pool/riffle alternations

•Stream channel changes over time—oxbow lakes, braided streams

Flood plain habitats greatly increase biodiversity in river systems. They contribute mostly temporary wetlands, but some remain as permanent because of groundwater inputs.•Important for•Waterfowl, mammals, amphibians, fish, aquatic insects, molluscs etc.

•Most of the species found in flood plain wetlands would be lost from the river if the floodplain were not allowed to be flooded regularly.

Flood plains depend on seasonal variability in river discharge

Oxbow lake and the Chippewa River. Eau Claire, Wisconsin.http://www.uoregon.edu/~millerm/meander.html

River flood plains often undergo extensive urban development

Many are also strongly impacted by agriculture

Meander pattern of the Sacramento River, CA, note old abandoned channels and oxbows, and the encroachment of agriculture on the river channel.http://www.uoregon.edu/~millerm/meander.html

http://vernalfishandwildlife.fws.gov/vernalfloodplain.jpg

Such restoration can involve breaching of levees and removal of dams with the aim of.re-establishing natural floodplain vegetation and habitat

Green River, Wyoming