HIGHER GEOGRAPHY Hydrosphere - Hydrographs

CAN DOs! √ describe and explain the patterns shown on a river hydrograph. GMTs construct and analyse river hydrographs to show the relationship between rainfall and river flow

Hydrograph River Discharge = cross sectional area X Record of River Discharge over a period of time River Discharge = cross sectional area rivers mean velocity X (at a particular point in its course)

The pattern of discharge over a period of time is the Annual hydrographs show river discharge over a year. Precipitation and discharge cover the full year. The pattern of discharge over a period of time is the RIVER REGIME

Colorado River, SW USA. Long term discharge of the Colorado

The Hydrograph as a measure of Annual Climate Change While many modern waterways are controlled by dams and reservoirs, in natural systems the hydrograph provides a climate signature that can be an important way of tracking seasonal fluctuations. The hydrograph year starts in October and ends at the end of September. The graph on the next slide shows the discharge of Boulder Creek in the Rocky Mountains near Boulder, Colorado, which is somewhat controlled by a dam upstream but still shows the basic pattern of a natural system. The unit of measure is in cubic feet per second, with the red line referring to daily mean streamflow.

Storm hydrographs show a few days only and show the changes in discharge that result from a period of rainfall.

Basic terms: Run off: the sum of all the rainwater that flows over the surface of the river basin (stream flow and overland flow) Through flow: the downslope movement of water through soil towards streams and rivers. Baseflow: groundwater movement, which often lags behind precipitation by weeks, months or even years. Stormflow: discharge that is not baseflow.

Terms to do with hydrographs: Approach segment: the discharge of a river before the storm.

Lag time: the time delay between max rainfall and max discharge. Lag time varies according to the type of drainage basin.

Rising limb: part of the graph which shows increasing discharge

Falling limb (recession limb): part of the graph which shows decreasing discharge

Bankfull discharge: the point at which a river is full. If the level increases any more, the river floods.

SUMMARY

SUMMARY

Discharge is the amount of water passing a sampling point SUMMARY Discharge is the amount of water passing a sampling point It is a combination of run off, throughflow and base flow. Overland flow is the volume of water reaching the river from surface run off Throughflow is the volume of water reaching the river through the soil.

Discharge is measured in cumecs (cubic metres per second) SUMMARY Discharge is measured in cumecs (cubic metres per second) Discharge of a river depends on many factors, including type of precipitation, drainage density, channel shape and slope and land use.

SUMMARY River discharge can increase dramatically during rain storms and during snow melt. River flow can be analysed in a storm hydrograph.

Factors influencing hydrographs

These influencing factors will: Change throughout the rivers course Remember! These influencing factors will: Influence each other Change throughout the rivers course

Why does a river flood? This river flooded after the second rainstorm, when the discharge reached 70 cumecs, although the first rainstorm had more precipitation. Why?

You would need to mention the following points when explaining this hydrograph. At the time of the first rainstorm: rain water does not all fall directly into the river water soaks into the ground run-off takes time to reach the river water can be intercepted by vegetation water can be stored in soil and rock But by the second rainstorm: the soil and ground water stores may be full the ground is saturated run-off will be faster water is still reaching the river from the first rainstorm

Comparison of impermeable mountain catchment with permeable lowland catchment area

Comparison of an area before and after urbanisation Explain the difference in the hydrographs before and after urbanisation.