Hydrographs

Question 1

Unit Hydrographs are a simple method of describing the catchment response to rainfall. List and comment on the fundamental assumptions of this method

Answer 1

1) Effective rainfall is uniformly distributed over the whole catchment and duration
- For large catchments, rainfall is likely to be variable, so analysis needs to be done on subcatchements

2) The rainfall runoff process is stationary (no change in time).
- Channel & catchment conditions change over time, so analysis may need to be redone after a period of time, and the unit hydrograph modified to reflect the change.

3) The direct runoff process is linear in superposition and proportionality
i. e if rainfall is doubled, the runoff is also doubled.
i. e. Any runoff from a later time can be added to the previous runoff
- These two principles describe the rainfall as linear. This is an over simplification since catchment processes are complex and non-linear

Question 2

What is the definition of a hydrograph?

Answer 2

A hydrograph is a graph showing changes in the discharge of a river over a period of time. It represents how a catchment responds to rainfall

Question 3

Basic Hydrograph terms

Answer 3

1) River runoff
River runoff is surface water in rivers. When rain falls on the earth, it starts to move according to the law of gravity.  There are four ways that rainfall contributes to river runoff: 
1) overland surface runoff; 
2) interflow (subsurface runoff), 
3) base flow from groundwater; and 
4) rainfall on river channel.

2) Infiltration excess runoff
When rainfall intensity is greater than ground surface infiltration rate, surface runoff is generated. Infiltration excess flow usually occurs during high intensity rainfall events. If the subsoil infiltration rate is lower than the top soil infiltration rate, infiltration excess runoff can occur below the ground surface and this will result in interflow (i.e., subsurface runoff).

3) Saturation excess runoff
When rainfall intensity is less than soil infiltration rate, prolonged rainfall will saturate the soil and no more water could be held. As a result, excess water is released from the soil into groundwater and river channels.

4) Direct runoff
Overland surface runoff and interflow travel much faster than groundwater, hence they are combined into a term ‘direct runoff’ in hydrology.

5) Hydrograph components
A hydrograph has a rising limb, a recession limb and a peak flow. The division between direct runoff and base flow is transitional and not very clear