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Discharge behaviour

Gina River outlet: computed hydrograph (July 2003, 4th)

Gina River outlet: computed hydrograph (July 2003, 4th)
Image Credit: after Beck et al. 2004, 48

As shown in the hydrological cycle a river receives its water from three main components:

  • overland flow;
  • interflow and
  • base flow.

Additionally, a tiny percentage is falling as precipitation into the river.

Stream discharge varies from watershed to watershed according to the interactions of geospheres. Discharge may also fluctuate over time within one and the same watershed (Bradshaw and Weaver 1995). Discharge rises after heavy rainfalls. In the Gina river this occurs during the two rainy seasons. It is falling in drier weather - a seasonally fluctuation of discharge. Local flooding, in general, occurs when the supply of water exceeds the capacity of the river channel to carry it away within the river banks (Bradshaw and Weaver 1995).

The response of a streamflow to a rainfall is portrayed by hydrographs. A hydrograph shows the progression of the flood wave, whether the runoff is concentrated on a short period of time or nearly constant during the period of rainfall (Beck et al. 2004). This depends on the nature of the watershed. In a well vegetated watershed with permeable soils and bedrock and slopes which are gentle, much of the rain will be intercepted and can infiltrate into the soil. This delays the passage of water through the watershed and balancing its arrival at the river. The level of discharge rises slowly and has a moderate peak. Between rainfalls baseflow maintains a high average flow in the river.

In contrast, in a watershed where vegetation is clear-cut, the inclination of slopes high and soils and bedrock which are relatively impermeable like in the Gina River catchment, a high proportion of precipitation becomes overland flow and reaches the river quickly after a rainfall event. The discharge level rises sharply to a high peak and then falls almost as rapidly (Bradshaw and Weaver 1995). This is shown by the hydrograph of a sub-watershed of the Gina River catchment beside. The discharge behaviour, of course, determines also the processes proceeding within the river bed.