Variations in River Discharge

• Short term fluctuations in discharge e.g. increased discharge following heavy rain or snow melt.
• Medium-term variations e.g. seasonal variations such as less discharge when interception is higher in a deciduous forest in summer.
• Long term changes in discharge e.g. as a result of climate change or a change in land use.

Spatial variations (changes from place to place):

• Rivers in different geographical locations have different patterns of discharge. The discharge is affected by the climate of the region and many other factors.

Temporal Variations

Rivers have seasonal fluctuations in their discharge. They are mist pronounced in climates with a wet and dry season e.g. countries with Mediterranean/ monsoon climate and where glaciers feed streams.

In Britain there is a maximum winter discharge and a minimum summer discharge which reflect low precipitation totals and high rates of evapotranspiration in the summer.

Short term fluctuations of channel discharge are related to individual rainstorms or snowmelt.

Spatial Variations

Due to the interplay of a variety of factors especially climate and vegetation there are big differences in river regimes for different regions.

        Tropical rainforests with equatorial climates have a large river discharge. A product of high rainfall (over 2000 mm per year). Despite high rates of evapotranspiration due to the high year round temperatures (over 28 C) and large amounts of vegetation in the forest, the river discharge is high.

        Sub tropical hot deserts – annual rainfall is less than 250 mm. There are high rates of evapotranspiration, resulting in minimal river discharge. Channel runoff is short-lived (as flash floods) after heavy rainstorms and covers short distances as it quickly infiltrates the sandy soil.

        Tundra Regions –North Canada and Russia – Annual precipitation is similar to hot deserts (250mm p/a) and most discharge is intense runoff in the early summer as the inter snow is melting. Discharge is high as the temperatures are low at this time and evapotranspiration is minimal as there is so little vegetation. Some areas have permanently frozen ground (permafrost) which restricts infiltration. There are shot term fluctuations diurnally. Discharges are low overnight and early morning as little melting takes place then. Discharges increase through the day as the ice melts and it peaks in the mid afternoon.