Factors Affecting River Discharge: The Storm Hydrograph.

Hydrograph = a graph of river discharge against time.

Storm hydrograph = graph of the discharge of a river up to and following a storm.

Discharge of a river = the volume of water that flows past a point in the river’s course per second.

Storm hydrographs can be described in terms of:

• Rising limb (the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing).
• Peak flow.
• Lag time (the time difference between the peak precipitation and the peak discharge. A long lag time indicates that it’s taking a long time for precipitation to enter the river).
• Recession limb (the opposite of the falling limb showing that the discharge is falling).
• Time taken to return to base flow.
• Factors affecting the shape of a hydrograph:
• Drainage basin size – large basins have high peak discharges (as they catch more precipitation) and longer lag times because the water takes longer to reach the rivers.
• Drainage basin slope gradient – steep basins have high peak discharges and short lag time because the water travels faster downhill.
• Drainage basin drainage density – those with high drainage density (containing lots of rivers and streams) have short lag time and steep falling limb because water drains quickly.
• Rock type – a river surrounded by impermeable rocks will have a short lag time. This is because water cannot percolate through impermeable rocks, so it travels by overland flow (faster than groundflow, interflow and throughflow), therefore reducing the lag time. A river surrounded by non-porous rocks will have a high peak discharge because the rock cannot store water, so more water enters the river instead of the drainage basin.
• Soil type – unconsolidated soilds allow water to infiltrate, acting as a store in a drainage basin. Soil also causes water to travel slowly via throughflow, reducing peak discharge and increasing lag time. Fine clay soils do not allow water to infiltrate, forcing overland flow and therefore reducing lag time.
• Storm intensity – higher storm intensity is likely to mean more rainwater and therefore a higher discharge. A winter storm will result in an increase in the river’s discharge when the snow melts, but this often won’t be for a long time, so the lag time will be large.
• Climate – waterlogged ground will result in reduced lag time due to inability for infiltration and forced overland flow. Ground that has been very hot and dry or freezing cold will be hard and not allow infiltration, resulting in overland flow, reducing lag time and increasing peak discharge.
• Vegetation – vegetation causes interception, increasing lag time. Water may evaporate from the vegetation, reducing peak discharge.
• Urbanisation – the growth of towns and cities leads to impermeable surfaces causing more surface run off, reducing the amount of water being stored, leading to an increased peak discharge and reduced lag time. A dense network of smooth drains is often built, transporting the water straight to rivers, leading to the same effects.

A river’s hydrograph may be described as “flashy” when water is transferred to a river quickly (with a steep rising limb and a high peak discharge). This often occurs in a circular drainage basin and can lead to flooding.