River Discharge

Question 1

Explain River Discharge.

What is River Discharge affected by?

Answer 1

River discharge is the volume of water (in cubic metres, m3) that flows in a river per second. It’s measured in cubic metres per second (m3/s) but it usually shortened to cumecs.

River discharge is affected by:

1. **Precipitation **- the more precipitation, the higher the discharge.
2. **Hot weather **- the higher the temperature, the lower the discharge because the rate of evaporation is higher.
3. Removal of water from the river (abstraction) - also reduces discharge.

Question 2

1. State and explain the features of a Storm Hydrograph.
2. Briefly explain a Flashy Hydrograph.

Answer 2

Features:

• **Peak discharge **- this is the highest point on the graph, when the river discharge it as its greatest.
• **Lag time **- this is the delay between peak rainfall and peak discharge. This delay happens because it takes time for the rainwater to flow into the river. A shorter lag time can increase peak discharge because more water reaches the river during a shorter period of time.
• **Rising limb **-this is the part of the graph up to peak discharge. The river discharge increases as rainwater flows into the river.
• **Falling limb **- this is the part of the graph after peak discharge. Discharge is decreasing because less water is flowing into the river. A shallow falling limb shows water is flowing in from stores long after it’s stopped raining.

The exact shape of the hydrograph varies with each river drainage basin and each individual storm event. A basin with rapid runoff and not much storage capacity gives a hydrograph with a short lag time and high peak discharge. This is called a “flashy” hydrograph - the graph has steep, roughly symmetrical rising and falling limbs.

Question 3

The Storm Hydrograph is affected by both Physical and Human Factors.

Explain the Physical factors.

Explain the Human factors.

Answer 3

Physical Factors:

• The **antecedent rainfall **- heavy rain falling on a soil which is saturated from a previous period of wet weather will produce a steep rising limb.
• **Rock type **- affects lag time and peak discharge.
  
  • Impermeable rocks don’t store water or let water flow through them. This reduces infiltration and increases surface runoff, reducing lag time. Peak discharge also increases are more water reaches the river in a shorter period of time.
• **Vegetation **- affects lag time and peak discharge.
  
  • Vegetation intercepts precipitation and slows its movement to the river channel, increasing lag time. Interception is highest when there’s lots of vegetation and deciduous trees have their leaves.
  • The more vegetation there is in a basin, the more water is lost (through transpiration and evaporation directly from the vegetation) before it reaches the river channel, reducing peak discharge.
• **Precipitation **affects peak discharge and lag time.
  
  • Intense storms will generate more precipitation and so greater peak discharges than light rain showers.
  • The type of precipitation also affects lag time - e.g. snow that’s fallen in a winter storm can melt (meltwater) and flow into the river in spring, giving a very long lag time.
• **Drainage basin characteristics **- affect lag time and peak discharge.
  
  • Size of drainage basin - a small drainage basin tends to respond more rapidly to a storm than a larger one, so the lag time is shorter.
  • Shape of drainage basin - rainfall reaches the river more quickly from a round basin than from an elongated basin, thus achieving a shorter lag time and an increase in peak discharge.

Human factors:

• In urban areas, much of the soil is covered with man-made impermeable materials like concrete. Water can’t infiltrate into the soil, which increases surface runoff, so water flows more quickly into the river. This makes the lag time short and increases peak discharge.
• Man-made drainage systems affect the hydrograph in a similar way. Water flows down drains into the river before it can evaporate or infiltrate into the soil, causing a shorter lag time and increased peak discharge.