Factors Affecting Discharge & Storm Hydrograph

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Nature of the Storm

Intensity and Length of Precipitation: More intense rain causes soil to become saturated, so more overland flow occurs. Similar situation can occur if rain is prolonged and soils store becomes filled producing shorter lag time and higher peak discharge of flashy hydrograph

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Nature of Drainage Basin

Basin Size: Smaller basins, precipitation has less to distance to travel before it reaches the mouth, so there will be a shorter lag time and hydrograph will be shorter and steeper. Opposite for larger basin

Basin Shape: Shorter, rounded basins are more likely to be flashy as the water from the basin tends to arrive more quickly at the mouth. In a thinner basin, water that falls near the source has much further to travel so it will produce a flatter hydrograph

Basin Relief: Steeper basin, under the influence of gravity, water makes its way to the mouth more quickly giving a flashy hydrograph. A less steep basin means water takes longer to arrive at the mouth, giving a flat hydrograph

Soil: Clay soils have smaller pore spaces so don't allow for much infiltration, as a result overland flow is more likely and water reaches channel quicker giving a flashy hydrograph. Opposite for sandy soils

Geology: Basalt is impermeable and less infiltration occurs, flashy hydrograph. Chalk is more permeable so there is more infiltration and a flatter hydrograph

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Land Use

Flashy Hydrograph

  • In urban areas, impermeable surfaces increase runoff and drains and sewers are designed to take surface water to river quickly
  • In plough fields where vegetation is removed for agri. purposes, it leaves soil bare so reduces interception and so water gets into channel more quickly

Flatter Hydrograph

  • Afforestation increases interception thus slowing down speed at which water reaches channel. Increased interception results in more evaporation, so total amount of water reaching channel is reduced, lowering peak discharge
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