Physical - drainage basin characteristics
Larger drainage basins catch more precipitation so have a higher peak discharge. But smaller basins have a smaller lag time as there is a lot less water to drain.
Steep-sided drainage basins have shorter lag times than shallower basins - water flows more quickly increasing peak discharge.
Circular basins are more likely to have a flashy hydrograph than long narrow basins. This is because at watershed is at roughly the same distance away from the point of discharge. This means water reaches measurement point at the samt time increasing discharge.
Basins with a high drainage density drain quickly and have shorter lag times.
Physical - antecedent moisture
If the ground is already waterlogged then infiltration is reduced and surface runoff increases.
Surface runoff is much faster than through flow or base flow, so rainwater reaches the river much more quickly, reducing lag time.
Physical - rock type
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 as more water reaches the river in a shorter time period.
Areas of permeable rock and soil allow more infiltration and less surface runn off.
Physical - soil type
Sandy soils allow more infiltration.
Clay soils allow not much infiltration.
Low infiltration rates increase surface runn off, reducing lag time and increasing peak discharge.
Physical - vegitation
Vegitation intercepts precipitation and slows its movement to the river channel, increasing lag time.
Interception is at its highest when there's lots of vegiation and deciduous tree have their leaves.
The more vegitation there is in a basin, the more water there is lost through transpiration and evaporation before it reaches the river channel, reducing peak discharge.
Physical - precipitation
Intense storms will generate more preciptation and allow greater peak discharges than in lighter showers.
The type also effects the discharge and lag time - snowmelt in the spring takes a long time to flow into the river making lag time long, a long sloping rising limb and a low peak discharge.
Physical - climate
Hot, dry conditions and cold freezing conditions both result in hard ground.
This reduces in filtration and increases surface run off.
This reduces lag time and increases peak discharge.
High temperatures increase evapotranspiration, so less water reaches the river channel, therfore reduces river discharge.
Human - urban areas
In urban areas, much soil is covered by impermeable surfaces such as tarmac.
Water therefore can't infiltrate into the soil, which increases surface run off, so water flows more quickely into the river.
This makes lag time short and increaeses peak discharge.
Human - man made drainage systems
Water flows down drains into the river before it can evaporate or infiltrate into the soil.
This causes a shorter lag time and an increased peak discharge.