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Factors affecting storm hydrographs - Coggle Diagram
Factors affecting storm hydrographs
Climate
Temperature, evaporation, transpiration and evapotranspiration
On the other hand, warm air can hold more water so the potential for high peak flows in hot areas is raised.
Increased vegetation cover intercepts more rainfall and may return a proportion of it through transpiration, thereby reducing the amount of water reaching stream channels
Not only does temperature affect the type of precipitation, it also affects the evaporation rate (higher temperatures lead to more evaporation and so less water getting into rivers).
The greater the return through evapotranspiration, the less water if able to reach stream channels, and therefore the peak of the hydrograph is reduced.
Antecedent moisture
If it has been raining previously and the ground is saturated or nearly saturated, rainfall will quickly produce overland flow, a high peak flow and short time lag.
Precipitation type and intensity
Low-intensity rainfall is likely to infiltrate into the soil and percolate slowly into the rock, thereby increasing the time lag and reducing the peak flow.
Precipitation that falls as snow sits on the ground until it melts.
Highly intensive rainfall is likely to produce overland flow, a steep rising limb and high peak flow
Sudden, rapid melting can cause flooding and lead to high rates of overland flow, and high peak flows.
Drainage Basin Characteristics
Rock type
Impermeable rocks such as granite and clay produce greater peak flows with a more flashy response.
In contrast, more permeable rocks such as chalk and limestone produce storm hydrographs with a much lower peak flow (if at all) and with a much delayed/less flashy response (greater time lag).
Slopes
Steeper slopes create more overland flow, shorter time lags and higher peak flows.
Porosity and impermeability of rocks and soils
Urban areas contain large areas of impermeable surfaces.
In contrast, rocks such as chalk and gravel are permeable and allow water to infiltrate and percolate. This reduces the peak flow and increases the time lag.
Impermeable surfaces cause more water to flow overland. This causes greater peak flows.
Sandy soils allow water to infiltrate, whereas clay is much more impermeable and causes water to pass overland.
Vegetation type
Forest vegetation intercepts more rainfall, especially in summer, and so reduces the amount of overland flow and peak flow and increases time lag.
In winter, deciduous trees lose their leaves and so intercept less.
Drainage density
Basins with a high drainage density, such as urban basins with a network of sewers and drains, respond very quickly.
Networks with a low drainage density have a very long time lag.
Land use
If more drainage channels are built (sewers, ditches, drains), the water is carried to rivers very quickly.
This means that peak flows are increased and time lags reduced.
Land uses that create impermeable surfaces, or reduce vegetation cover, reduce interception and increase overland flow.
Drainage basin size and shape
In contrast, the Mississippi River is over 3700km long - it takes much longer for the lower part of the river to respond to an event that might occur in the upper course of the river.
Circular basins respond more quickly than linear basins, where the response is more drawn out.
Smaller drainage basins respond more quickly o rainfall conditions. For example, the Boscastle (UK) floods of 2004 drained an area of less than 15km2.
This meant that the peak of the flood occurred soon after the peak of the storm.