Factors influencing storm hydrographs

Overview

How the geology of a basin affects its storm hydrograph and drainage density.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How does relief affect a storm hydrograph?

   Model answer

   Steep relief shortens lag time and raises peak discharge. On steep slopes water moves downhill quickly under gravity as overland flow and has less time in contact with the ground to infiltrate, so it reaches the channel sooner and in greater quantity, giving a steep rising limb and a sharp peak. Gentle relief slows overland flow, allows more time for infiltration, and produces a longer lag time and lower peak.

2. 2

   What difference does basin size make to the hydrograph?

   Model answer

   A large basin intercepts more precipitation overall, so its total and peak discharge are greater in absolute terms, but its lag time is longer because water falling in the distant headwaters has much further to travel to the gauging point. A small basin has a shorter lag time, since all the water is close to the channel, and a lower absolute peak discharge, but it responds more sharply because the whole basin can be covered by a single storm.

3. 3

   Why does the shape of a drainage basin matter?

   Model answer

   A circular basin has all its points roughly the same distance from the gauging station, so water from every part of it arrives at about the same time. The flows combine, giving a short lag time and a high, sharp peak. An elongated basin delivers water from its nearest parts first and from its distant parts much later, so the arrival is staggered. This produces a longer lag time and a lower, flatter, more drawn-out peak.

4. 4

   How does soil type influence the shape of a storm hydrograph?

   Model answer

   Soil controls infiltration capacity. A coarse, sandy, well-structured soil has large pore spaces and a high infiltration capacity, so rain soaks in and travels slowly as throughflow, lengthening lag time and lowering the peak. A fine clay soil has tiny pores, a low infiltration capacity and swells when wet, so rain cannot enter fast enough and runs off the surface, shortening lag time and raising the peak. Thin soils fill and saturate quickly, which also produces a flashy response.

5. 5

   Explain how the geology of a basin affects its storm hydrograph.

   Model answer

   Permeable rock, whether porous like chalk and sandstone or well-jointed like carboniferous limestone, allows water to percolate down into groundwater storage. This removes water from the rapid surface routes, producing a long lag time, a low peak and a high, well-sustained baseflow. Impermeable rock such as granite, shale or clay prevents percolation, so water is forced to travel over the surface or through the thin soil, giving a short lag time, a high peak and a low baseflow that falls away quickly between storms.

6. 6

   What effect does land use have on a storm hydrograph?

   Model answer

   Urban land use makes the hydrograph flashy. Tarmac, concrete and roofs are impermeable, gutters and storm drains deliver water to the channel in minutes, and rivers are often straightened and lined to move water on quickly, so lag time is short and peak discharge is high. Forested land use does the opposite: the canopy intercepts rain, transpiration removes water, the litter layer slows overland flow and roots open the soil and improve infiltration, so lag time is long and the peak is low. Bare arable land, compacted by machinery and left without cover in winter, behaves more like an urban surface than a wooded one.

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