Which factor would most likely cause a flood hydrograph with a higher peak and shorter lag time?

Which factor would most likely cause a flood hydrograph with a higher peak and shorter lag time?

• A. A basin with impermeable surfaces and rapid surface runoff
• B. A highly vegetated, permeable catchment
• C. A broad, circular basin with uniform permeability
• D. A region with slow infiltration and deep groundwater storage

Answer: (A)

The key idea is how easily rainfall is converted into surface runoff and how quickly that runoff is delivered to the stream. When the land surface is largely impermeable, rain can’t infiltrate or be stored in the soil. Almost all of it becomes surface runoff and quickly concentrates into channels, so the river discharge responds rapidly. This produces a flood hydrograph with a steep rise, a high peak, and a short lag time between rainfall and the peak discharge. If the catchment is highly vegetated and permeable, water infiltrates and is stored in the soil and groundwater, which slows the response. The hydrograph rises more gradually, has a lower peak, and the lag time is longer. A broad basin with uniform permeability and slow infiltration or deep groundwater storage also tends to delay and dampen the response, not produce the sharp, high peak associated with rapid surface runoff. So, the factor most likely to yield a higher peak and shorter lag time is impermeable surfaces that generate rapid surface runoff.

The key idea is how easily rainfall is converted into surface runoff and how quickly that runoff is delivered to the stream. When the land surface is largely impermeable, rain can’t infiltrate or be stored in the soil. Almost all of it becomes surface runoff and quickly concentrates into channels, so the river discharge responds rapidly. This produces a flood hydrograph with a steep rise, a high peak, and a short lag time between rainfall and the peak discharge.

If the catchment is highly vegetated and permeable, water infiltrates and is stored in the soil and groundwater, which slows the response. The hydrograph rises more gradually, has a lower peak, and the lag time is longer.

A broad basin with uniform permeability and slow infiltration or deep groundwater storage also tends to delay and dampen the response, not produce the sharp, high peak associated with rapid surface runoff.

So, the factor most likely to yield a higher peak and shorter lag time is impermeable surfaces that generate rapid surface runoff.