How does hillslope gradient influence runoff velocity and lag time?

How does hillslope gradient influence runoff velocity and lag time?

• A. Steeper slopes increase runoff velocity and decrease lag time; gentler slopes slow flows and increase lag time.
• B. Slopes do not affect runoff velocity; lag time is constant across slopes.
• C. Gentler slopes increase runoff velocity and decrease lag time.
• D. Steeper slopes slow flows and increase lag time.

Answer: (A)

The main idea being tested is how the steepness of a hillslope controls how fast water moves over the surface and how long it takes for that water to reach a downstream outlet. On steeper slopes, gravity pulls the water more strongly along the surface, overcoming friction more effectively. This makes overland flow move faster, so runoff velocity increases and the water reaches the outlet sooner, shortening the lag time between rainfall and peak discharge. On gentler slopes, the gravitational driving force along the slope is smaller and friction has a greater relative influence. Water moves more slowly, gives more time for infiltration and temporary storage in small depressions, and the runoff peak is delayed. That lengthens the lag time. So, steeper slopes lead to higher runoff velocity and shorter lag time, while gentler slopes slow the flow and increase lag time.

The main idea being tested is how the steepness of a hillslope controls how fast water moves over the surface and how long it takes for that water to reach a downstream outlet. On steeper slopes, gravity pulls the water more strongly along the surface, overcoming friction more effectively. This makes overland flow move faster, so runoff velocity increases and the water reaches the outlet sooner, shortening the lag time between rainfall and peak discharge.

On gentler slopes, the gravitational driving force along the slope is smaller and friction has a greater relative influence. Water moves more slowly, gives more time for infiltration and temporary storage in small depressions, and the runoff peak is delayed. That lengthens the lag time.

So, steeper slopes lead to higher runoff velocity and shorter lag time, while gentler slopes slow the flow and increase lag time.