Question

What causes surface runoff to increase?

Answer 1

Cities, for one.

Explanation:

In places where there is a lot of paved ground, runoff has nowhere to soak in.

Water is a powerful force which always travels from the highest point to the lowest. If rain cannot soak into the ground where it falls, it will continue downhill to the lowest point in that drainage area (a river or the ocean).

Hard rains at higher elevations can overwhelm a watershed, causing a flash flood at the lower elevations of that watershed (drainage area).

Long, drawn-out rainfall at lower elevations can overwhelm the capacity of both the smaller tributaries of a river, and then overwhelm the major river the tributaries feed into. (Flooding of cities in the Mississippi river basin, for example.)

Building on flood plains of rivers and putting up levees of dirt or concrete to protect those houses also increases runoff, since the water in that river (which would normally spread out onto the flood plain at times of heavy flow) now cannot. At times of hard rainfall (either there or farther upstream), the river can overtop the levees or break through them.

Desert landscapes do not soak up as much water as one might think.

A long, hard rainfall will result in the least amount of runoff if it falls on forests or grasslands that have no human development.

Connie