Basin Slope

The slope of a basin affects the amount and the timing of runoff.

As the slope of the land increases several factors come into play. The first is that water contact to the surface is no longer perpendicular. With the land sloping, gravity no longer pulls the water directly into the ground, so more water is likely to become surface runoff.

Another factor is the movement of water across the land surface. As the ground becomes increasingly steep, water will move faster and will have less time in contact with the ground surface, reducing the time during which it could infiltrate.

Incised channel after the Cerro Grande Fire near Los Alamos, NM

Also important is the amount of sediment carried by flowing water. Erosion occurs when water removes sediment from the ground surface. Although it is dependent on soil type and ground cover, erosion generally increases with increasing slope. With higher amounts of sediment in the water, the surface pores in the soil which the water might otherwise enter, can become plugged, reducing infiltration.

In general, the steeper the hill slope and the steeper the drainage channels, the quicker the flow response and the higher the peak discharges.