Surface Properties

Water running off of a parking lot.

Soil surface properties can be the most important influence on runoff in some areas. If water is unable to infiltrate the soil surface, the characteristics of the soil profile below become unimportant. Impermeable surface materials, soil compaction, deforestation, and fire are factors that affect infiltration into the soil profile.

Cross-section showing effect of urban surface cover vs. natural forest on runoff.

Covering ground surfaces with impermeable materials such as concrete and asphalt means less water can infiltrate. In addition, as soil becomes more compacted, the infiltration, percolation, and soil moisture storage all decrease. In general, human activity results in less infiltration, less storage, and greater surface runoff.

Cross-section showing effect of deforestation on runoff, groundwater and sediment transport.

Deforestation can have an impact on infiltration and runoff. Without plant matter on the surface, water will move more quickly. This means water will have less time to infiltrate the soil. In addition, runoff from deforested areas is likely to contain more sediment. The sediment load can take up space in the stream channel that would otherwise be available for increased flow. Thus, deforested areas are more likely to result in enhanced runoff with large sediment loading into the stream channels.

Channels draining burned areas have zones of erosion and zones of deposition. The peeled bark indicates the highest level of water and debris during a flash flood. Sediment is coarse sand and gravel. The view is downstream and the blue backpack is about 1 meter tall.  Photo by John A. Moody

Fires are a type of deforestation that can have more severe consequences on subsequent runoff. In addition to the typical deforestation problems, fires can alter the soil surface and make it temporarily hydrophobic, that is, unable to absorb water. This is especially noted in pine forest areas because the oils and resins from the trees vaporize and get infused into the soil. This creates a hydrophobic layer at or near the surface. The severity of runoff and sediment loading can often be seen by the scour on the tree trunks and the alluvial deposits left after the floodwater passes.