Figure 5.8. The fat of precipitation at the land surface determines whether water infiltrates or runs off the surface

An important function of soil is to absorb water at the land surface, and either store it for use by plants or slowly release it to groundwater through gravitational flow (figure 5.8). When rainfall hits the ground, most water will infiltrate the soil; but some may run off the surface, and some may stand in ruts or depressions before infiltrating or evaporating. The maximum amount of rainwater that can enter a soil in a given time, called infiltration capacity, is influenced by the soil type, structure, and moisture content at the start of the rain.

Early in a storm, water usually enters a soil readily, as it is literally sucked into the dry ground. As the soil wets up during a continuing intense storm, water entry into the soil is reduced and a portion of rainfall begins to run downhill over the surface to a nearby stream or wetland. The ability of a soil to maintain high infiltration rates, even when saturated, is related to the sizes of its pores. Since sandy and gravelly soils have more large pores than do fine loams and clays, they maintain better infiltration during a storm. But soil texture is also important in governing the number of pores and their sizes: When finer-textured soils have strong aggregates due to good management, they can also maintain high infiltration rates.

When rainfall exceeds a soil’s infiltration capacity, runoff is produced. Rainfall or snowmelt on frozen ground generally poses even greater runoff concerns, as pores are blocked with ice. Runoff happens more readily with poorly managed soils, because they lack strong aggregates that hold together against the force of raindrops and moving water and, therefore, have few large pores open to the surface to quickly conduct water downward. Such runoff can initiate erosion, with losses of nutrients and agrochemicals as well as sediment.