The continuous movement of water on, above, and below the Earth’s surface is described by the water cycle. When precipitation reaches the ground, infiltration occurs as water enters the soil. Percolation is the subsequent downward movement of this water, traveling below the root zone to recharge deep groundwater stores or aquifers. If rainfall exceeds the soil’s absorption capacity or the ground is saturated, the water cannot percolate deeply. This excess water follows alternative pathways: flowing across the land surface, returning to the atmosphere as vapor, or moving laterally underground before resurfacing.
Surface Runoff and Overland Flow
The most immediate alternative path for non-percolating water is surface runoff, also known as overland flow. Runoff occurs when the volume of water arriving at the surface exceeds the soil’s absorption rate, or when the soil is completely saturated. This excess water moves horizontally, usually downslope, and is a major component of the water cycle.
Land characteristics significantly influence the amount of runoff generated. Impervious surfaces, such as concrete and rooftops, prevent infiltration almost entirely, leading to immediate and substantial runoff. This rapid flow is often directed into engineered storm drains, increasing the likelihood of urban flooding. Runoff flowing across the land is the primary agent of soil erosion, moving large amounts of sediment and reshaping landscapes.
Surface runoff acts as a nonpoint source of pollution by collecting various contaminants as it moves. Water flowing over roads and agricultural fields picks up petroleum, pesticides, and fertilizers. These pollutants are transported directly into surface water bodies like rivers and lakes. This leads to downstream impacts such as nutrient pollution and water quality degradation. The initial surge of runoff during a storm event, often termed the “first flush,” carries the highest concentration of pollutants.
Water Vapor Return to the Atmosphere
Water that does not percolate returns to the atmosphere through evapotranspiration, a combined process converting liquid water back into vapor. This involves two mechanisms: evaporation and transpiration. Evaporation is the direct movement of water from liquid to gas, occurring from open water bodies, wet soil, and plant leaves.
Transpiration involves plants taking up water through their roots and releasing it as vapor through tiny pores in their leaves called stomata. A large percentage of the water plants draw up is released back into the air this way. Both evaporation and transpiration are driven by solar energy, which provides the heat necessary for the phase change.
Environmental conditions such as temperature, wind speed, and humidity greatly influence the rate of evapotranspiration. Higher air temperatures increase the atmosphere’s capacity to hold moisture. Wind helps transport the water vapor away from the surface, accelerating the rate of loss. Evapotranspiration is responsible for approximately two-thirds of the precipitation that falls over land. This atmospheric return flow drives cloud formation and the continuation of the global water cycle.
Lateral Movement and Temporary Storage
Water that infiltrates the soil but fails to reach the deep groundwater table can be held in temporary storage. This storage primarily occurs in the soil moisture zone, the unsaturated layer above the water table. Water held here is available for plant uptake or is subject to evaporation from the soil surface.
When the soil is saturated, or downward movement is blocked by a less permeable layer, the water begins to move sideways. This horizontal flow within the upper soil layers is known as interflow or throughflow. Interflow is an intermediate component of runoff, positioned between surface overland flow and deep groundwater flow. It is noticeable on hillslopes where gravity guides the water downslope along restrictive layers.
This subsurface lateral movement eventually discharges into streams, rivers, and wetlands, contributing to streamflow. When interflow emerges at the surface, typically at the base of a slope or near a stream bank, it is referred to as “return flow.” These lateral pathways are significant contributors to stream discharge, especially during and immediately following intense storm events.