When precipitation reaches the land surface, it initiates the terrestrial phase of the hydrologic cycle. Water molecules immediately engage in a competition for movement, simultaneously being pulled downward by gravity into the soil and flowing across the landscape. This moment begins several processes that determine where the water will be stored, how it will travel, and how quickly it will return to the atmosphere. The fate of any drop is determined by the interplay of weather, geology, and land cover.
The Initial Split: Infiltration and Surface Runoff
The immediate fate of precipitation is decided by the balance between infiltration and surface runoff. Infiltration is the process where water sinks into the soil through pore spaces, while surface runoff is the flow of water across the land surface. These two pathways are mutually exclusive, and the rate of infiltration determines the amount of runoff.
The maximum rate at which soil can absorb water is known as the infiltration capacity. This capacity is heavily influenced by soil characteristics; for instance, sandy soils generally have a higher capacity than dense clay soils. The moisture already present in the soil, known as antecedent moisture, also plays a large part, as saturated soil cannot absorb much more water, leading to increased runoff.
If the rate of precipitation exceeds the soil’s infiltration capacity, the excess water becomes surface runoff. The slope of the land also causes water to flow more quickly, reducing the time available for infiltration and resulting in a greater proportion of runoff, especially in steep terrain.
In developed areas, impervious surfaces such as roads and parking lots entirely block infiltration, forcing nearly all precipitation to become urban runoff. This alteration significantly increases the volume and speed of surface flow, contributing to urban flooding and the rapid transport of pollutants. Conversely, vegetation increases infiltration by protecting the soil from compaction and creating natural pathways for water through root systems.
Below the Surface: Groundwater Storage and Movement
Water that successfully infiltrates the soil continues its downward journey through percolation. It first passes through the unsaturated zone (vadose zone), where soil pores contain both air and water. The water eventually reaches the water table, the upper boundary of the saturated zone where all rock and soil pores are completely filled with water.
The water stored in this saturated zone is known as groundwater. Large, permeable geologic formations that can yield a usable quantity of water are called aquifers. Groundwater movement is a slow process, typically measured in feet per day, moving through the tiny, interconnected spaces in the rock and sediment. This subsurface flow is driven by gravity, moving from areas of high hydraulic pressure to areas of lower pressure.
Groundwater provides a steady supply of water to the surface through natural discharge points. This discharge occurs at springs, seeps, or by feeding the base flow of streams and rivers, sustaining their flow even during dry periods. This reservoir represents a significant long-term storage component of the hydrologic cycle, with residence times ranging from days to many centuries.
Surface Water Bodies: Streams, Rivers, and Lakes
The water that flows over the surface as runoff is channeled by gravity, collecting in increasingly larger pathways. Initially, water may flow in thin sheets, but it quickly gathers into small rills and gullies, which feed into streams and creeks. These streams merge to form rivers, creating a network that drains a specific geographical area known as a watershed or drainage basin.
The function of a river system is to collect and transport water, sediment, and dissolved materials toward a discharge point. Surface runoff provides a quick input of water to these channels, causing water levels to rise rapidly after precipitation. Rivers are dynamic features, with flow constantly fluctuating based on recent rainfall and input from groundwater base flow.
Lakes and reservoirs act as temporary storage pools within this surface network, slowing the water’s journey toward the ocean. These bodies are fed by surface flow and direct precipitation, and they lose water through outflow to a downstream river. The surface water system acts as the visible transport mechanism for water moving across the land.
Completing the Cycle: Evaporation and Transpiration
Regardless of where water is stored—in the soil, a river, or a lake—it is consistently subject to the final terrestrial process: the return to the atmosphere. This is accomplished through the dual processes of evaporation and transpiration, collectively termed evapotranspiration.
Evaporation is the physical process where liquid water absorbs energy, primarily from the sun, and changes into water vapor, returning directly to the atmosphere. This occurs from all exposed liquid water surfaces, including lakes, rivers, and the top layer of the soil. Warmer temperatures, increased wind speed, and lower humidity accelerate the rate of evaporation.
Transpiration is the biological release of water vapor from plants through tiny pores on their leaves called stomata. Plants draw up liquid water through their roots and release it to the air as part of their cooling and nutrient transport processes. While evaporation accounts for the majority of moisture returned to the atmosphere globally, transpiration contributes a significant portion.