Water is constantly moving through Earth's systems in the hydrologic cycle. From evaporation to precipitation, surface runoff to groundwater flow, water changes form and location as it cycles between the atmosphere, land, and oceans. Understanding these processes is the foundation for everything else in this unit on freshwater systems.
The hydrologic cycle has several linked components: evapotranspiration, condensation, precipitation, infiltration, runoff, and groundwater flow. Each process transfers water between reservoirs (oceans, atmosphere, ice, surface water, groundwater), and together they circulate water at both global and local scales.
Atmospheric Water Processes
Evaporation and Transpiration
Evaporation converts liquid water into water vapor. It happens primarily at the surface of oceans, lakes, and rivers, where solar energy provides the heat needed to break hydrogen bonds between water molecules.
Transpiration is the release of water vapor from plants through tiny pores called stomata on their leaves. Plants pull water from the soil through their roots and lose it to the atmosphere during gas exchange for photosynthesis.
Because evaporation and transpiration are difficult to measure separately in the field, scientists typically combine them into a single term: evapotranspiration. Together, they transfer roughly 505,000 km³ of water per year from Earth's surface into the atmosphere (about 434,000 km³ from ocean evaporation and 71,000 km³ net from land surfaces).
Four main factors control evapotranspiration rates:
- Temperature: warmer air holds more moisture, so evaporation increases with heat
- Humidity: drier air creates a steeper moisture gradient, pulling water vapor away from surfaces faster
- Wind speed: moving air carries away water vapor, maintaining that gradient
- Solar radiation: provides the energy that drives evaporation
Condensation and Precipitation
Condensation occurs when water vapor cools enough to change back into liquid water droplets, forming clouds or fog. Two conditions are needed:
- Air must cool to its dew point temperature, where relative humidity reaches 100% and the air is saturated.
- Condensation nuclei must be present. These are tiny particles (dust, sea salt, pollen, or even ice crystals) that give water molecules a surface to collect on.
Precipitation happens when water droplets or ice crystals in clouds grow heavy enough to fall. Forms include rain, snow, sleet, and hail, depending on temperature profiles in the atmosphere.
A useful number to remember: about 77% of global precipitation falls over the oceans, while roughly 23% falls over land. This matters because land surfaces depend heavily on atmospheric moisture transport from the oceans. Wind patterns carry evaporated ocean water inland, and without that moisture delivery, continental interiors would be far drier than they are.
Surface Water Processes
Infiltration and Runoff
When precipitation reaches the ground, it either soaks in or flows across the surface. These two fates are infiltration and runoff, and the balance between them shapes landscapes and water availability.
Infiltration is water entering the soil from the surface. Once in the soil, that water can:
- Be stored in soil pore spaces (soil moisture)
- Be taken up by plant roots
- Percolate deeper to recharge groundwater
Infiltration rates depend on soil texture (sandy soils infiltrate faster than clay because they have larger pore spaces), soil moisture content (already-wet soil absorbs less), vegetation cover (roots and organic matter create pathways for water to enter), and slope (water on steep surfaces has less time to soak in before gravity pulls it downhill).
Runoff is water flowing over the land surface toward streams, rivers, and lakes. It occurs in two main situations:
- Precipitation falls faster than the soil can absorb it. This is called infiltration-excess runoff (or Hortonian overland flow), and it's common during intense storms.
- The soil is already completely saturated, so any additional water has nowhere to go (saturation-excess runoff). This tends to happen in low-lying areas near streams or after prolonged wet periods.
Steeper slopes, impervious surfaces (pavement, rooftops), and intense rainfall all increase runoff. This is why urbanization tends to increase flood risk: replacing soil and vegetation with concrete dramatically reduces infiltration.
Water Balance
The water balance equation is a simple but powerful accounting tool. Over any area and time period, water inputs must equal water outputs plus any change in storage:
where is precipitation, is evapotranspiration, is runoff (including both surface and subsurface flow), and is the change in water storage (soil moisture, groundwater, snowpack, etc.).
If is positive, the area is gaining water. If negative, it's losing stored water. Over long time periods, tends toward zero for most regions, meaning inputs roughly equal outputs.
This equation is how hydrologists predict what happens when conditions change. For example, deforestation removes trees that would normally return water to the atmosphere through transpiration. With reduced, more water ends up as (runoff), increasing flood risk downstream while also reducing local humidity and rainfall recycling. You can apply the same logic to urbanization, climate shifts, or irrigation withdrawals.
Groundwater Processes
Groundwater Flow and Water Table
Water that percolates below the soil eventually reaches the saturated zone, where all pore spaces in rock or sediment are completely filled with water. The upper boundary of this zone is the water table. Above it sits the unsaturated zone (or vadose zone), where pore spaces contain a mix of air and water.
Groundwater moves through aquifers, which are layers of porous and permeable rock or sediment capable of storing and transmitting significant quantities of water. Two properties define how well an aquifer functions: porosity (the percentage of open pore space in the material) and permeability (how well those pore spaces are connected, allowing water to flow through). A material can be porous but not very permeable if the pores aren't well connected, like some clays.
There are two main aquifer types:
- Unconfined aquifers (also called water table aquifers): the water table forms their upper boundary, and they're recharged directly from above by infiltrating water. These are more vulnerable to surface contamination.
- Confined aquifers (also called artesian aquifers): sandwiched between layers of low-permeability material called aquitards (like clay or shale). Water in these aquifers can be under pressure, which is why some artesian wells flow without pumping.
Groundwater flow is driven by differences in hydraulic head (a measure of the total energy of water at a given point). Water flows from high hydraulic head to low hydraulic head. This relationship is described by Darcy's Law:
where is the volumetric flow rate, is hydraulic conductivity (how easily water moves through the material), is the cross-sectional area of flow, and is the hydraulic gradient. The negative sign indicates flow goes in the direction of decreasing head.
The depth to the water table varies by location and season, depending on the balance between recharge (water entering the aquifer from infiltration) and discharge (water leaving it). In humid regions, the water table roughly mirrors surface topography: higher under hills, lower near valleys. In arid regions, the water table can sit much deeper below the surface.
Groundwater returns to the surface through springs, seeps, and as baseflow to streams and rivers. Baseflow is why many streams keep flowing even during dry periods when there's no recent rainfall. Groundwater can also be extracted through wells for irrigation, drinking water, and industrial use. When extraction exceeds recharge over time, the water table drops, a problem known as groundwater depletion.