APES 1.7-1.8: Hydrologic Cycle and Energy Flow

Water cycle

The water cycle is the continuous movement and phase-changing of water among reservoirs on, above, and below Earth's surface. It is a biogeochemical cycle involving liquid water, ice, and water vapor.

Why does the total amount of water on Earth remain approximately constant during the water cycle?

Water is transferred among reservoirs and changes phase, but it is generally neither created nor destroyed. Thus, Earth's total water mass is nearly constant even though the amounts stored in oceans, ice, freshwater, and the atmosphere vary.

Major reservoirs of Earth's water

The major reservoirs include oceans, ice caps and glaciers, groundwater, lakes and rivers, soil moisture, the atmosphere, and living organisms. Water continually moves among these reservoirs through physical processes.

Why is the ocean central to global water cycling?

The ocean contains about $97\%$ of Earth's water, receives about $78\%$ of global precipitation, and supplies approximately $86\%$ of global evaporation. It is therefore the dominant source and storage reservoir for water in the cycle.

What fraction of Earth's freshwater is stored in ice caps, glaciers, and permanent snow?

Ice caps, glaciers, and permanent snow contain about $1.7\%$ of Earth's total water but approximately $68.7\%$ of its freshwater. Thus, frozen reservoirs are disproportionately important to freshwater storage.

What are the main processes that move water between reservoirs?

The main fluxes are evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow. These processes move water among reservoirs and may involve changes between liquid, solid, and vapor.

What powers the water cycle, and what drives much of water's downward movement?

Solar energy powers evaporation and other upward movements of water vapor. Gravity drives precipitation, surface runoff, river flow, percolation, and much subsurface flow.

Evaporation

Evaporation is the endothermic phase change $\mathrm{H_2O(l) \rightarrow H_2O(g)}$. It primarily absorbs energy from solar radiation and removes the highest-energy molecules from a liquid.

How do evaporation and transpiration differ?

Evaporation is the conversion of liquid water at surfaces into water vapor. Transpiration is the release of water vapor from plants; evaporation and transpiration together are called evapotranspiration.

What happens to energy when liquid water evaporates?

Evaporation absorbs heat, specifically the latent heat of vaporization. The energy increases the potential energy of intermolecular separation rather than simply increasing the temperature of the vapor.

Condensation

Condensation is the conversion of water vapor into liquid droplets, represented by $\mathrm{H_2O(g) \rightarrow H_2O(l)}$. It is exothermic and releases latent heat to the surroundings.

Why does condensation release energy?

As water molecules come closer together during condensation, intermolecular attractions form or strengthen. The system releases potential energy as heat, making condensation the reverse energetics of evaporation.

Sublimation in the water cycle

Sublimation is the direct phase change from solid ice to water vapor, $\mathrm{H_2O(s) \rightarrow H_2O(g)}$, without passing through the liquid phase. It is an endothermic process.

Precipitation

Precipitation is condensed water that falls from the atmosphere to Earth's surface. It can occur as rain, snow, sleet, hail, graupel, or fog drip.

How are clouds formed in the water cycle?

As moist air rises, pressure and temperature generally decrease. Water vapor cools and condenses into microscopic liquid droplets or ice particles; a sufficiently concentrated region of these particles forms a cloud.

Fog

Fog is condensation of water vapor into liquid droplets near Earth's surface. Chemically, it is the same phase change as cloud formation but occurs close to ground level.

Why does atmospheric cooling favor condensation?

Cooling lowers the saturation vapor pressure of water and reduces the amount of vapor the air can hold at equilibrium. Once the air becomes saturated, excess water vapor condenses into liquid droplets or solid particles.

Clausius–Clapeyron relationship as applied to the water cycle

The saturation vapor pressure of water in the atmosphere increases by approximately $7\%$ for each $1\ ^\circ\mathrm{C}$ increase in temperature. A warmer atmosphere can therefore hold more water vapor, affecting evaporation and precipitation.

Advection

Advection is the transport of water or water vapor through the atmosphere. It allows moisture evaporated over oceans to travel long distances and precipitate over land.

What is the approximate global annual volume of evaporation and precipitation?

Approximately $505{,}000\ \mathrm{km^3}$ of water evaporates and a similar amount precipitates globally each year. About $434{,}000\ \mathrm{km^3}$ of evaporation and $398{,}000\ \mathrm{km^3}$ of precipitation occur over the oceans.

Why can precipitation exceed evapotranspiration over land while evaporation exceeds precipitation over oceans?

Atmospheric transport moves water vapor from oceans to land. Consequently, oceans have a net loss of water to the atmosphere, while land receives more precipitation than it returns through evapotranspiration; runoff and groundwater flow return the excess to oceans.

Infiltration versus percolation

Infiltration is the movement of water from the land surface into soil. Percolation is the subsequent downward movement of water through soil and rock, largely driven by gravity.

Why does liquid water flow downhill in the water cycle?

Gravity provides the driving force for surface runoff, river flow, percolation, and much subsurface flow. Pressure gradients caused by gravity can also move groundwater toward lower elevations and discharge points.

Runoff

Runoff is water moving across land or through channels toward lower elevations and ultimately toward bodies of water. It may enter rivers, infiltrate soil, evaporate, collect in lakes, or be diverted for human use.

Subsurface flow

Subsurface flow is the movement of water below Earth's surface through the unsaturated vadose zone and saturated aquifers. Groundwater may emerge at springs, discharge to rivers or oceans, or be extracted by pumping.

How does soil moisture participate in the water cycle?

Infiltrated water can remain in soil as soil moisture, where it may evaporate, be taken up by plants and transpired, flow into streams, or percolate downward to recharge groundwater. Soil moisture has a relatively short average residence time of about $1$ to $2$ months.

Residence time of a water reservoir

Residence time is the average time a water molecule remains in a reservoir. Under approximately steady-state conditions, it can be estimated using $\text{residence time} = \frac{\text{reservoir volume}}{\text{rate of inflow or outflow}}$.

Which water-cycle reservoir has the shortest average residence time listed in the source?

The atmosphere has an average residence time of about $9$ days. This short time reflects the rapid cycling of atmospheric water vapor into clouds and precipitation.

Compare the average residence times of oceans, glaciers, and seasonal snow.

Ocean water averages about $3{,}200$ years, glacier water about $20$ to $100$ years, and seasonal snow about $2$ to $6$ months. These differences reflect the rates at which each reservoir exchanges water with the rest of the cycle.

Groundwater residence time

Groundwater can remain in aquifers for years to thousands of years because it generally moves and is replenished slowly. Deep groundwater may have an average residence time of about $10{,}000$ years.

How can phase changes in water transfer heat around Earth?

Evaporation absorbs heat at Earth's surface, while condensation releases that latent heat in the atmosphere. This process helps transport energy from the tropics toward higher latitudes, along with ocean circulation.

Why is evaporation considered a purification process?

During evaporation, water molecules enter the gas phase while most dissolved salts and nonvolatile particles remain in the liquid. Therefore, evaporation can separate water from many dissolved impurities, although it does not remove volatile contaminants.

How does the water cycle transport and redistribute dissolved substances?

Runoff and groundwater flow carry dissolved ions and suspended particles from land to rivers, lakes, and oceans. These flows transport minerals and contribute to the salinity of seawater.

How does runoff contribute to ocean salinity?

Weathering and erosion dissolve minerals and salts from rocks and soils. Runoff and groundwater transport these dissolved ions to the oceans, where they accumulate and contribute to seawater salinity.

How does the water cycle shape Earth's surface?

Moving water causes weathering and erosion and transports sediment. When the flow slows, sediment can be deposited, reshaping river valleys, floodplains, coastlines, and other geological features.

How do living organisms connect the water cycle to other biogeochemical cycles?

Plants remove water from soil and release it through transpiration, while runoff and groundwater transport nutrients and dissolved substances. Water movement therefore links hydrologic cycling with carbon, nitrogen, phosphorus, and sediment cycling.

How can global warming intensify the water cycle?

Increased greenhouse-gas concentrations warm the atmosphere, increasing evaporation and the atmosphere's capacity to contain water vapor. This can produce stronger precipitation events while also contributing to greater drying, drought risk, and changes in soil moisture.

What does water-cycle intensification mean?

Water-cycle intensification means that exchanges of water between Earth's surface, atmosphere, and reservoirs become stronger or more variable. Observed and projected effects include heavier precipitation, more frequent extreme weather, altered rainfall timing, and increased drought in some regions.

How does deforestation affect the local water cycle?

Deforestation generally reduces transpiration, soil moisture, evaporation, and local rainfall or snowfall. Removing vegetation can also alter infiltration and increase the fraction of water leaving as runoff.

How does urbanization alter runoff and infiltration?

Buildings, roads, and other impervious surfaces prevent water from entering soil, reducing infiltration and increasing surface runoff. Soil compaction has a similar effect and can increase flood risk while reducing groundwater recharge.

How can dams alter local hydrologic conditions?

Dams change the timing and rate of natural water flow and can modify downstream sediment transport and water quality. They may also contribute to habitat loss for aquatic species.

How can groundwater extraction disrupt the water cycle?

Pumping groundwater faster than natural recharge depletes aquifers. This can reduce groundwater discharge to streams and ecosystems and threaten long-term freshwater availability.

Why can leakage from sewage pipes affect groundwater and streams?

Leaking sewage pipes can artificially increase groundwater recharge and stream baseflow. They can also introduce contaminants into groundwater.

How does the water cycle contribute to eutrophication?

Fertilizer nutrients, especially phosphorus and nitrogen compounds, can be transported from agricultural land by runoff or groundwater. Their accumulation in water bodies can promote excessive biological growth and oxygen depletion.

How can nitrate runoff create a coastal dead zone?

Nitrate from fertilizers can enter rivers and reach coastal waters, stimulating algal growth. Decomposition of the resulting biomass consumes dissolved oxygen, producing hypoxic conditions known as a dead zone.

What is the long-term geological loss pathway for water from Earth?

In the upper atmosphere, light atoms—especially hydrogen—can reach the exosphere and escape into space. This slow atmospheric escape produces a small net loss of water over geologic time.