Lake Ice and 
Snow Science 

BASIC CONCEPTS: Hydrological Cycle Definitions

There are some definitions of important elements of the hydrological cycle.

Condensation: The process of water changing from a vapor to a liquid. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. The water vapor then has a change of state into liquid or ice.

Evaporation: The process by which water changes from a liquid to a gas and is transferred from the earth’s surface to the atmosphere. Radiant energy from the sun heats water causes water molecules to become so active that some of them rise into the atmosphere as vapor. Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.

Groundwater: The underground water trapped between layers of rock or clay. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers , which empty into lakes, rivers and the oceans. The flow of groundwater is much slower than run-off with speeds usually measured in centimetres per day, metres per year or even centimetres per year.

Infiltration: The portion of the precipitation that reaches the Earth's surface and seeps into the ground. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints), the more infiltration occurs. Water that doesn't infiltrate the soil flows on the surface as runoff.

Percolation: The gravity flow of water within soil from the surface or surface groundwater into the groundwater table. In general, water flow in which gravitational forces predominate.

Precipitation: The primary mechanism for transporting water from the atmosphere to the surface of the earth. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air currents and falls from clouds as rain, sleet, snow, or hail. A proportion of atmospheric precipitation evaporates.

Surface Runoff: Precipitation that reaches the surface of the Earth but does not infiltrate the soil. Runoff can also come from melted snow and ice. When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.

Transpiration: The process by which water absorbed by plants, usually through the roots, is transferred as water vapor to the atmosphere from a single leaf, or the amount of water so transferred.

Transport: The movement of water through the atmosphere, specifically from over the oceans to over land. Most water is transported in the form of water vapour, which is actually the third most abundant gas in the atmosphere. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets.
Clouds are propelled from one place to another by either the jet stream, surface-based circulations like land and sea breezes or other mechanisms. However, a typical cloud 1 km thick contains only enough water for a millimetre of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater than this.

Water Table: The level below the land surface at which the subsurface material is fully saturated with water. The depth of the water table reflects the minimum level to which wells must be drilled for water extraction.

For more meteorlogical definitions click here.