Water and Energy Balance

Evapotranspiration

The sum of surface evaporation and transpiration.

Transpiration

The loss of water from plant leaves.

Water vapor feedback

Greenhouse effect provided by water vapor; occurs when the atmosphere warms and increases its water vapor content.

Net radiation

The balance between the inputs and outputs of shortwave and longwave radiation.

Direct radiation

Radiation that comes directly from the sun without scattering or reradiation by the atmosphere or objects in the environment.

Diffuse radiation

Shortwave radiation that is scattered by particles and gases in the atmosphere.

Reflected radiation

Shortwave radiation that is reflected from clouds and objects in the landscape.

Albedo

Fraction of the incident shortwave radiation reflected from a surface.

Snow/Ice albedo feedback

Atmospheric warming caused by warming-induced decrease in albedo due to earlier snowmelt.

Emissivity

Coefficient that describes the max rate at which a body emits radiation, relative to a perfect (black body) radiator.

Ground heat flux

Heat transferred from the surface into the soil.

Latent heat of vaporization

Energy required to change a gram of a substance from a liquid to a vapor without change in temperature.

Turbulence

Irregular velocities of air or water movement that can transport heat and materials more readily than by diffusion.

Bulk air

Air above the canopy that isn’t strongly influenced by the canopy.

Convective turbulence

Caused by the increased buoyancy of surface air due to heat transfer from the surface.

Mechanical turbulence

Caused by the uneven slowing of air by a rough surface.

Roughness element

Obstacle to air flow (i.e. a tree) that creates mechanical turbulence.

Bowen ratio

Ratio of sensible to latent heat flux.

Specific heat

Energy required to warm a gram of a substance by 1°C.

Vapor pressure

Partial pressure exerted by water molecules in the air.

Vapor pressure deficit

Difference in actual vapor pressure and the vapor pressure in air of the same temperature and pressure that is saturated with water vapor; loosely used to describe the difference in vapor pressure in air immediately adjacent to an evaporating surface and the bulk atmosphere, although strictly speaking the air masses are at different temperatures.

Conductance

Flux per unit driving force (e.g., concentration gradient); inverse of resistance.

Closed basin lakes

Lakes in dry climates that have such high evaporation rates that outflow seldom occurs.

Blue water

Liquid water in rivers, lakes, reser- voirs, and groundwater aquifers that is poten- tially available to society.

Green water

Water that evaporates from the soil surface or is transpired by plants.

Phreatophytes

Deep-rooted plant that taps groundwater.

Canopy interception

Fraction of precipitation that does not reach the ground.

Throughfall

Water that drops from the canopy to the ground.

Stemflow

Water that flows down stems to the ground.

Water potential

Potential energy of water rela- tive to pure water at the soil surface.

Pressure potential

Component of water poten- tial generated by gravitational forces and by physiological processes of organisms.

Osmotic potential

Component of water potential due to the presence of substances dissolved in water.

Matric potential

Component of water potential caused by adsorption of water to surfaces; it is considered a component of pressure potential in some treatments

Hydraulic conductivity

Capacity of a given volume of a substance (such as soil) to con- duct water; this defines the relationship between discharge and the hydraulic gradient causing it.

Infiltration

Movement of water into the soil.

Macropores

Large pores between soil aggregates that allow rapid movement of water, roots, and soil animals

Field capacity

Water held by a soil after gravitational water has drained

Permanent wilting point

Water held by soil that cannot be extracted by plant uptake

Hydraulic lift

Upward movement of water through roots from deep moist soils to dry surface soils along a gradient in water potential

Cavitation

Breakage of water columns under tension in the xylem

Stomata

Pores in the leaf surface through which water and CO2 per unit driving force between the interior of a leaf and the atmosphere

Isohydric plants

Plants from moist sites that close their stomata at relatively high soil moisture before they experience large changes in plant water potential

Ansiohydric plant

Plants from dry sites that show little response of stomatal conductance to soil drying and therefore continue to photosynthesize and to absorb and lose water as the soil dries

Surface roughness

Vertical irregularities in the height of the canopy surface

Sublimation

Vaporization of a soild such as snow

Leaf area index

Projected (i.e. one side of a flat leaf) leaf area per unit ground area

Surface conductance

Potential of the leaf and soil surfaces in the ecosystem to lose water. Similar to stomatal conductance but applied at a canopy scale.

Aerodynamic conductance

Conductance of water vapor through a canopy from the vegetation or soil surface to the bulk atmosphere. Sometimes termed the boundary layer conductance of a canopy

Decoupling coefficient

Measure of the extent to which the canopy is decoupled from the bulk atmosphere

Base flow

Background stream flow from groundwater input in the absence of recent storm events.