Glossary

10. Glossary#

A list of terms used in the QES course.

Ablation#: The loss of ice or snow from a glacier or ice sheet, by melting, sublimation or the calving of icebergs.
Abstraction#: The removal of water from groundwater or surface water, e.g. by pumping from wells.
Accumulation#: The addition of mass to a glacier or ice sheet, mainly by snowfall.
Albedo#: The fraction of incoming solar radiation that a surface reflects, from 0 (perfectly absorbing) to 1 (perfectly reflecting). Snow and sea ice have an albedo of roughly 0.8, compared to less than 0.1 for open ocean.
Aquiclude#: Sometimes referred to as an aquifuge; a rock with very low hydraulic conductivity that acts as an impermeable barrier to groundwater flow.
Aquifer#: A body of permeable rock or sediment that can store and transmit significant quantities of water.
Aquitard#: A rock with low values of hydraulic conductivity, but that still allows some flow of groundwater through it.
Artesian Well#: A well that flows without pumping because the hydraulic head in the confined aquifer that feeds it is greater than that at ground level.
Average Linear Velocity#
Seepage Velocity#: The average speed at which water moves through the pores of a rock, \(v = q/n_e\), where \(q\) is the specific discharge and \(n_e\) is the effective porosity. It is always greater than the specific discharge.
Brine Rejection#: The expulsion of salt from growing sea ice, which makes the surrounding seawater colder, saltier and denser. It is an important driver of the global overturning circulation.
Buttressing#: The resistance to flow that an ice shelf exerts on the grounded ice upstream of it, when the shelf is held back by contact with fjord walls, islands or other pinning points.
Cone of Depression#: The cone-shaped drawdown of the water table or potentiometric surface around a pumping well.
Confined Aquifer#: An aquifer that is sealed by impermeable rock above and below.
Confining Layer#: A layer of low permeability (an aquitard or aquiclude) that bounds a confined aquifer.
Darcy’s Law#: The law that the flow rate of water through a porous medium is proportional to the hydraulic gradient and the cross-sectional area, \(Q = -KA\dv{h}{l}\), where \(K\) is the hydraulic conductivity.
DIC#: Dissolved Inorganic Carbon - the sum of \(CO_2\), \(H_2CO_3\), \(HCO_3^-\) and \(CO_3^{2-}\) in water.
Discharge#: The volume of water flowing past a particular point per unit time (e.g. in a river). In groundwater systems, discharge also refers to water leaving the groundwater system, e.g. into a river, lake, the ocean, or at a spring.
Drawdown#: The lowering of the water table or potentiometric surface caused by pumping.
Elevation Head#: The height in m above the reference datum, which is usually mean sea level in groundwater systems.
Equipotential Line#: A line joining points of equal hydraulic head. In an isotropic aquifer, groundwater flow lines cross equipotential lines at right angles.
Eutectic#: The lowest temperature at which a mixture can remain liquid, and the composition at which this happens. For salt (NaCl) and water the eutectic is about \(-21\ ^\circ\mathrm{C}\) at about 23 wt% salt; below it, both ice and salt crystals form.
Eutrophication#: The process by which a body of water becomes enriched in dissolved nutrients (such as phosphates) that stimulate the growth of aquatic plant life, usually resulting in the depletion of dissolved oxygen.
Evapotranspiration#: The combined loss of water to the atmosphere by evaporation from surfaces and transpiration from plants.
Grounding Line#: The boundary at which ice flowing from a grounded ice sheet or glacier begins to float, becoming an ice shelf.
Groundwater Depletion#: The long-term decline in the amount of groundwater stored in an aquifer, because abstraction exceeds recharge.
Hydraulic Conductivity#: A combination of the physical properties of a fluid and the permeability of the medium it is flowing through which describes how quickly the fluid can move through it, termed \(K\). This is a key property of hydrogeologic systems which is highly variable (12 orders of magnitude!) depending on rock type.
Hydraulic Gradient#: The difference in hydraulic head per unit distance, \(\dv{h}{l}\).
Hydraulic Head#: The total mechanical energy per unit weight of water at a point, expressed as a height (in m) above a reference datum (usually mean sea level). It is the sum of the elevation head and the pressure head, \(h = z + \frac{P}{\rho g}\) (there is also a kinetic or velocity head, but this is negligible in groundwater systems). Groundwater flows from high to low hydraulic head.
Ice Divide#: The line of highest elevation on an ice sheet, from which ice flows away in opposite directions.
Ice Shelf#: A thick, floating platform of ice formed where a grounded ice sheet or glacier flows out onto the ocean.
Ice-Albedo Feedback#: A positive feedback in which a reduction in ice cover lowers the albedo, so that more solar radiation is absorbed, causing further warming and further loss of ice.
Infiltration#: The downward entry of water into soil.
Laminar Flow#: Smooth flow in which the fluid moves in parallel layers with no mixing between them. Flow through porous media is laminar at low Reynolds numbers, and this is the regime in which Darcy’s law holds. Compare turbulent flow.
Latent Heat#: The energy absorbed or released when a material changes phase at constant temperature, e.g. \(334\ \mathrm{kJ~kg^{-1}}\) to melt ice.
Liquidus#: The line on a phase diagram giving the temperature at which a liquid of a given composition begins to freeze.
Marine Ice Sheet Instability#: The potential for runaway retreat of the grounding line of an ice sheet that rests on a bed that deepens inland (a retrograde slope).
Ocean Acidification#: A reduction in ocean pH caused by the dissolution of \(\mathrm{CO_2}\) into water.
Percolation#: The downward movement of water through soil.
Permeability#: The physical characteristics of a porous medium that allow water to flow through it. Related to porosity, but not identical - a porous rock is not necessarily permeable.
Porosity#: The total sum of all void spaces in a rock. Often expressed as a percentage, or a number between 0 and 1. Not all these spaces may be connected, however, so for water flow, the effective porosity is the amount of void space that can hold and transmit water.
Potentiometric Surface#
Piezometric Surface#: The level to which water would rise in a well that taps an aquifer; a surface showing the hydraulic head across the aquifer. In an unconfined aquifer it coincides with the water table.
Pressure Head#: The pressure of the water at a point divided by \(\rho g\), expressed in m. It is the height of the column of water that the pressure at that point would support.
Pumping Test#: A field test in which water is pumped from a well at a known rate while the drawdown is measured in nearby observation wells, used to determine the hydraulic conductivity and transmissivity of an aquifer.
Recharge#: The downward movement of water from the soil to the water table.
Residence Time#: The average time that a given substance remains in a reservoir.
Reynolds Number#: The ratio of inertial forces to viscous forces within a fluid. Fluids with a large Reynolds Number are dominated by inertial forces, and fluids with a small Reynolds Number are dominated by viscous drag.
Saline Intrusion#: The movement of saline groundwater into a freshwater aquifer, usually caused by abstraction near the coast or by sea-level rise.
Snow Line#: The elevation on a glacier or ice sheet above which there is net accumulation of snow over a year, and below which there is net ablation. Also called the equilibrium line.
Solubility Pump#: The movement of carbon around the ocean by physical-chemical processes. Surface water cools as it travels to higher latitude, allowing it to store more C and DIC. This downwells into the deep ocean, where it remains until that water is upwelled. The warming of upwelling water releases the stored C back into the atmosphere.
Specific Discharge#
Darcy Velocity#: The flow rate through a system divided by its cross sectional area, \(q = Q/A\).
Steady State#: A condition in which the amount of something stored in a system does not change with time, because the inputs balance the outputs.
Stefan Number#: The ratio of the latent heat released on solidification to the sensible heat associated with a temperature change \(\Delta T\), \(\mathcal{S} = L / c_p \Delta T\). When \(\mathcal{S} \gg 1\) the growth of a solid is limited by how quickly the latent heat can be removed.
Storativity#: The volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer.
Thermal Diffusivity#: A measure of how quickly temperature changes spread through a material, \(\kappa = k / \rho c_p\), in \(\mathrm{m^2~s^{-1}}\).
Total Alkalinity#: A conservative quantity that can be used to track the equilibrium state of weak acids in water. Calculated as the sum of the negative ions of weak acids, minus \(\mathrm{[H^+]}\), this does not change as acid-base equilibria shift with temperature because it includes both the acid and base component of the system.
Transmissivity#: The ability of an aquifer to transmit groundwater throughout its entire saturated thickness, equal to the hydraulic conductivity multiplied by the saturated thickness, \(T = Kb\).
Turbulent Flow#: Chaotic flow with eddies and mixing, which occurs at high Reynolds numbers. In porous media, the onset of non-linear and turbulent flow means Darcy’s law no longer holds. Compare laminar flow.
Unconfined Aquifer#: An aquifer where the water table defines the upper water limit.
Water Footprint#: The total amount of water it takes to manufacture something.
Water Table#: The upper surface of groundwater, or the level below which an aquifer is permanently saturated with water.