Glossary
Anisotropic medium
A medium having properties that vary depending on the direction of measurement. An example would be hydraulic conductivity that may be different in a vertical and lateral direction due to layering and alignment of the soil grains.
Antecedent moisture conditions
Soil-moisture content preceding a given storm.
Baseflow
Stream discharge derived from groundwater seepage; a time-based definition relating to runoff sustained without precipitation, largely composed of groundwater; outflow from extensive groundwater aquifers, which are recharged by water percolating down through the soil mantle to the water table.
Bulk density
The dry density of the soil;the mass of the solid mineral and organic components of soil divided by the total volume.
Capillary fringe
The unsaturated zone containing water in direct hydraulic contact with the water table, and held above the water table by capillary forces resulting in a negative pressure potential in the soil matrix.
Contributing area
The area upslope of any point on a watershed or topographic surface; the area of a catchment contributing to storm runoff, dimensioned as [L2].
Darcy’s Law
An experimentally-derived relationship stating that rate of fluid flow through a permeable medium is directly proportional to the hydraulic gradient and to the hydraulic conductivity.It is valid only for flow velocities within the laminar range.Being originally stated for saturated flow, it was extended by Richards in 1931 to embrace unsaturated flow.
Depression storage
The volume of water, forming part of surface detention, which is contained in small natural depression in the land surface during or shortly after rainfall, none of which runs off.
Distributed hydrologic model
A hydrologic model that allows for spatial variability of model parameters and inputs.The spatial resolution of distributed parameters and inputs depends on available physical data.
Drainable porosity
The difference between moisture content at saturation and at field capacity; quantifies the porosity of the soil that gravity drains within a time frame of a few days. Quantitatively, it is defined as porosity minus the field capacity moisture content corresponding to a pressure head between -100 and -500 cm.
Elevation head
The elevation above an arbitrary horizontal datum, also called gravitational head.
Field Capacity
The moisture content remaining in soil after a few days of gravity drainage.Quantitatively, it is defined as the moisture content corresponding to a pressure head between -100 and -500 cm..
Hydraulic conductivity
A coefficient of proportionality describing the rate at which water can move through a porous medium under a hydraulic gradient.A function of both the porous medium and fluid properties. Hydraulic conductivity depends upon the pore geometry determined by soil texture and structure and the fluid viscosity and density. The hydraulic conductivity is at its maximum when the soil is saturated and decreases with decreasing water content or increasing water tension.
Hydraulic head
The equivalent height of a liquid column corresponding to a given pressure; usually called simply “head” of the fluid.Hydraulic head is measured with respect to an arbitrary horizontal datum and is the sum of pressure head and elevation head.
Hydraulic gradient
The gradient of hydraulic head that induces flow of water, expressed as head drop per unit distance in the direction of flow.
Hydrograph
A graph or table of stream discharge versus time.
Hyetograph
A graph or table of water input (rainfall or snowfall) or runoff generated versus time.
Hysteresis
A phenomenon that occurs during the draining and wetting of soils whereby the relationship between soil moisture content and negative pressure head depends upon the history of drying and wetting.
Infiltration capacity, fc
The maximum rate at which a given soil can absorb falling rain (or melting snow), when it is in a specified condition.
Infiltration excess overland flow
Overland flow that occurs when the infiltration capacity drops below the water input rate from rainfall or snowmelt.Also known as Hortonian overland flow and saturation from above.
Infiltration rate, f
The volume rate of the passage of water through the surface of the soil, via pores or small opening, into the soil.
Interception
Water retained in the vegetation canopy for some period, however short, after rain has struck the vegetative material above the soil surface.
Interflow
An intermediate component of runoff, between overland flow and groundwater flow.Interflow is made up of subsurface flow, which never reaches the water table but instead returns to form surface runoff.
Intrinsic permeability
A property of a porous medium which determines the ease with which a fluid will move through the matrix. Intrinsic permeability depends on the pore geometry determined by soil texture and structure. It does not contain any fluid properties so is more general than hydraulic conductivity because it applies to the flow of all fluids through the porous medium.It is equal to Nd2 where N is a pore shape factor and d is a pore size scale measure (such as the average pore diameter or grain size).
Kinetic energy
The energy associated with the motion of a substance.For fluid flow kinetic energy is proportional to the square of the velocity.
Lateral moisture flux [L2/T]
The flow rate in a lateral or horizontal direction through a soil profile.This is normalized by the corresponding width so is expressed as volume per unit width per time. This is integrated over the full depth of the soil profile conducting flow laterally.
Lateral flow capacity [L2/T]
The capacity of a soil profile to conduct flow in a lateral or horizontal direction. When lateral flow is driven by the hydraulic gradient this maximum capacity is generally slope or topographic gradient times the transmissivity.
Lumped hydrologic model
A hydrologic model with spatially averaged parameters and inputs. Lumped model parameters often must be developed through optimization or calibration rather than calculating directly from field measurements or existing data.
Manometer
A device for measuring pressure, one end of a U-shaped tube containing liquid is open to the atmosphere, and the other end is connected to a system of unknown pressure.
Overland flow
Part of streamflow which originates from rain which fails to infiltrate the soil surface at any point as it flows over the land surface to stream channels.
Partial area concept
Storm runoff generated by only a part of the surface of a catchment.
Piezometer
A tube used to measure the head of fluids of constant density.The hydraulic head at any “point” in a ground-water or porous medium flow can be measure as the height above the selected arbitrary datum to which water rises in a tube connecting the “point” to the atmosphere.
Ponding time
Time to the first occurrence of ponding from the beginning of a surface water input event (such as a rainstorm).
Porosity
The volume of voids or pore spaces in a soil or rock expressed as a fraction of the bulk volume.
Potential energy
Gravitational potential energy, is the energy of an object resulting from its position in a gravitational field.
Precipitation excess
The surface water input that does not infiltrate and ponds on the surface contributing to depression storage or overland flow runoff.
Pressure head
The equivalent height of a liquid column corresponding to a given pressure. Pressure head is measured relative to the height at which pressure is measured and is pressure divided by the weight density of water.
Return flow
Infiltrated water which returns to the land surface after having flowed for some distance in the subsurface.
Runoff
Overland and subsurface flow components that contribute to the quickflow in a stream, leaving a watershed within a time scale of about a day following surface water input. Runoff is also used to refer to all water leaving a watershed, the sum of quick flow, base flow and groundwater outflow.
Saturation excess overland flow
Surface runoff occurring when the soil is saturated. This is also called the Dunne mechanism or saturation from below and occurs most commonly in humid and vegetated areas with shallow water tables, where infiltration capacities of the soil surface are high relative to normal rainfall intensities. Saturation excess overland flow is most common on near-channel wetlands.
Soil particle density
The weighted average density of the mineral grains making up the
soil; mass of the soil divided by the volume of mineral grains.
Soil texture
The classification of a soil based on the distribution of particle sizes within the soil.Clay is defined as particles with diameter less than 0.002 mm.Silt has a particle diameter range from 0.002 mm to 0.05 mm and sand has particle diameter range from 0.05 to 2 mm.The USDA soil texture triangle assigns names, such as sandy loam, silty clay loam, sandy clay based upon the relative fractions of particles in these size ranges.
Soil water diffusivity
A property that quantifies the flux of water per unit gradient of water content.This is a quantity that appears in Richard's equation describing the flow of water in unsaturated soil.
Sorptivity
A parameter expressing the macroscopic balance between capillary forces pulling water in to a soil and hydraulic conductivity that limits the flow rate. This parameter appears in Philip's solution to Richards equation for unsaturated flow and is the proportionality constant in the expression indicating that in the absence of other forces the quantity of water absorbed is proportional to the square root of time.
Specific catchment area
Contributing area per unit contour width; dimensioned as [L].
Specific discharge
The volume rate of flow per unit area through a porous medium.
Specific moisture capacity
A parameter representing the rate of change of soil moisture content with respect to pressure head, that appears in Richard's equation
Subsurface runoff
The movement of subsurface storm water within the soil layers to stream channels at a rate more rapid than the usual groundwater flow.
Subsurface stormflow
The part of streamflow which derives from the lateral subsurface flow of water which discharges into the stream channel so quickly as to become part of the stream flow associated directly with a given rainstorm.
Surface detention
That portion of rainwater, other than depression storage, which remains in temporary storage on the land surface as it moves downslope by overland flow and either runs off, is evaporated or is infiltrated after the rain ends.
Surface runoff
The stream outflow from a region.
Tensiometer
A device used to directly measure the capillary tension of soil moisture under field conditions. A tensiometer consists of a hollow metal tube, of which one end is closed off by a cup of porous ceramic material and the other end is fitted with a removable airtight seal. A manometer, vacuum gage, or pressure transducer is attached to the end of the tube. The tube is completely filled with water and inserted into the soil to the depth of the measurement. Since the water in the tube is initially at a pressure somewhat above atmospheric, there is a pressure-induced flow through the porous cup into the soil that continues until the tension inside the tube equals that in the soil. When this equilibrium is reached, the manometer or gage gives the tension in the tube and in a roughly spherical region immediately surrounding the cup.
Throughfall
The portion of rainfall which penetrates the vegetation and reaches the surface through spaces in the vegetative canopy and as drip from leaves, twigs and stems.Throughfall is precipitation that is not retained as interception.
Throughflow
Downslope flow of water occurring physically within the soil profile, usually under unsaturated conditions except close to flowing streams, occurring where permeability decreases with depth.
Topmodel
An approach for predicting saturation overland flow based on the idea that
the location and size of zones of surface saturation that generate saturation overland flow can be predicted based on the distributed topographic attributes and soil properties of a catchment.
Topographic wetness index
The ration of specific catchment area to slope or its natural logarithm, denoted ln(a/S), or ln(a/tan b).The topographic wetness index quantifies the dependence of soil moisture deficit on catchment area and slope.The probability distribution of the topographic index can be used to describe the hydrologic response of watersheds.
Transmissivity
The integral over soil depth of hydraulic conductivity. If the soil is relatively homogenous and flow paths are horizontal, transmissivity may be defined as the depth times the hydraulic conductivity.
Variable source area
That portion of a watershed contributing to saturation excess overland flow.
Viscosity
Used in fluid flow to characterize the degree of internal friction in the fluid. This internal friction or viscous force is associated with the resistance of two adjacent layers of the fluid against moving relative to each other.
Volumetric soil moisture content
The ratio of water volume to soil volume.
