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permeability and porosity of the channel filling are thus reduced, and runoff is diverted to the
periphery of the channel fill, where new gullies occur only to be filled by more debris. In this
manner, an entire hillslope can become veneered with coarse rock debris. H
Habitat is the living space for one or more organisms; it is described by the combined
environmental parameters of biotic and abiotic factors.
Halophytes are plants that have the ability to survive despite the presence of large amounts of
salt or alkali in the soil water on which they depend.
Hard-water effect is the tendency of the dissolved inorganic carbon in many “hard-water” lakes
to be in disequilibrium with atmospheric CO2 owing to a short residence time. A result is that
radiocarbon in the water, carbonate minerals formed in the water, and organisms living in the
water, is deficient relative to that of the atmosphere, thereby yielding computed ages older
than the true ages. Similarly, streams that receive significant amounts of ground water with
substantial subsurface residence times contain dissolved carbon that also is deficient relative to
atmospheric radiocarbon, and radiocarbon dates derived from the water and its biota are
greater than the true ages.
Headcut, a type of knickpoint, is a vertical or near-vertical face, or drop, on the bed of a stream
channel that interrupts the channel gradient and, through processes of channel erosion,
progressively moves up-channel.
Herbs are plants whose stems develop very little wood but consist mostly of soft tissue, and
which generally die each year. Perennial herbs are those plants whose tops generally die each
year but whose roots survive for two or more years.
Histogram (history gram) is a bar graph, without open intervals separating bars along the
horizontal axis, for which the area or vertical height of each bar is proportional to the frequency
or relative frequency of the variable represented. Histograms typically are used by hydrologists
to illustrate particle-size distributions of sediment samples by representing the proportional
amount of sediment in a specified size class as the bar height; the summation of the areas or
heights of the bars must equal the total (100 percent) sample volume or weight.
Humus is a complex mixture of brown to dark brown components of soil organic matter, mostly
amorphous and colloidal substances, that are decomposed to the extent that the sources of the
material cannot be identified.
Hydraulic conductivity is a measure of the ease by which a fluid, generally water, will pass
through a porous medium, often soil or rock. 24 Hydraulic geometry describes, for a given cross section of a stream channel, the graphical
relations among plots of hydraulic characteristics (width, depth, velocity, gradient, roughness
coefficient, particle sizes) as simple power functions of river discharge; hydraulic-geometry
pertains to the water in a channel as opposed to the geometry of the channel. Hydraulicgeometry relations can be developed both for the at-a-station condition and the downstreamdirection condition.
Hydraulic gradient is the gradient (often termed slope) of the energy grade line – the line
representing the sum of kinetic and potential energy along the flow path; for uniform flow, the
hydraulic gradient and the slope of the water surface are equal. An analogous term of groundwater hydrology is potentiometric surface.
Hydraulic radius, R, of a stream channel is the ratio of its cross-sectional area, A, to its wetted
perimeter, WP: R = A/WP.
Hydric refers to an environment (habitat) that is characterized by abundant moisture.
Hydrograph is the graphical representation of a hydrologic variable, such as the stage of a
stream or the water level in a well, as a function of time; a hydrograph for runoff (streamflow)
is a graph of the time-rate distribution of flowing water passing a site on the landscape,
generally at a stream channel and often for a specific flow or runoff event.
Hydrologic budget is a quantitative accounting of the various components of water or stored in
drainage basins or watersheds; when the amount of water entering a system equals that
leaving or stored in the system, the hydrologic budget is assumed, over reasonably long time
periods, to be in balance (see water balance).
Hydrologic cycle is the cycle of water movement (in the liquid, solid, and vapor phases) from
the atmosphere to land, surface-water, and ground-water bodies, including movement among
land and water bodies, before returning to the atmosphere.
Hydrology, a composite science, is that part of the earth sciences that is concerned with the
origin, circulation, distribution, and properties of water; important elements of hydrology
include the measurement of fluxes of water (as streamflow, ground-water discharge, etc.) and
the manners by which the fluxes affect the landscape (erosion, plant growth, etc.).
Hydrometer is a bulb that indicates the specific gravity of a water-sediment mixture by the
height at which it floats above the mixture surface; the hydrometer is commonly used to
compute concentrations of fluvial sediment too fine to fall from suspension, and to determine
particle sizes of fine sediment in soi...
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