When applied to the adjusted perennial streams for

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Unformatted text preview: applied to the adjusted perennial streams for which it was defined, dominant discharge may have geomorphic significance, but when applied to intermittent- and ephemeral-stream channels formed by reduced rates of precipitation, runoff, and streamflow, the concept is of questionable value and may be inappropriate. Drainage basin is an area of land surface, upslope from a specified channel site to topographic divides separating the basin from adjacent drainage basins, over which water that results from precipitation moves and converges through a system of channels to (and past) the specified channel site. Drainage density is the ratio of the combined stream lengths of a drainage basin to the basin area (km km-2); drainage density is indicative of the facility by which excess rainfall moves from an upland surface. High values of drainage density are indicative of high transfer efficiency and therefore of high peak discharge. Drainage network is the system of channels and other paths of conveyance for water and sediment moving downslope through a drainage basin. Drift is a term with different meanings depending on whether the topic is caves, coastal studies, geophysics, hydraulics, or surface-water hydrology; as applied to glacial geology and geomorphology, drift is rock debris of any size deposited by ice of a glacier or sediment-laden meltwater flowing from a glacier. Related terms are outwash and till. Dry wash is a channel in which streamflow occurs inconsistently or infrequently and, except during periods of streamflow, is directly underlain by unsaturated alluvium; dry washes are most common in arid and semiarid regions and typically have a rectangular to steeply sided trapezoidal cross section, banks a meter or more in height formed of fine-grained, poorly consolidated over-bank sediment, and a nearly flat, sandy bed. Synonyms are ephemeralstream channel, arroyo (northern Mexico and southwestern United States), and wadi (southwestern Asia, Arabian peninsula, and northern Africa) 17 Duff is fresh to moderately decomposed organic matter that veneers the surface in forested areas; duff is derived from fallen trees, failed shrubs and saplings of the understory, remains of herbaceous vegetation, seeds, especially of conifers, and, to a minor degree, scat and remains of animals. Duff absorbs and stores moisture and thereby increases evaporation at the expense of runoff and ground-water recharge; because runoff mostly flows through duff, it slows the runoff rate and minimizes the potential of runoff to entrain and transport sediment downslope and to rills and gullies. Dune, including dune field, is an accumulation, or concentration, by depositional processes of water or wind as a low, small-scale mound, ridge, or, more commonly, a complex (field, or zone) of mounds and ridges, of loose, well sorted granular material (generally sand) that, if active, may be bare or, if inactive, partially to fully vegetated; dunes are subject to translocation, without a basic loss of scale or structure, by the action of streamflow, waves, or wind. Duricrust is an accumulation of mineral precipitates at or near the land surface of generally semiarid areas; a calcium-rich duricrust is a calcrete and a silica-rich duricrust is termed a duripan. E Ecohydrology is that branch of natural science that describes interactions between ecosystems and hydrologic processes by considering how those processes affect the distributions, functions, and dynamics of biota, and by identifying feedbacks from biota to the hydrologic cycle. Ecology, a composite science, is the study of organisms relative to the environment in which they live. Ecosystem describes the complex of biotic populations, the biophysical (environmental) constraints on the biotic populations, and the ability of the complex to function as an ecological unit within a specified area or part of a watershed. Ecosystem services are the production of renewable natural resources through processes yielding clean water, soil, vegetation, and wildlife. Ecosystem-services water is that portion of the total water resource that is essential for ecosystem function and as such has intrinsic value for physical and biotic processes; in contrast, commodity water is that portion of the accessible water resource that is viewed by humans, and economists in particular, to be an article of commerce and thus to have a monetary value as an economic good. Edaphology, a composite science, when viewed as a division or sub-discipline of soil science, is the study of interactions of soils with those organisms dependent on the soils, and thus has a long history of application to agriculture; if viewed as a division of ecology, edaphology is the study of the controls that soil characteristics place on plant ecology and plant growth. 18 Effective precipitation is that portion of rainfall, generally measured in millimeters (mm), resulting in runoff and sustaining soil moisture available for plant growth; owing to elevated rates of evapotranspiration, effective precipitation in arid and semiarid regions is generally lower than it is in high-latitude areas with the same measured precipitation but lower evapotranspiration. El Niňo-S...
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