LABORATORY BLANK — An artificial sample, usually distilled water, introduced to a chemical analyzer to observe the response of the instrument to a sample that does not contain the material being measured. The blank can also detect any contamination occurring during laboratory processing of the sample.

LACUSTRINE — Pertaining to, produced by, or inhabiting a lake.

LACUSTRINE DEPOSITS — Stratified materials deposited in lake waters and later become exposed either by the lowering of the water level or by the elevation of the land.

LAGOON — (1) A shallow lake or pond, especially one connected with a larger body of water. (2) The area of water enclosed by a circular coral reef, or atoll. (3) An area of shallow salt water separated from the sea by sand dunes. (4) (Water Quality) Lagoons are scientifically constructed ponds in which sunlight, algae, and oxygen interact to restore water to a quality equal to effluent from a secondary treatment plant.

LAGOON SYSTEM — (Water Quality) A system of scientifically construction Lagoons or ponds in which sunlight, algae, and oxygen interact to restore water to a quality equal to effluent from a Secondary Treatment Plant.

LAKE — A considerable body of inland water or an expanded part of a river.

LAKES, SEEPAGE — Lakes whose ecology is determined primarily by ground water rather than surface water.

LAMINAR FLOW — A flow in which fluid moves smoothly in streamlines in parallel layers or sheets. The stream lines remain distinct and the flow directions at every point remain unchanged with time. It is characteristic of the movement of ground water. Contrasts with turbulent flow. Synonymous with Streamline Flow and Viscous Flow.

LAND — The entire complex of surface and near surface attributes of the solid portions of the surface of the earth, which are significant to man. Water bodies occurring within land masses are included in some land classification systems.

LAND APPLICATION — The reuse of reclaimed water or the utilization or disposal of effluents on, above, or into the surface of the ground through spray fields or other methods.

LAND BREEZE — The land-to-sea surface wind that occurs in coastal areas at night. It is caused by the rising of the air above the ocean, which is warmer than the land due to the rapid cooling of the land after sunset. Contrast with Sea Breeze.

LAND CAPABILITY — The suitability of land for use without permanent damage. Land capability, as ordinarily used in the United States, is an expression of the effect of physical land conditions, including climate, on the total suitability for use, without damage, for crops that require regular tillage, for grazing, for woodland, and for wildlife. Land capability involves consideration of the risks of land damage from erosion and other causes and the difficulties in land use owing to physical land characteristics including climate.

LAND DEVELOPMENT (for Irrigation) — The following constitutes a partial listing of land development and improvement activities normally associated with irrigation projects. While these techniques have been prevalent in the past and have had a beneficial effect on land from an agricultural productivity standpoint, more recent concerns over wetland preservation and restoration and wildlife habitat concerns have tended to more recently restrict their widespread usage.
[1] Land Leveling and Smoothing — Leveling to a more uniform grade to permit more efficient gravity irrigation and to facilitate surface drainage on undulating lands developed for sprinkler or drip irrigation;
[2] Artificial Drainage — Installation of tile drains or ditches or installation of drainage wells to facilitate the removal of excess water from lands prone to accumulate excess water;
[3] Deep Ripping — When some lands, especially older terrace deposits, develop hardpan or cemented layers, if relatively thin in nature, they can often be ripped by powerful equipment thereby deepening the effective root zone and improving internal soil drainage;
[4] Soil Amendments — In order to improve the chemical and physical properties of the soil, it is sometimes necessary to apply gypsum, sulfur, or farm manure to the land;
[5] Terracing — A technique specifically designed for sloping land to prevent accelerated erosion on exposed slopes;
[6] Land Clearing — Initially and periodically, brush and trees must be cleared from irrigated lands to improve overall productivity;
[7] Surface Outlet Excavation — A technique to provide an outlet for standing water, most common in prior glacial areas where many closed depressions allow for the accumulation of runoff during wet periods;
[8] Tailwater Return Systems — Allow for the recycling and reuse of farm runoff water for additional irrigation;
[9] Rock Removal — Particularly prevalent in glacial areas, initial and periodic surface rock is especially important for development and on-going use of irrigated lands;
[10] Shelter Belt Planting — Mitigates wind erosion and provides cover for livestock and wildlife by planting tree lines in strategically located areas in and around irrigated fields.

LAND FARMING — A technique for the controlled biodegradation of organic waste that involves the mixture of waste sludges with soil. Microorganisms in the soil degrade the organic wastes. The biodegradation is enhanced by tilling the soil-waste mixture to ensure adequate oxygen and the control of moisture content, nutrient levels, and soil pH.

LANDFILL — (Water Quality) A disposal site which disposes of solid wastes on land. Wastes are deposited and compacted. At specific intervals, a layer of soil covers the waste and the process of deposit and compaction is repeated. The purpose is to confine the wastes to the smallest practical area and volume without creating nuisances or hazards to public health and safety, for example through leaching into the groundwater below the waste site.

LANDFORM — (Geography) A discernible natural landscape that exists as a result of wind, water or geological activity, such as a plateau, plain, basin, mountain, etc.

LANDLOCKED — (1) Enclosed or nearly enclosed by land, as a landlocked country without access to the sea or ocean. (2) Confined to fresh water by some barrier, as salmon.

LAND PAN — An evaporation pan located on land.

LAND RECLAMATION — Making land capable of more intensive use by changing its general character, as by drainage of excessively wet land, irrigation of arid or semiarid land, or recovery of submerged land from seas, lakes, and rivers.

LAND RECONSTRUCTION — (Mining) (1) Restoring land and water areas adversely affected by past mining practices and increasing the productivity of the areas for a beneficial use. (2) Restoring currently mined land to an acceptable form and for a planned use.

LAND RETIREMENT — (Agriculture) Taking land out of agriculture production by leaving it fallow or letting it return to a natural state.

LANDS — References to federally owned lands are defined as follows:
[1] Federal — All classes of land owned by the federal government, which includes Public Domain (Lands), withdrawn and acquired federal lands;
[2] Acquired — Lands acquired by the federal government through purchase, condemnation, or gift;
[3] Withdrawn — Federal lands for which formal withdrawal action has been taken which restricts the disposition of specific public lands and which holds them for specific public purposes; also, public lands which have been dedicated to public purposes;
[4] National Forest — Federal lands which have been designated by executive order or statute as national forests or purchase units, and other lands under the administration of the Forest Service, including experimental areas and Bankhead-Jones Title III lands; and
[5] Public Domain — Original public lands which have never left federal ownership. Also includes lands in federal ownership which were obtained by the federal government in exchange for public lands, or for timber on public lands.

LANDSCAPE — (Geography) All the natural features, such as fields, hills, forests, and water that distinguish one part of the earth's surface from another part. Usually refers to that portion of land or territory which the eye can comprehend in a single view, including all of its natural characteristics. These characteristics are a result not only of natural forces but of human occupancy and use of the land as well.

LANDSCAPE IMPOUNDMENT — A body of reclaimed water which is used for aesthetic enjoyment or which otherwise serves a function not intended to include contact recreation.

LANDSLIDE — A mass of material that has slipped downhill under the influence of gravity, frequently occurring when the material is saturated with water.

LAND SPREADING — The disposal of solid effluents derived from wastewater treatment facilities on the surface of the ground for dilution or dispersal.

LAND SUBSIDENCE — The sinking or settling of land to a lower level in response to various natural and man-caused factors, for example:
[1] earth movements;
[2] lowering of fluid pressure (or lowering of ground water level);
[3] removal of underlying supporting materials by mining or solution of solids, either artificially or from natural causes;
[4] compaction caused by wetting (Hydrocompaction);
[5] oxidation of organic matter in soils; or
[6] added load on the land surface.

With respect to ground water, subsidence most frequently results from overdrafts of the underlying water table or aquifer and its inability to fully recharge, a process termed Aquifer Compaction. Also see Subsidence.

LAND TREATMENT MEASURES — The application of vegetative tillage, structural and land management measures, individually or in combination, to alter runoff, to reduce erosion and sediment production, to increase fertility, and to improve drainage and irrigation applications. Also refers to the land disposal of sludge from sewage treatment plants.

LANDTYPE — A land system with a designated soil, vegetation, geology, topography, climate, and drainage situation.

LAND USE — The primary or primary and secondary uses of land, such as cropland, woodland, pastureland, etc. The description of a particular land use should convey the dominant character of a geographic area, and thereby establish the types of activities which are most appropriate and compatible with primary uses.

LAND USE PLAN — A coordinated composite of information, ideas, policies, programs, and activities related to existing and potential uses of land within a given area and frequently the key element in a comprehensive plan for an area under development for public and private land uses, such as residential, commercial, industrial, recreational, and agricultural activities.

LAND USE PLANNING — The process of inventorying and assessing the status, potentials, and limitations of a particular geographic area and its resources, interacting with the populations associated and/or concerned with the area to determine their needs, wants, and aspirations for the future.

LAND VOIDING — The process of damaging land by gully action causing this land to be unproductive for agricultural uses and relegating its use primarily to wildlife and recreation.

LANGELIER INDEX (LI) — An expression of the ability of water to dissolve or deposit calcium carbonate scale in pipes. The index has important implications in industrial water system where the formation of scale or sludge can cause equipment or process failure. The index is calculated from direct measurements of the following in the water system: pH, alkalinity, calcium concentrations, total dissolved solids, and temperature. A positive value indicates a tendency to form scale, and a negative value means the water will dissolve scale and may be corrosive.

LAP — (1) To wash or slap against with soft liquid sounds. (2) A watery food or drink.

LARGE WATER SYSTEM — A water system that services more than 50,000 customers.

LASER LAND LEVELING — The use of instruments featuring laser beams to guide earth-moving equipment for leveling land for surface-type irrigation.

LATE SERAL CONDITION — Synonymous with good ecological conditions.

LATENT HEAT — (1) The quantity of heat absorbed or released by a substance undergoing a change of state, such as ice changing to water or water to steam, at constant temperature and pressure. (2) The heat released or absorbed per unit mass of water in a reversible, isobaric-isothermal change of phase.

LATENT HEAT OF CONDENSATION — The amount of heat released by a unit mass of substance, without change in temperature, while passing from the vapor to the liquid state.

LATENT HEAT OF VAPORIZATION — The amount of heat absorbed by a unit mass of substance, without change in temperature, while passing from the liquid to the vapor state.

LATENT HEAT TRANSFER — The removal or addition of heat when a substance changes state. In the environment, this almost always refers to the release of heat from water upon condensation and the absorption of heat by water upon evaporation.

LATERAL — (1) A branch canal or pipeline that diverges from the main canal or other branches. (2) (Irrigation) A water project or irrigation conveyance structure, smaller than a canal intended to convey water away from the main canal or ditch. (3) (Water Quality) A municipal wastewater drain pipe that connects a home or business to a branch or main line.

LATERAL LINE — A series of sensory pores along the head and sides of fish and some amphibians by which water currents, vibrations, and pressure changes are detected.

LATERAL SEWERS — Pipes that run under city streets and receive the sewage from homes and businesses, as opposed to domestic feeders and main trunk lines.

LATERITIC SOIL — Land that consist of minerals that are rich in iron and aluminum compounds, other minerals having been removed by Leaching. The land is hard and unsuitable for agricultural use.

LEACHATE — Liquid which has percolated through the ground, such as water seeping through a sanitary landfill, wastes, pesticides, or fertilizers. Leaching may occur in farming areas, feedlots, and landfills, and may result in hazardous substances entering surface water, ground water, or soil.

LEACHATE COLLECTION SYSTEM — An arrangement of reservoirs and pipes underlying a waste disposal site designed to accumulate and remove Leachate, water that migrates through the waste, and pump it to the surface for treatment.

LEACHED LAYER (SOIL) — A soil layer or an entire soil profile from which the soluble materials (CaCO₃ and MgCO₃ and material more soluble) have been dissolved and washed away by percolating waters.

LEACHING — The process by which soluble materials in the soil, such as salts, nutrients, pesticide chemicals or contaminants, are washed into a lower layer of soil or are dissolved and carried away by water.

LEACHING EFFICIENCY — The ratio of the average salt concentration in drainage water to an average salt concentration in the soil water of the root zone when near field capacity (also defined as the hypothetical fraction of the soil solution that has been displaced by a unit of drainage water).

LEACHING FIELD — The area used for disposal of liquid through a non-water-tight artificial structure, conduit, or porous material by downward or lateral drainage, or both, into the surrounding permeable soil.

LEAF AREA INDEX — The area of one side of leaves per unit area of soil surface.

LEAKAGE — (1) (Hydrology) The flow of water from one Hydrogeologic Unit to another. This may be natural, as through a somewhat permeable confining layer, or Anthropogenic, as through an uncased well. It may also be the natural loss of water from artificial structures, as a result of Hydrostatic Pressure. (2) (Dams) The uncontrolled loss of water by flow through a hole or crack.

LEAKY AQUIFER — An artesian or water table aquifer that loses or gains water through adjacent semipermeable Confining Units.

LEFT BANK — The left-hand bank of a stream viewed when the observer faces downstream.

LENTIC WATERS — Ponds or lakes (standing water).

LEVEE — A natural or man-made earthen obstruction along the edge of a stream, lake, or river. Also, a long, low embankment usually built to restrain the flow of water out of a river bank and protect land from flooding. If built of concrete or masonry, the structure is usually referred to as a flood wall. The term Dike is commonly used to describe embankments that block an area on a reservoir rim that are lower than the top of the main dam.

LEVEE (NATURAL) — Bank of sand and silt built by a river during floods, where the Suspended Load is deposited in greatest quantity close to the river. The process of developing natural levees tends to raise river banks above the level of the surrounding flood plains. A break in a natural levee is sometimes called a Crevasse.

LEVEE (MANMADE) — An embankment, generally constructed on or parallel to the banks of a stream, lake or other body of water, for the purpose of protecting the land side from inundation by flood water or to confine the stream flow to its regular channel.

LEVEE SYSTEM — A flood protection system which consists of a levee, or levees, and associated structures, such as closure and drainage devices, which are constructed and operated in accord with sound engineering practices.

LEVEL OF DEVELOPMENT — In a planning study, the practice of holding constant the population, irrigated acreage, industry, and wildlife so that hydrologic variability can be studied to determine adequacy of supplies.

LIMITED WATER-SOLUBLE SUBSTANCES — (Water Quality) Water pollution chemicals that are soluble in water at less than one milligram of substance per liter of water.

LIMNOLOGY — The branch of Hydrology pertaining to the study of freshwater, the aquatic environment and its life; the study of the physical, chemical, hydrological, and biological aspects of fresh water bodies. Related terms include Limnological, Limnologic, and Limnologist.

LIMNOLOGY HYDROBIOLOGIST — A person who undertakes the biological study of bodies of water.

LINED WATERWAY OR OUTLET — A waterway or outlet with an erosion-resistant lining of concrete, stone, or other permanent material. The lined section extends up the side slopes to a designed height.

LINER — (1) (Water Quality) A low-permeability material, such as clay or high-density polyethylene, used for the bottom and sides of a landfill. The liner retards the escape of Leachate from the landfill to the underlying groundwater. (2) An insert or sleeve for sewer pipes to prevent leakage or infiltration.

LINING — With reference to a canal, tunnel, shaft, or reservoir, a coating of asphaltic concrete, reinforced or unreinforced concrete, shotcrete, rubber or plastic to provide water tightness, prevent erosion, reduce friction, or support the periphery of the structure. May also refer to the lining, such as steel or concrete, of an outlet pipe of conduit of a dam or reservoir.

LINING (HYDRAULICS) — A protective covering over all or part of the perimeter of a reservoir or a conduit to prevent seepage losses, withstand pressure, resist erosion, reduce friction, or otherwise improve conditions of flow.

LIQUID — A state of matter in which the molecules are closer and held more tightly by one another than in the gaseous state. Has a definite volume, but indefinite shape.

LIQUOR — (Water Quality) A liquid solution containing dissolved substances. A concentrated solution of process chemicals or raw materials added to an industrial process.

LITER — The basic unit of measurement for volume in the Metric System equivalent to 0.001 cubic meters (10^-3 m³); also equal to 61.025 cubic inches or 1.0567 liquid quarts.

LITTORAL ZONE — (1) The shallow area near the shore of a non-flowing body of water; that portion of a body of fresh water extending from the shoreline lakeward to the limit of occupancy of rooted plants. (2) A strip of land along the shoreline between the high and low water levels.

LIVESTOCK WATER USE — Water use for stock watering, feed lots, dairy operations, fish farming, and other on-farm needs. Livestock as used here includes cattle, sheep, goats, hogs, and poultry. Also included are such animal specialties as horses, rabbits, bees, pets, fur-bearing animals in captivity, and fish in captivity.

LOAD — The amount of material that a transporting agency, such as a stream, a glacier, or the wind, is actually carrying at a given time. Also, the amount of power delivered to a given point. In this respect:
[1] Base Load - The minimum load in a stated period of time.
[2] Firm Load - That part of the system load which must be met on demand.
[3] Peak Load - Literally, the maximum load in a stated period of time. Sometimes the term peak load is used in a general sense to describe that portion of the load above the base load.

LOAD ALLOCATION (LA) — (Water Quality) The portion of the pollution Load of a stream attributable to human Nonpoint Sources (NPS) of pollution. The amount of pollution from each point source is the Wasteload Allocation.

LOADING — Synonym for the pollution Load of a stream.

LOADING CAPACITY — The greatest amount of chemical materials or thermal energy that can be added to a stream without exceeding water quality standards established for that stream.

LOCAL FLOODING — Flood conditions which occur over a relatively limited area.

LOSING STREAM — A stream or reach of a stream that is losing water by seepage into the ground.

LOSSES INCIDENTAL TO IRRIGATION — The quantity of water depleted by irrigation in excess of the beneficial irrigation consumptive use.

LOTIC — (1) Of, relating to, or living in moving water. (2) Referring to a running water Ecosystem (streams and rivers).

LOTIC ENVIRONMENT — Characterizing aquatic communities found in running water.

LOTIC SYSTEM — A flowing body of fresh water, such as a river or stream.

LOTIC WATERS — Describing the waters of rivers and streams (flowing waters) as compared to Lentic Waters of ponds or marshes (standing waters).

LOUGH — (Irish) (1) A lake. (2) A bay or an inlet of the sea.

LOW — (1) Situated below the surrounding surfaces as in water standing in low spots. (2) Of less than usual or average depth; shallow, as in the river is low.

LOW FLOW FREQUENCY CURVE — A graph showing the magnitude and frequency of minimum flow for a specified period of time (duration).

LOWLAND FLOODING — Inundation of the very lowest portions of floodplain areas near a river, stream or lake, which are normally subject to frequent flooding; usually considered nuisance flooding.

LOW-LEVEL DRAWDOWN — A discharge feature of a dam allowing water to be removed from the bottom of a reservoir.

LOW-LEVEL OUTLET — An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases.

LOW-LYING — Lying close to water or ground level as low-lying coastal areas.

LOW-PRESSURE/LOW-VOLUME IRRIGATION — Irrigation systems that apply water directly, or very near to the soil surface, either above the ground or into the air, in discrete drops, continuous drops, small streams, mist, or sprays. These include drip systems, spray systems, jet systems, and bubbler systems. Also referred to as Micro or Trickle Irrigation. The efficiencies of these low pressure irrigation systems range from 75 to 95 percent; however, the average of 80 percent is commonly used.

LOW WATER (LW) — (1) The lowest level of water in a body of water, such as a river, lake, or reservoir. (2) (Navigation) The depth of a navigation channel is generally referenced to the low water stage which coincides with the lowest sustained flow over a 15-day period. On most streams this is referred to as "adopted low water"; on the lower Columbia, for example, it is the "Columbia River Datum".

LOW-WATER MARK (LWM) — The lowest level attained by a varying water surface level.

LYSIMETER — A field-situated tank or container filled with soil and planted to a crop. Crop consumptive use is measured by weighing or volumetrically monitoring this tank. Also a device for measuring the percolation of water through soils and for determining the soluble constituents removed in the drainage.