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Habitat — The part of the physical environment where plants and animals live.

Head — Difference in elevation between intake and discharge points for a liquid. In geology, most commonly of interest in connection with the movement of underground water.

Head Cut — A break in slope at the top of a gully or section of gully that forms a “waterfall,” which in turn causes the underlying soil to erode and the gully to expand uphill.
Head Cutting —
(1) The action of a bedscarp or headward erosion of a locally steep channel or gully.
(2) A natural process of active erosion in a water channel caused by an abnormal and abrupt change in channel gradient. This change causes a “waterfall” action as water tumbles from the upper level vertically to the lower. The turbulence erodes the channel by undercutting the substrate material. This causes the collapse of the upper level (head). The “undercut-collapse” process advances up the channel whenever water is present or until bedrock is reached.

Headgate — The gate that controls water flow into irrigation canals and ditches; the control works or gate at the entrance to a canal or conduit system. A watermaster regulates the headgates during water distribution and posts headgate notices declaring official regulations. Headgate also refers to a diversion structure which controls the flow rate from a conveyance system (canals and laterals) into the farm conveyance system.

Headwater(s) —
(1) The source and upper reaches of a stream; also the upper reaches of a reservoir.
(2) The water upstream from a structure or point on a stream.
(3) The small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.

Hierarchical — (Ecology) A description of Ecosystems referring to their nested and scale-dependent organization.

Higher Aquatic Plants — Those plants whose seeds germinate in the water phases or substrate of a body of water and which must spend part of their life cycle in water. Includes plants which grow completely submerged as well as a variety of emersed and floating leaf types.

Highest Annual Mean — A value used for river flow readings representing the highest total annual volume (in acrefeet per year) and the corresponding highest annual average rate of flow (in cubic feet per second) recorded at a specific gaging station location over a specific period of record. Also referred to as the High Water Year.

High-Grading — (Ecology) A harvesting practice in which the most valuable trees are removed with little provision for regeneration or subsequent entries.

Historic Variability — (Ecology) The variation in spatial, structural, compositional, and temporal characteristics of ecosystem elements during a reference period prior to intensive resource use and management. In the Southwest United States, this reference period is typically considered the recent climatic and ecological era before the territorial period (resource use and management by native and Hispanic cultures are integrated with other ecological process).

Histosols — Organic soils.

Holding Medium — (Water Quality) A special fluid employed for maintaining fecal bacteria in a viable state between the time that water samples are processed by filtration and the time that the filters used to remove the bacteria from water can be incubated properly. The medium protects viability between sampling and analysis.

Holistic — Of, concerned with, or dealing with wholes or integrated systems rather than with their parts. With respect to water-related issues, the term most typically describes an analytical and planning approach which examines and considers the inter-related linkages and interdependencies of a socioeconomic system with resource use, pollution, environmental impacts, and preservation of an entire ecosystem.

Homeostasis — (Ecology) The maintenance of a steady state by use of feedback control processes. In homeostatic systems, a change outside the normal range is seen as a decline in the health of that system.

Hundred-Year Flood — The magnitude of a flood which has one chance in one hundred (i.e., one percent) of occurring in any one-year period. As the occurrence of floods is random in time, there is no guarantee that there will not be two one hundred-year floods within a given year, or that there will be one such flood within a given century (100 years). The boundary of the one hundred-year flood zone is used by the Federal Emergency Management Agency (FEMA) to designated Special Flood Hazard Areas. These areas are plotted on Flood Insurance Rate Maps (FIRMs), which are used in determining the flood risk to structures in the Flood Plain for flood insurance purposes.

Hydraulic Gradient (I) —
(1) The slope of the water surface.
(2) The gradient or slope of a water table or Piezometric Surface in the direction of the greatest slope, generally expressed in feet per mile or feet per feet. Specifically, the change in static head per unit of distance in a given direction, generally the direction of the maximum rate of decrease in head. The difference in hydraulic heads (h1 – h2), divided by the distance (L) along the flowpath, or, expressed in percentage terms:
I = (h1 – h2) / L X 100 A hydraulic gradient of 100 percent means a one foot drop in head in one foot of flow distance.

Hydrilla — An exotic (nonnative) aquatic weed, hydrilla (Hydrilla verticillata) has come to represent a serious threat to lakes, reservoirs, streams and canals in the United States. A native of Asia, Africa, and Australia, hydrilla is part of a group of rooted aquatic plants well adapted to growth under water. Hydrilla was first introduced to the U.S. in Florida during the 1950s, probably for use in aquariums. Hydrilla is capable of prodigious growth, expanding from a few acres to several thousand acres in two to three years. Due to its ability to photosynthesize under very low light conditions, hydrilla becomes easily established in turbid waters and at greater depths than other aquatic plants. Most populations in the U.S. comprise dioecious female plants; reproduction is asexual, including fragmentation and the formation of hardy, long-lived propagules, called turions or tubers, which form deep in the sediment and remain viable for as long as ten years. Once hydrilla has invaded a site, the diversity of other rooted, submersed aquatic plants plummets, resulting in monospecific stands of hydrilla and loss of
biodiversity. At is peak population, hydrilla has reduced water flows in canal systems by up to 80 percent and in small, standing-water impoundments, such as lakes and ponds, hydrilla can often completely cover the water surface within two to three years.

Hydrograph — Graph showing variation of water elevation, velocity, streamflow, or other property of water with respect to time.

Hydrographic Area — In its most general sense, may refer to an defined geographic area, sub-area, sub-basin, basin, region or watershed encompassing the drainage area or catchment area of a stream, its tributaries, or a portion thereof. Typically defined as a study area for analysis or planning purposes in which the land or undersea contours results in surface water flows or measures of elevation draining to a single point. At its smallest extent, a hydrographic area may encompass a single valley containing a single stream system, or a portion of a valley or stream system with distinctive drainage characteristics. At its greatest extent, a hydrographic area may encompass
the entire drainage area of a major river system, e.g., the Mississippi River hydrographic area, including all tributary rivers, streams and other sources of surface water flow. Conventionally, a number of hydrographic subareas comprise a hydrographic area whereas a number of hydrographic areas comprise a hydrographic basin or region.

Hydrography —
(1) The study, description, and mapping of oceans, lakes, and rivers, especially with reference to their navigational and commercial uses. Specifically includes the measurement of flow and investigation of the behavior of streams, especially with reference to the control or utilization of their waters, as well as the surveying, sounding, and charting of water bodies.
(2) That branch of surveying which embraces the determination of the contour of the bottom of a harbor or other sheet of water, the dept of soundings, the position of channels and shoals, with the construction of charts exhibiting these particulars.

Hydrologic Benchmark — A hydrologic unit, such as a basin or a ground-water body, that because of its expected freedom from the effects of man, has been designated as a benchmark. Data from such basins may provide a standard with which data from less independent basins can be compared so that changes wrought by man’s interference can be distinguished from changes caused by variations in the natural regimen.

Hydrologic Regions [California] — For water planning and conservation purposes, the California Department of Water Resources (DWR) and the State Water Resources Control Board (SWRCB) have divided the state into 10 Hydrologic Regions, also referred to as a Hydrologic Study Area (HSA), and are based on the Watershed or Water Basin concept. These California HSAs include:
[1] North Coast Region — Comprises all of the California area tributary to the ocean from the mouth of Tomales Bay north to the Oregon border and east along the border to a point near Goose Lake,
consisting of 19,590 square miles (12 percent of the state’s total area), 571,750 persons (1.9 percent of the state’s total population — all populations as of 1990), with average annual precipitation of 53 inches (range: 15 to over 100 inches), and average annual runoff of 28,886,000 acre-feet (40.8 percent of total state runoff);
[2] San Francisco Bay Region — Extends from Pescadero Creek in southern San Mateo County to the mouth of Tomales Bay in the north and inland to the confluence of the Sacramento and San Joaquin rivers near Collinsville, consisting of 4,400 square miles (3 percent of the state’s total area), 5,484,000 persons (18 percent of the state’s total population), with average annual precipitation of 31 inches (range: 14 to almost 48 inches), and average annual runoff of 1,245,500 acre-feet (1.8 percent of total state runoff);
[3] Central Coast Region — Encompasses the area adjacent to the Pacific Ocean including Santa Cruz County in the north through Santa Barbara County in the south to the Diablo and Temblor mountain ranges on the east, consisting of 11,280 square miles (7 percent of the state’s total area), 1,292,900 persons (4 percent of the state’s total population), with average annual precipitation of 20 inches (range:
14 to 45 inches), and average annual runoff of 2,477,000 acre-feet (3.5 percent of total state runoff);
[4] South Coast Region — Extending eastward from the Pacific Ocean, the region is bounded by the Santa Barbara–Ventura county line and the San Gabriel and San Bernardino mountains on the north, the Mexican border on the south, and a combination of the San Jacinto Mountains and low-elevation
mountain ranges in central San Diego County on the east, consisting of 10,950 square miles (7 percent of the state’s total area), 16,292,800 persons (54 percent of the state’s total population), with average annual precipitation of 18.5 inches (range: 10 to 45 inches), and average annual runoff of 1,227,000 acre-feet (1.7 percent of total state runoff);
[5] Sacramento River Region — Contains the entire drainage area of the Sacramento River and its
tributaries and extends almost 300 miles from Collinsville in the Sacramento–San Joaquin Delta north
to the Oregon border to the crest of the Sierra Nevada and Cascade Ranges which form the eastern
border to the crest of the Coast Range forming the western side, consisting of 26,960 square miles (17 percent of the state’s total area), 2,208,900 persons (7 percent of the state’s total population), with average annual precipitation of 36 inches (range: 10 to 80 inches), and average annual runoff of
22,389,700 acre-feet (31.6 percent of total state runoff);
[6] San Joaquin River Region — Located in the heart of California bordered on the east by the crest of the Sierra Nevada and on he west by the coastal mountains of the Diablo Range, extending from the Delta and the Cosumnes River drainage south to include all of the San Joaquin River watershed, consisting of 15,950 square miles (10 percent of the state’s total area), 1,430,200 persons (5 percent of the state’s total population), with average annual precipitation of 13 inches (range: 9 to 35 inches), and average annual runoff of 7,933,300 acre-feet (11.2 percent of total state runoff);
[7] Tulare Lake Region — Including the southern San Joaquin Valley from the southern limit of the San Joaquin River watershed to the crest of the Tehachapi Mountains, stretching from the Sierra Nevada Crest in the east to the Coast Range in the west, consisting of 16,520 square miles (10 percent of the state’s total area), 1,554,000 persons (5 percent of the state’s total population), with average annual precipitation of 14 inches, and an average annual runoff of 3,313,500 acre-feet (4.7 percent of total state runoff);
[8] North Lahontan Region — Comprises the eastern drainages of the Cascade Range and the eastern Sierra Nevada, north of the Mono Lake drainage, consisting of 3,890 square miles (less than 3 percent of the state’s total area), 78,000 persons (less than 0.3 percent of the state’s total population), with average annual precipitation of 32 inches (range: 4 to 70 inches), and average annual runoff of 1,842,000 acre-feet (2.6 percent of total state runoff);
[9] South Lahontan Region — Encompassing the area from the mountain divide north of Mono Lake to the divide south of the Mojave River, which runs through the Mojave Desert, bordered on the east by the Nevada state line and on the west by the crest of the Sierra Nevada, consisting of 29,020 square miles (18 percent of the state’s total area), 599,900 persons (2 percent of the state’s total population), with average annual precipitation of 8 inches (range: 4 to 10 inches, with extremes of 1.9 inches in Death Valley and over 120 inches at Mammoth Lakes), and an average annual runoff of 1,334,000 acre-feet (1.9 percent of total state runoff);
[10] Colorado River Region — Encompassing the southeastern corner of California with the region’s
northern boundary, a drainage divide, beginning along the southern edge of the Mojave River watershed in the Victor Valley area of San Bernardino County and extending northeast across the Mojave Desert to the Nevada state line. The southern boundary is the Mexican border while a drainage divide forms the jagged western boundary through the San Bernardino, San Jacinto, and Santa Rosa mountains and the Peninsular ranges and the Nevada state line and the Colorado River form the region’s eastern boundary, consisting of 19,730 square miles (12 percent of the state’s total area), 464,200 persons (less than 2 percent of the state’s total population), with average annual precipitation of 5.5 inches (range: 3 to 36 inches), and an average annual runoff of 178,700 acre-feet (less than 0.3 percent of total state runoff).

Hydrologic Soil Groups — The classification of soils by their reference to the intake rate of infiltration of water, which is influenced by texture, organic matter content, stability of the soil aggregates, and soil horizon development.

Hydrologic Unit Maps [USGS] — A set of maps developed by the U.S. Geological Survey (USGS) that present information on drainage, culture, hydrography, and hydrologic boundaries and codes of
(1) the 21 major water resources regions and the 222 subregions designated by the U.S. Water Resources Council, (2) the 352 accounting units of the U.S. Geological survey’s National Water Data Network, and (3) the 2,149 cataloging units of the U.S. Geological Survey’s “Catalog of Information on Water Data.” The hydrologic unit map series was initiated in the fall of 1972 by the U.S. Geological Survey’s Office of Water Data Coordination, in cooperation with the U.S. Water Resources Council and supported by the U.S. Geological Survey’s Resources and Land Information program. These maps and associated codes provide a standardized base for use by water-resources organizations in locating, storing, retrieving, and exchanging hydrologic data, in indexing and inventorying hydrologic data and information, in cataloging water-data acquisition activities, and in a variety of other applications. Because the maps have undergone extensive review by all principal federal, regional and state water-resource agencies, they are widely accepted for use in planning and describing water-use and related land-use activities, and in geographically organizing hydrologic data. The maps depict a hydrologic system that divides the United States into 21 major regions. These regions are further subdivided into 222 subregions, 352 accounting units, and finally, into 2,149 cataloging units. These four levels of subdivisions, used for the collection and organization of hydrologic data, are referred to as Hydrologic Units.

Hydrology —
(1) The science of waters of the earth, their occurrence, distribution, and circulation; their physical and chemical properties; and their reaction with the environment, including living beings.
(2) The study of the movement and storage of water in the natural and disturbed environment.
(3) The condition of the aquatic environment at some specified time and place. Most frequently, the term is used in reference to water on the surface of the land, in the soil and underlying rocks, and in the atmosphere.

Hydrophyte —
(1) A perennial vascular aquatic plant having its overwintering buds under water.
(2) A plant growing in water or in soil too waterlogged for most plants to survive.
(3) A plant adapted to grow in water.
(4) Any plant growing only in water or very wet earth, requiring large quantities of water for growth.

Hydrostatic Loads —
(1) Forces imposed on a flooded structure due to the weight of the water.
(2) (Floods) Those loads or pressures resulting from the static mass of water at any point of floodwater contact with a structure. They are equal in all directions and always act perpendicular to the surface on which they are applied. Hydrostatic loads can act vertically on structural members such as floors, decks, and roofs, and can act laterally on upright structural members such as walls, piers, and foundations.

Hygroscopic Coefficient — The level of tension at which water is considered to be “bound” to the soil particles, 31 atmospheres.

Hypereutrophic (Water) — Pertaining to a lake or other body of water characterized by excessive nutrient concentrations such as nitrogen and phosphorous and resulting high productivity. Such waters are often shallow, with algal blooms and periods of oxygen deficiency. Slightly or moderately eutrophic water can be healthful and support a complex web of plant and animal life. However, such waters are generally undesirable for drinking water and other needs. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll–a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll–a, maximum phosphorus).

Hypolimnetic — Refers to the cold bottom water zone below the Thermocline in a lake.

Hypolimnion — The lowermost, non-circulating layer of cold water in a thermally stratified lake or reservoir that lies below the Thermocline, remains perpetually cold and is usually deficient of oxygen. Also see Thermal Stratification.

Hyporheic Zone — Zone of substrate in a stream bottom extending 1 to 2 meters (3.3 to 6.6 feet) below the surface of the stream bed. In gaining and in losing streams, water and dissolved chemicals can move repeatedly over short distances between the stream and the shallow subsurface below the streambed. The hyporheic zone is comprised of the resulting subsurface environments, which contain variable proportions of water from groundwater and surface water. These zones can be active sites for aquatic life, for example, the spawning success of fish may be greater where flow from the stream brings oxygen into contact with eggs that were deposited within the course bottom sediment or where stream temperatures are modulated by groundwater flows.