Los Angeles County, California
Los Angeles County the County of Los Angeles, in the Los Angeles metropolitan area of the U. S. state of California, is the most populous county in the United States, with more than 10 million inhabitants as of 2017. As such, it is the largest non–state level government entity in the United States, its population is larger than that of 41 individual U. S. states. It is the third-largest metropolitan economy in the world, with a Nominal GDP of over $700 billion—larger than the GDPs of Belgium and Taiwan, it has 88 incorporated cities and many unincorporated areas and, at 4,083 square miles, it is larger than the combined areas of Delaware and Rhode Island. The county is home to more than one-quarter of California residents and is one of the most ethnically diverse counties in the U. S, its county seat, Los Angeles, is California's most populous city and the nation's second largest city with about 4 million people. Los Angeles County is one of the original counties of California, created at the time of statehood in 1850.
The county included parts of what are now Kern, San Bernardino, Inyo, Tulare and Orange counties. In 1851 and 1852, Los Angeles County stretched from the coast to the border of Nevada; as the population increased, sections were split off to organize San Bernardino County in 1853, Kern County in 1866, Orange County in 1889. Prior to the 1870s, Los Angeles County was divided into townships, many of which were amalgamations of one or more old ranchos, they were: Azusa El Monte Azusa and El Monte Townships were merged for the 1870 census. City of Los Angeles Los Angeles Township Los Nietos San Jose San Gabriel Santa Ana. For the 1870 census, Annaheim district was enumerated separately. San Juan. San Pedro. Tejon When Kern County was formed, the portion of the township remaining in Los Angeles County became Soledad Township According to the U. S. Census Bureau, the county has an area of 4,751 square miles, of which 4,058 square miles is land and 693 square miles is water. Los Angeles County borders 70 miles of coast on the Pacific Ocean and encompasses mountain ranges, forests, lakes and desert.
The Los Angeles River, Rio Hondo, the San Gabriel River and the Santa Clara River flow in Los Angeles County, while the primary mountain ranges are the Santa Monica Mountains and the San Gabriel Mountains. The western extent of the Mojave Desert begins in the Antelope Valley, in the northeastern part of the county. Most of the population of Los Angeles County is located in the south and southwest, with major population centers in the Los Angeles Basin, San Fernando Valley and San Gabriel Valley. Other population centers are found in the Santa Clarita Valley, Pomona Valley, Crescenta Valley and Antelope Valley; the county is divided west-to-east by the San Gabriel Mountains, which are part of the Transverse Ranges of southern California, are contained within the Angeles National Forest. Most of the county's highest peaks are in the San Gabriel Mountains, including Mount San Antonio 10,068 feet ) at the Los Angeles-San Bernardino county lines, Mount Baden-Powell 9,399 feet, Mount Burnham 8,997 feet and Mount Wilson 5,710 feet.
Several lower mountains are in the northern and southwestern parts of the county, including the San Emigdio Mountains, the southernmost part of Tehachapi Mountains and the Sierra Pelona Mountains. Los Angeles County includes San Clemente Island and Santa Catalina Island, which are part of the Channel Islands archipelago off the Pacific Coast. East: Eastside, San Gabriel Valley, portions of the Pomona Valley West: Westside, Beach Cities South: South Bay, South Los Angeles, Palos Verdes Peninsula, Gateway Cities, Los Angeles Harbor Region North: San Fernando Valley, Crescenta Valley, portions of the Conejo Valley, portions of the Antelope Valley and Santa Clarita Valley Central: Downtown Los Angeles, Mid-Wilshire, Northeast Los Angeles Angeles National Forest Los Padres National Forest Santa Monica Mountains National Recreation Area Los Angeles County had a population of 9,818,605 in the 2010 United States Census; the racial makeup of Los Angeles County was 4,936,599 White, 1,346,865 Asian, 856,874 African American, 72,828 Native A
Flood control methods are used to reduce or prevent the detrimental effects of flood waters. Flood relief methods are used to reduce the effects of high water levels. Floods are caused by many factors or a combination of any of these prolonged heavy rainfall accelerated snowmelt, severe winds over water, unusual high tides, tsunamis, or failure of dams, retention ponds, or other structures that retained the water. Flooding can be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce the supply of vegetation that can absorb rainfall. Periodic floods occur on many rivers. During times of rain, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, the rest travels over the land as surface runoff. Floods occur when ponds, riverbeds and vegetation cannot absorb all the water. Water runs off the land in quantities that cannot be carried within stream channels or retained in natural ponds and man-made reservoirs.
About 30 percent of all precipitation becomes runoff and that amount might be increased by water from melting snow. River flooding is caused by heavy rain, sometimes increased by melting snow. A flood that rises with little or no warning, is called a flash flood. Flash floods result from intense rainfall over a small area, or if the area was saturated from previous precipitation; when rainfall is light, the shorelines of lakes and bays can be flooded by severe winds—such as during hurricanes—that blow water into the shore areas. Coastal areas are sometimes flooded by unusually high tides, such as spring tides when compounded by high winds and storm surges. Flooding has many impacts, it endangers the lives of humans and other species. Rapid water runoff causes soil erosion and concomitant sediment deposition elsewhere; the spawning grounds for fish and other wildlife habitats can become polluted or destroyed. Some prolonged high floods can delay traffic in areas. Floods can interfere with drainage and economical use such as interfering with farming.
Structural damage can occur in bridge abutments, bank lines, sewer lines, other structures within floodways. Waterway navigation and hydroelectric power are impaired. Financial losses due to floods are millions of dollars each year, with the worst floods in recent U. S. history having cost billions of dollars. There are many disruptive effects of flooding on economic activities. However, flooding can bring benefits, such as making soil more fertile and providing nutrients in which it is deficient. Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River, among others; the viability for hydrologically based renewable sources of energy is higher in flood-prone regions. This is the method used for remote sensing the disasters. Detection of disasters such as floods and explosions are quite complex in previous days and range of detection is inappropriate. But, it came to possibilities by using Multi temporal visualization of Synthetic Aperture Radar images.
But to obtain the good SAR images perfect spatial registration and precise calibration are necessary to specify changes that have occurred. Calibration of SAR is complex and a sensitive problem. Errors may occur after calibration that involves data fusion and visualization process. Traditional image pre-processing cannot be used here due to the on-Gaussian of radar back scattering, but a processing method called "cross calibration/normalization" is used to solve this problem; the application generates a single disaster image called "fast-ready disaster map" from multitemporal SAR images. These maps are generated without user interaction and helps in providing immediate first aid to the people; this process provides image enhancement and comparison between numerous images using data fusion and visualization process. This proposed processing includes histogram truncation and equalization steps; the process helps in identifying the permanent waters and other classes by combined composition of pre-disaster and post-disaster images into a color image for better identity.
Some methods of flood control have been practiced since ancient times. These methods include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, the construction of floodways. Other techniques include the construction of levees, dams, retention ponds to hold extra water during times of flooding. Many dams and their associated reservoirs are designed or to aid in flood protection and control. Many large dams have flood-control reservations in which the level of a reservoir must be kept below a certain elevation before the onset of the rainy/summer melt season to allow a certain amount of space in which floodwaters can fill. Other beneficial uses of dam created reservoirs include hydroelectric power generation, water conservation, recreation. Reservoir and dam construction and design is based upon standards set out by the government. In the United States and reservoir design is regulated by the US Army Corps of Engineers. Design of a dam and reservoir follows guidelines set by the USACE and covers topics such as design flow rates in consideration to meteorological, topographic and soi
Coast Line (UP)
The Coast Line is a railroad line between Burbank and the San Francisco Bay Area along the Pacific Coast. It is the shortest rail route from Los Angeles to the Bay Area; the San Francisco and San Jose Railroad built the first segment of the line from San Francisco to San Jose between 1860 and 1864. The founders of the SF&SJ incorporated as the Southern Pacific Railroad, authorized by Congress in 1866 to connect the line from San Jose south to Needles, where it would meet the Atlantic & Pacific Railroad. However, SP had built to Tres Pinos by 1873 and abandoned efforts to continue the line to Coalinga, instead choosing a route from Lathrop. By 1871, SP had completed a line south from San Jose through Gilroy and Pajaro, arriving at Salinas in 1872 and Soledad in 1873. SP halted southward work at Soledad for thirteen years and started building north from Los Angeles in 1873, completing a line to Burbank. In 1886, SP had pushed south from Soledad to King City, Paso Robles, Templeton. By 1894, SP had extended the line south from Templeton to San Luis Obispo further south to Guadalupe in 1895 and Surf in 1896.
The 80-mile gap between Surf and Santa Barbara was closed with the last spike driven on December 28, 1900. The first version of the Coast line, via Saugus and Santa Paula through the Santa Clara River Valley, was completed by the Southern Pacific Railroad on December 31, 1900; the work between Ventura and Carpenteria allowed the construction of the Rincon sea-level road for automobile traffic to travel this impassible section of coastline. Work on the Montalvo Cutoff, which crossed the Santa Clara River to serve the farmers in the Oxnard Plain and was extended to Santa Susana in Simi Valley, began in 1898; the Santa Susana Tunnel opened in 1904 connecting with the Chatsworth cutoff from Burbank and thereafter was the main line. In 1907, the Bayshore Cutoff opened from San Bruno to San Francisco. In the golden era of passenger service SP trains on the San Francisco leg of this route ran from the Third and Townsend Depot in San Francisco to the Union Station in Los Angeles; the Oakland-Los Angeles trains originated from the 16th Street Station in Oakland.
The line has several subdivisions. Ownership is split into three segments: Caltrain north of Santa Clara Union Pacific Railroad, which merged with the Southern Pacific in 1996, from there to the north end of Moorpark Metrolink south of Moorpark. In 1992, Southern Pacific granted the Los Angeles County Transportation Commission an option to purchase the entire Coast Line for passenger train operations at 110 mi/h. Upgrades to signals and tracks to enable higher-speed operations were estimated to cost $360 million at the time. Union Pacific freight trains run on the route, although the San Joaquin Valley route is the primary north–south California route; the Coast Line is an important link for one of the busiest passenger routes in the nation. The route hosts passenger trains for Amtrak and Metrolink trains: Amtrak's Pacific Surfliner from San Luis Obispo to San Diego and Metrolink's Ventura County Line from Los Angeles Union Station to east Ventura, as well as Amtrak's Coast Starlight which connects Los Angeles to San Jose Diridon Station, Oakland Jack London Square station, points north.
Local agencies along with the host railroads formed the Los Angeles–San Diego-San Luis Obispo Rail Corridor Agency in 1989 to work together on upgrading the route that extends from San Luis Obispo to San Diego. Millions in enhancements to improve the reliability and safety of this 351-mile-long railroad corridor have been proposed by Caltrans and federal railroad officials. Ventura County would get rail curve realignments near Seacliff, the Santa Clara River and Montalvo in the near term for an estimated $300 million. Future rail service could include a Ventura–Santa Barbara commuter rail service. Long-range plans including commuter service between Ventura and Santa Clarita along the original route through the Santa Clara River Valley; the Ventura County Transportation Commission purchased the Santa Paula Branch Line within Ventura County from Southern Pacific. While a portion of the line was abandoned after being washed out in Los Angeles County, the proposed Newhall Ranch development will provide for a route through the community.
Amtrak Pacific Surfliner Amtrak Coast Starlight Metrolink Ventura County Line Caltrain Commuter rail Surf Line, the continuation south to San Diego Cal-P Line, the continuation north to Sacramento History of rail transportation in California South Pacific Coast Railroad Notes Citations BibliographySignor, John R.. Southern Pacific's Coast Line. Signature Press. ISBN 0-9633791-3-5. Retrieved 13 June 2018. "Coast Line History". The Ferroequinologist. June 1984. Retrieved 23 May 2018
Union Pacific Railroad
Union Pacific Railroad is a freight hauling railroad that operates 8,500 locomotives over 32,100 route-miles in 23 states west of Chicago and New Orleans. The Union Pacific Railroad system is the second largest in the United States after the BNSF Railway and is one of the world's largest transportation companies; the Union Pacific Railroad is the principal operating company of the Union Pacific Corporation. Union Pacific is known for pioneering multiple innovative locomotives the most powerful of their era; these include members of the Challenger-type, the Northern-type, as well as the famous Big Boy steam locomotives. Union Pacific ordered the first streamliner, the largest fleet of turbine-electric locomotives in the world, still owns the largest operational diesel locomotive; the Union Pacific legacy began in 1862 with the original company, called the Union Pacific Rail Road, part of the First Transcontinental Railroad project known as the Overland Route. The railroad would subsequently be reorganized thrice: as the Union Pacific Railway, as the Union Pacific "Railroad", as a renamed Southern Pacific Transportation Company.
The current Union Pacific corporation began in 1969 as the Southern Pacific Transportation Company, itself created in a reorganization of a railroad whose legacy dated to 1865. Over the years it would grow to include the Denver and Rio Grande Western Railroad and the St. Louis Southwestern Railway, in addition to its eponymous railroad; the 1998 Union Pacific-Southern Pacific merger was not UP's first: Union Pacific had merged with Missouri Pacific Railroad, the Chicago and North Western Transportation Company, the Western Pacific Railroad and the Missouri–Kansas–Texas Railroad. However, because the merger with Southern Pacific changed the scope of the Union Pacific railroad, this article will refer to the unmerged system as Union Pacific, the merged system as Union Pacific. Union Pacific's main competitor is the BNSF Railway, the nation's largest freight railroad by volume, which primarily services the Continental U. S. west of the Mississippi River. Together, the two railroads have a duopoly on all transcontinental freight rail lines in the U.
S. The original company, the Union Pacific Rail Road was incorporated on July 1, 1862, under an act of Congress entitled Pacific Railroad Act of 1862; the act was approved by President Abraham Lincoln, it provided for the construction of railroads from the Missouri River to the Pacific as a war measure for the preservation of the Union. It was constructed westward from Council Bluffs, Iowa to meet the Central Pacific Railroad line, constructed eastward from Sacramento, CA; the combined Union Pacific-Central Pacific line became known as the First Transcontinental Railroad and the Overland Route. The line was constructed by Irish labor who had learned their craft during the recent Civil War. Under the guidance of its dominant stockholder Dr. Thomas Clark Durant, the namesake of the city of Durant, the first rails were laid in Omaha; the two lines were joined together at Promontory Summit, Utah, 53 miles west of Ogden on May 10, 1869, hence creating the first transcontinental railroad in North America.
Subsequently, the UP purchased three Mormon-built roads: the Utah Central Railroad extending south from Ogden to Salt Lake City, the Utah Southern Railroad extending south from Salt Lake City into the Utah Valley, the Utah Northern Railroad extending north from Ogden into Idaho. The original UP was entangled in the Crédit Mobilier scandal, exposed in 1872; as detailed by The Sun, Union Pacific's largest construction company, Crédit Mobilier, had overcharged Union Pacific. In order to convince the federal government to accept the increased costs, Crédit Mobilier had bribed congressmen. Although the UP corporation itself was not guilty of any misdeeds, prominent UP board members had been involved in the scheme; the ensuing financial crisis of 1873 led to a credit crunch, but not bankruptcy. As boom followed bust, the Union Pacific continued to expand; the original company was purchased by a new company on January 24, 1880, with dominant stockholder Jay Gould. Gould owned the Kansas Pacific, sought to merge it with UP.
Thusly was the original "Union Pacific Rail Road" transformed into "Union Pacific Railway."Extending towards the Pacific Northwest, Union Pacific built or purchased local lines that gave it access to Portland, Oregon. Towards Colorado, it built the Union Pacific and Gulf Railway: both narrow gauge trackage into the heart of the Rockies and a standard gauge line that ran south from Denver, across New Mexico, into Texas; the Union Pacific Railway would declare bankruptcy during the Panic of 1893. Again, a new Union Pacific "Railroad" was formed and Union Pacific "Railway" merged into the new corporation. In the early 20th century, Union Pacific's focus shifted from expansion to internal improvement. Recognizing that farmers in the Central and Salinas Valleys of California grew produce far in excess of local markets, Union Pacific worked with its rival Southern Pacific to develop a rail-based transport system, not vulnerable to spoilage; these efforts came culminated in the 1906 founding of
Big Tujunga Creek
Big Tujunga Creek is a major stream in Los Angeles County in the U. S. state of California. From its headwaters high in the San Gabriel Mountains, it flows southwest for 28.8 miles, joining Little Tujunga Creek to form the Tujunga Wash near Pacoima. The stream is sometimes considered as one with the Tujunga Wash, the continuation of Big Tujunga to the Los Angeles River, bringing the total length to more than 40 miles; the creek rises near the Angeles Crest Highway in Upper Big Tujunga Canyon, deep within the Angeles National Forest. Its upper course is steep and rocky, sprinkled with rapids and small waterfalls, it flows west northwest, receiving Alder Creek and Lynx Gulch from the right and Wildcat Gulch and Wickiup Creek from the left. As the stream cuts deeper into its gorge, it receives a major tributary, Mill Creek, from the right, Fall Creek from the right just before emptying into Big Tujunga Reservoir, formed by the Big Tujunga Dam. While part of the reservoir, it receives Fox Creek from the right.
Below the dam, the stream flows through a steep rocky gorge, receiving Clear Creek from the left, before turning northwest into a broader valley. It flows through this valley for several miles before receiving Trail Canyon Creek from the right, swinging southwards around a few ridges spills out of the mountains near Sunland. Part of the stream is diverted at Sunland into spreading grounds in order to recharge the local aquifer; the rest of Big Tujunga Creek continues west into the dry Hansen Flood Control Basin, formed by the Hansen Dam. Here it receives Little Tujunga Creek from the right, becomes the Tujunga Wash, which runs south about 8.5 miles to the Los Angeles River near Studio City. In 1931, the Big Tujunga Dam was built by the Los Angeles Department of Water and Power deep within the creek's canyon to provide flood control and block silt. With a capacity of just 5,960 acre feet, the dam is unable to control large floods, has overflowed many times since its construction. During the Los Angeles Flood of 1938, Big Tujunga Creek reached a maximum flow of more than 50,000 cubic feet per second, washing down thousands of tons of silt from the mountains and jumping its banks destroying hundreds of buildings and floodworks.
Following the floods, the 97-foot -high Hansen Dam was built across the lower creek by the U. S. Army Corps of Engineers in 1940 in the hope of controlling future floods. List of rivers of California
U.S. Route 101 in California
U. S. Route 101 in the state of California is one of the last remaining and longest U. S. Routes still active in the state, the longest highway of any kind in California. US 101 was one of the original national routes established in 1926. Significant portions of US 101 between the Los Angeles area and the San Francisco Bay Area follow El Camino Real, the historic road connecting the former Alta California's 21 missions. Although the highway has been superseded in overall importance for transportation through the state by Interstate 5, US 101 continues to be the major coastal north–south route that links the Greater Los Angeles Area, the Central Coast, the San Francisco Bay Area, the North Coast. Referred to as "101" by residents of Northern California, in Southern California it is called "The 101"; the highway has portions designated as the Santa Ana Freeway, the Hollywood Freeway, the Ventura Freeway, South Valley Freeway, Bayshore Freeway. The Redwood Highway, the 350-mile-long northernmost segment of the highway, begins at the Golden Gate and passes through the world's tallest and only extensive preserves of virgin, old-growth coast redwood trees.
US 101 is part of the California Freeway and Expressway System, is part of the National Highway System, a network of highways that are considered essential to the country's economy and mobility by the Federal Highway Administration. The south terminus of US 101 is in Los Angeles, about one mile east of downtown Los Angeles at the East Los Angeles Interchange known as the "Commuters' Complex"; this southernmost portion is named the Santa Ana Freeway, inheriting that title as the northerly extension of the roadway now known as I-5. After merging with westbound traffic from the San Bernardino Freeway, US 101 proceeds northwest via the Downtown Slot under the northern edge of Los Angeles' Civic Center to State Route 110 at the Four Level Interchange. From here, US 101 becomes the Hollywood Freeway, it heads to Hollywood and up through the Cahuenga Pass before reaching the San Fernando Valley. US 101 intersects with SR 134 and SR 170 at the interchange known as the Hollywood Split. Here, the alignment of US 101 shifts to the alignment of SR 134 and thereafter is referred to as the Ventura Freeway until it reaches Ventura.
Though confusing, the "Hollywood Freeway" name continues northward from this interchange on SR 170, the "Ventura Freeway" name continues eastward to SR 134. From the Hollywood Split, US 101 is an east–west highway, it meets with I-405 in Sherman Oaks, an interchange which holds claim to the most traveled intersection in the nation. The east–west geographical alignment of the Ventura Freeway and the north–south designation which appears on the freeway signs can be confusing to visitors. After the Conejo Grade, a 7% grade incline, the freeway enters the Oxnard Plain and runs concurrent with SR 1 for the first time. Upon reaching Ventura, there is an interchange with SR 126. North of Santa Barbara, US 101 switches intermittently between freeway and expressway status, but there are no traffic signals until San Francisco; the last traffic signals along this stretch of the route were removed in 1991 when the section through downtown Santa Barbara was constructed to freeway standards after years of disagreement over the impact that the original elevated design would have on the community.
From Ventura and through Santa Barbara, US 101 follows the Pacific coastline until Gaviota State Park, about 23 miles west of Goleta. At Gaviota State Park, the highway shifts back from an east–west highway to a north–south alignment. About one mile north of this point, US 101 passes through the Gaviota Tunnel. A few miles north of the Gaviota Tunnel, SR 1 splits from US 101 and heads northwest, running along the Pacific coastline parallel and to the west of US 101. US 101 passes through Buellton, Los Alamos, Santa Maria, Nipomo. South of Santa Maria, US 101 widens from a four-lane highway to a six-lane freeway. SR 166 joins US 101 for about 3 miles before splitting just north of the city limits, while US 101 continues as a four-lane freeway before reverting to expressway status north of Nipomo. Farther north, SR 1 rejoins US 101 between San Luis Obispo. US 101 takes an inland route through the Salinas Valley, while Highway 1 heads northwest, running along the Pacific coastline in California, parallel and to the west of US 101.
A steep segment between San Luis Obispo and Atascadero is known as the Cuesta Grade. North of Atascadero, the highway joins SR 46 for about three miles through Paso Robles. From Paso Robles to Salinas, US 101 is an expressway known as the Salinas River Valley Highway, since the Salinas River Valley extends from Santa Margarita to the SR 156 junction in Prunedale. US 101 resumes freeway status between San Miguel and King City, passing through the smaller towns of Camp Roberts and San Ardo, as well as the San Ardo Oil Field about five miles south of San Ardo. Near this point, the wide agricultural bottomlands of the Salinas Valley begins. North of King City, US 101 once again switches intermittently between freeway and
Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water; the depth at which soil pore spaces or fractures and voids in rock become saturated with water is called the water table. Groundwater is recharged from and flows to the surface naturally. Groundwater is often withdrawn for agricultural and industrial use by constructing and operating extraction wells; the study of the distribution and movement of groundwater is hydrogeology called groundwater hydrology. Groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can contain soil moisture, immobile water in low permeability bedrock, deep geothermal or oil formation water. Groundwater is hypothesized to provide lubrication that can influence the movement of faults, it is that much of Earth's subsurface contains some water, which may be mixed with other fluids in some instances.
Groundwater may not be confined only to Earth. The formation of some of the landforms observed on Mars may have been influenced by groundwater. There is evidence that liquid water may exist in the subsurface of Jupiter's moon Europa. Groundwater is cheaper, more convenient and less vulnerable to pollution than surface water. Therefore, it is used for public water supplies. For example, groundwater provides the largest source of usable water storage in the United States, California annually withdraws the largest amount of groundwater of all the states. Underground reservoirs contain far more water than the capacity of all surface reservoirs and lakes in the US, including the Great Lakes. Many municipal water supplies are derived from groundwater. Polluted groundwater is less visible and more difficult to clean up than pollution in rivers and lakes. Groundwater pollution most results from improper disposal of wastes on land. Major sources include industrial and household chemicals and garbage landfills, excessive fertilizers and pesticides used in agriculture, industrial waste lagoons and process wastewater from mines, industrial fracking, oil field brine pits, leaking underground oil storage tanks and pipelines, sewage sludge and septic systems.
An aquifer is a layer of porous substrate that transmits groundwater. When water can flow directly between the surface and the saturated zone of an aquifer, the aquifer is unconfined; the deeper parts of unconfined aquifers are more saturated since gravity causes water to flow downward. The upper level of this saturated layer of an unconfined aquifer is called the water table or phreatic surface. Below the water table, where in general all pore spaces are saturated with water, is the phreatic zone. Substrate with low porosity that permits limited transmission of groundwater is known as an aquitard. An aquiclude is a substrate with porosity, so low it is impermeable to groundwater. A confined aquifer is an aquifer, overlain by a impermeable layer of rock or substrate such as an aquiclude or aquitard. If a confined aquifer follows a downward grade from its recharge zone, groundwater can become pressurized as it flows; this can create artesian wells that flow without the need of a pump and rise to a higher elevation than the static water table at the above, aquifer.
The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur. In general, the more productive aquifers occur in sedimentary geologic formations. By comparison and fractured crystalline rocks yield smaller quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater; the high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a steady temperature. In some places where groundwater temperatures are maintained by this effect at about 10 °C, groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather cool groundwater can be pumped through radiators in a home and returned to the ground in another well.
During cold seasons, because it is warm, the water can be used in the same way as a source of heat for heat pumps, much more efficient than using air. The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well-drilling to determine the extent and thickness of water-bearing sediments and rocks. Before an investment is made in production wells, test wells may be drilled to measure the depths at which water is encountered and collect samples of soils and water for laboratory analyses. Pumping tests can be performed in test wells to determine flow characteristics of the aquifer. Groundwater makes up about twenty percent of the world's fresh water supply, about 0.61% of the entire world's water, including oceans and permanent ice. Global groundwater storage is equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles; this makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought.
Groundwater is replenished b