Term infiltration. Definition water soaking into ground. Term transpiration. Definition plants release water vapor. Term laminar flow. Definition straight-line paths, slow, parallel.
Term turbulent flow. Definition swirling motion. Term factors that determine stream velocity. Term gradient. Definition vertical drop of a stream over a specified distance. Term discharge. Term longitudinal profile. Definition cross-section along the length of a stream gradient decreases downstream towards mouth.
Term base level. Definition level below which a stream cannot erode. Term temporary base level. Definition lakes, resistant layers of rock, and main streams that act as base level for their tributaries.
Term ultimate base level. Don't have an account? Sign in via your Institution. You could not be signed in, please check and try again. Sign in with your library card Please enter your library card number. Related Content Related Overviews gauging station stream groundwater canal See all related overviews in Oxford Reference ». Show Summary Details Overview discharge.
Generally, water underground is more like water in a sponge. It occupies the spaces between soil and rock particles. At a certain depth below the land surface, the spaces between the soil and rock particles can be totally filled with water, resulting in an aquifer from which groundwater can be pumped and used by people. Some of the precipitation that falls onto the land infiltrates into the ground to become groundwater. If the water meets the water table below which the soil is saturated , it can move both vertically and horizontally.
Water moving downward can also meet more dense and water-resistant non-porous rock and soil, which causes it to flow in a more horizontal fashion, generally towards streams, the ocean , or deeper into the ground. If groundwater wants to be a member in good standing of the water cycle, then it can't be totally static and stay where it is.
As the diagram shows, the direction and speed of groundwater movement is determined by the various characteristics of aquifers and confining layers of subsurface rocks which water has a difficult time penetrating in the ground.
Water moving below ground depends on the permeability how easy or difficult it is for water to move and on the porosity the amount of open space in the material of the subsurface rock. If the rock has characteristics that allow water to move relatively freely through it, then groundwater can move significant distances in a number of days.
But groundwater can also sink into deep aquifers where it takes thousands of years to move back into the environment, or even go into deep groundwater storage , where it might stay for much longer periods. If an aquifer is under enough pressure, an artesian well tapping the aquifer can result in pressurized water shooting above the land surface. Bottled water is a very popular beverage nowadays all over the world. Sometimes it is because the local drinking water is of lower quality and sometimes it is just a convenience.
Some bottled water is advertised as "artesian well water". Is the water really any different than other groundwater? Artesian well water is not really different from non-artesian well water - but it comes to the surface in a different manner. In the diagram above, you can see that there are unconfined and confined aquifers in the ground.
The confinement of water in an aquifer, which can result in pressure, determines if water coming from it is artesian or not. Wells drilled into confined aquifers can yield artesian water. So, in what way is bottled artesian well water different from other well water? Mainly, the company that bottles it doesn't have to go to the expense of installing a pump in their well. As these charts show, even though the amount of water locked up in groundwater is a small percentage of all of Earth's water , it represents a large percentage of total freshwater on Earth.
The pie chart shows that about 1. As the bar chart shows, about 5,, cubic miles mi 3 , or 23,, cubic kilometers km 3 , of groundwater exist on Earth. About 54 percent is saline, with the remaining 2,, mi 3 10,, km 3 , about 46 percent, being freshwater. Source: Gleick, P. The stage is then measured inside the stilling well using a float or a pressure, optic, or acoustic sensor.
The measured stage value is stored in an electronic data recorder on a regular interval, usually every 15 minutes. At some streamgage sites, a stilling well is not feasible or is not cost effective to install. As an alternative, stage can be determined by measuring the pressure required to maintain a small flow of gas through a tube and bubbled out at a fixed location under water in the stream.
The measured pressure is directly related to the height of water over the tube outlet in the stream. As the depth of water above the tube outlet increases, more pressure is required to push the gas bubbles through the tube. Streamgages operated by the USGS provide stage measurements that are accurate to the nearest 0. Stage at a streamgage must be measured with respect to a constant reference elevation, known as a datum.
Sometimes streamgage structures are damaged by floods or can settle over time. To maintain accuracy, and to ensure that stage is being measured above a constant reference elevation, the elevations of streamgage structures, and the associated stage measurement, are routinely surveyed relative to permanent elevation benchmarks near the streamgage.
Although stage is valuable information for some purposes, most users of streamgage data are interested in streamflow or discharge—the amount of water flowing in the stream or river, commonly expressed in cubic feet per second or gallons per day. However, it is not practical for a streamgage to continuously measure discharge. Fortunately, there is a strong relation between river stage and discharge and, as a result, a continuous record of river discharge can be determined from the continuous record of stage.
Determining discharge from stage requires defining the stage-discharge relationship by measuring discharge at a wide range of river stages. Discharge is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic feet per second or gallons per day.
In general, river discharge is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section:.
The USGS uses numerous methods and types of equipment to measure velocity and cross-sectional area, including the following current meter and Acoustic Doppler Current Profiler.
The most common method used by the USGS for measuring velocity is with a current meter. However, a variety of advanced equipment can also be used to sense stage and measure streamflow. In the simplest method, a current meter turns with the flow of the river or stream. The current meter is used to measure water velocity at predetermined points subsections along a marked line, suspended cableway, or bridge across a river or stream.
The depth of the water is also measured at each point. These velocity and depth measurements are used to compute the total volume of water flowing past the line during a specific interval of time.
Usually a river or stream will be measured at 25 to 30 regularly spaced locations across the river or stream. One method that has been used for decades by the USGS for measuring discharge is the mechanical current-meter method.
In this method, the stream channel cross section is divided into numerous vertical subsections. In each subsection, the area is obtained by measuring the width and depth of the subsection, and the water velocity is determined using a current meter. The discharge in each subsection is computed by multiplying the subsection area by the measured velocity. The total discharge is then computed by summing the discharge of each subsection.
Numerous types of equipment and methods are used by USGS personnel to make current-meter measurements because of the wide range of stream conditions throughout the United States. Subsection width is generally measured using a cable, steel tape, or similar piece of equipment.
Subsection depth is measured using a wading rod, if conditions permit, or by suspending a sounding weight from a calibrated cable and reel system off a bridge, cableway, or boat or through a hole drilled in ice.
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