Groundwater is usually spoken of as being found in a Water Table.
The water table is a saturated zone that does not have a sharply defined boundary.
The depth of a water table depends upon the saturated zone.
The more porous the volume of rock or sediment (open space), the greater the total volume of available water. The porosity of sediment and rocks are as follows:
Sand and gravel (well sorted) - 25 - 50%
Sand and gravel (mixed) - 22 - 35%
Mud (clay) – 33-60%
Sandstone or conglomerate - 5 to 30%
Shale - 10%
Lithification (as found in igneous and metamorphic rocks) reduces porosity, and water flow is very low, unless the formation is fractured.
Fractures are important for permeability, which is the ease of water flow, and is usually dependent upon the porosity and connectivity of pores. For instance, sand, sandstone, have high porosity and high permeability. Clay, on the other hand, has a high porosity, but very low permeability. Physically, fractures connect pores.
Water underground generally moves through small, constricted passages, often along a tortuous route. The flow velocity increases as the slope of the water table increases (gravity).
Back in 1856, Henri Darcy, studied the “downhill” velocity of groundwater, and determined that it must be related to the hydraulic gradient (water table slope) and the earth density. This study became the generally accepted theory.
So, how fast does groundwater flow? The quick answer, based on studies by Skinner and Porter, 2004, is: Very slowly in clays, maybe only 5 to 10 feet a year. In tight sand, about 3-feet a day, but in gravel, shale or highly mixed rock and sand, the flow can exceed 1,000-feet daily.
Movement of groundwater in a water saturated zone will flow from where the water table is high to where it is low. Differences in the height of the water table causes pressure gradient (pressure change/distance).
The elevation difference between the water table at a recharge and discharge point is called the hydraulic head.
- Water Measurements -
Water at rest; i.e., ponds, lakes, reservoirs, and in the soil, is measured in units of volume C gallon, cubic foot, acre-inch, and acre-foot.
The volume of water that would cover one acre one foot deep. A single acre-foot of water is the equivalent of 325,851 gallons of water covering an area of one acre at a depth of one foot.
One cubic foot = 7.481 gallons (7.5 for ordinary calculations) weights about 62.5 pounds.
One acre-inch = 27,154 gallons (27,200 for ordinary calculations), weighs around 113.1 tons.
One acre-foot = 325,851 gallons, weighs approximately 1,357 tons.
Volume Units (additional):
Cubic Foot - The volume of water that would be held in a container one foot wide by one foot long by one foot deep.
Acre-lnch - The volume of water that would cover one acre (43,560 square feet) one inch deep.
Acre-Foot - The volume of water that would cover one acre one foot deep, or enough to meet the demands of three families of four for one year.
Rate of Flow Units:
One cubic foot per second = 448.83 gallons per minute (450 for ordinary calculations), or 1 acre-inch in 1 hour and 30 seconds (1 hour for ordinary calculations). And,
One acre-foot in 12 hours and 6 minutes (12 hours for ordinary calculations), or 1.984 acre-feet per (24 hours) day (2 acre-feet for ordinary calculations).
Liter vs Cup Measurement:
Liter Measures in cups: Liters to cups volume measurement units conversion factors are listed below. To find out how many cups in liters, multiply by the right factor or simply use the converter below.
1 Liter = 4.2267528377 Cups [US]
1 Liter = 4 Cups [Metric] Liter is volume unit which equals to 1 cubic decimeter (decimetre). It is widely used in daily life to measure the fluids. The abbreviation is "l".
Cup is a volume unit and used mostly in cooking to measure liquid and bulk, dry foods. There is no international standard for the sizes, but mostly United States customary and metric cups are used. The abbreviation is "c".
For cups to liters converter, please go to cups to liters.
For other volume unit conversions, please go to Volume Conversion.
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