Groundwater Definitions
Acre-foot (acre-ft, ac-ft): the volume of water required to cover 1 acre of land to a depth of 1 foot. An acre-foot of water is equal to 325,851 gallons, nearly enough to cover the bottom of the UA football stadium to a depth of one foot.
MG: Million gallons or number of gallons times 106.
Overdraft: pumping water out of an aquifer faster than it is being naturally or artificially recharge; pumping more water out than is being put back in.
Safe yield: Aquifers achieve safe yield when no more water is pumped from the aquifer than is replaced through either natural or artificial recharge; the amount of groundwater withdrawn on an average annual basis must not exceed the amount that is naturally or artificially recharged.
Arizona Groundwater Management Code: passed in 1980, says that Tucson Active Management Area must attain safe-yield by the year 2025.
Natural recharge: the addition of precipitation and streamflows to aquifers.
Incidental recharge: water that reaches the water table after human use, e.g. water from irrigated agriculture, mining, and discharge of effluent into stream channels.
Artificial recharge: water added to aquifers through deliberate human intervention; usually involves constructing facilities to control the movement and rate of infiltration; may be done by injecting water through wells, or spreading water over recharge basins; e.g. CAP water, sewage effluent.
Effluent: water that flows from a treatment plant after it has been treated; can be treated sewage or industrial waste.
Graywater or greywater: usually refers to sewage effluent that has been treated to a high enough extent so it can be used in some industrial application or to irrigate crops or golf courses; can also refer to household wastewater (water from sinks, bath tubs, washing machines, etc.) that can be intercepted and used for other purposes, such as irrigating lawns and gardens.
Subsidence: a gradual settling or sudden sinking of the Earth's surface due to the underground movement of earth materials; commonly the result of excessive groundwater pumping or the de-watering of aquifers. Water occupying the spaces between soil particles is removed and the water level drops. Without the water’s buoyancy supporting them, the soil particles collapse on each other and compact, thus leaving little to no void space between particles anymore. Once compacted, the soil cannot become “un-compacted”; water storage space is forever lost. Water can no longer enter the aquifer because there is no void space left to enter into.
CAP (Central Arizona Project) : aqueduct that transport Colorado River water to Tucson.
Central Wellfield: The central area of town where the majority of active wells are located; this is the area of town where water levels have declined the most and where it is most likely that subsidence will occur. This is also the most difficult area to access in order to recharge water back into the aquifer.
Hydrograph: some measure of water versus time.
Clearwater Renewable Resource Facility: Tucson Water’s construction project built in Avra Valley; Currently consists of 3 basins that recharge 5 billion gallons/yr of CAP water into the aquifer; CAP water blends with aquifer water; this blend is pumped out at a rate of 18 million gallons/day and put into the Tucson Water System. Additional basins will be constructed in the future and it is planned that by 2003, 54 million gallons per day of blended water will pumped out of the aquifer and put in the system, amounting to about 54% of the total water demanded by Tucson Water customers. This introduction of a renewable water resource should allow Tucson Water to shut down some wells in the central well-field, allowing them the water levels to recover.