water. Well yield is the volume of water per unit time that is produced by the well from pumping. Methods for measuring drawdown Transducers are used to measure water levels in groundwater wells, rivers, streams, tanks, open channels and lift stations. Acoustic well sounders or echometers are a simple, cost effective, and minimally intrusive tool used to measure subsurface pressures and levels. Electric sounders are a practical land cost-effective method used to measure well water levels. This method uses a weight attached to a stranded insulated wire and an ammeter to indicate a closed circuit. Current supplied from a small battery flows through the circuit when the tip of the wire is in contact with the surface of the water. Air line method is a convenient and nonintrusive method used to measure water levels that is often used for the repeated testing of wells over 300 feet deep. This method obtains water table depth using a pressure gauge and water displacement. Wetted tape method is a commonly-used method for measuring water levels up to roughly 90 feet deep. This method uses a lead weight attached to a steel measuring tape. Ecological impacts of groundwater drawdown Groundwater drawdown due to excessive water extraction can have adverse ecological impacts. Groundwater environments often have high biodiversity, however, drawdown alters the amount and types of nutrients released to surrounding organisms. In addition, nearby wetlands, fisheries, terrestrial and aquatic habitats may be altered with a reduction in the water available to these ecosystems, sometimes altering species ecophysiology. Extracting groundwater at a rate that is faster than it can be naturally replenished is often referred to as overdrafting. Overdrafting may decrease the amount of groundwater that naturally feeds surrounding water bodies, including wetlands, lakes, rivers and streams. Additionally, when a cone of depression is formed around a pumping well due to groundwater extraction, nearby groundwater sources may flow toward the well to replenish the cone, taking water from local streams and lakes. This may result in poor water quality in these local water bodies as baseflow water contribution is reduced, which could result in perennial streams becoming more intermittent, and intermittent streams becoming more ephemeral. Finally, drawdown from groundwater extraction may lead to an increased sensitivity of the ecosystem to climate change and may be a contributing factor to sea-level rise and land subsidence. Related Subsidence References Hydrology Aquifers Water Water wells Hydraulic engineering