Author
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Bouwer, Herman |
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Submitted to: Biennial Symposium on the Artificial Recharge of Groundwater
Publication Type: Proceedings Publication Acceptance Date: 6/2/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Almost all of the liquid fresh water in the world occurs underground as groundwater forms and is recharged by water from rain and snow that infiltrates into the soil and moves down to water-bearing formations called aquifers. The sustainable pumping rate is controlled by the natural recharge of groundwater, which may vary from less than 1% of long-term precipitation in hot, dry climates to more than 40% in temperate wet climates. In many areas, groundwater pumping exceeds natural recharge so that groundwater is being depleted, and serious problems can develop when the wells go dry. Natural recharge rates are equal to precipitation minus surface runoff and minus evaporation and transpiration. Natural recharge can be enhanced by cloud seeding, reducing areal surface runoff by storing and spreading it for infiltration, reducing evaporation and transpiration as achieved by replacing deep-rooted plants with shallow rooted plants, and dby using plants that intercept less rain water on their leaves. Induced recharge is obtained by pumping groundwater from wells near streams, thus "pulling" more surface water into aquifers. Other forms of recharge are incidental recharge from irrigated fields, septic tanks, and urban developments where streets, driveways, and roofs produce less evapotranspiration and more runoff that can be used for artificial recharge of groundwater with surface infiltration systems (basins), subsurface infiltration systems (trenches and shafts), and aquifer wells. Knowledge about natural and other forms of groundwater recharge is important for water planners, engineers, hydrogeologists, regulators, water managers, etc., who must develop sustainable water supplies. Technical Abstract: Of all the fresh, liquid water on this planet, 98% is groundwater and 2% is surface water in streams and lakes. Sources of groundwater are rain and snow that percolate down through the soil to form aquifers below. Natural recharge rates are equal to precipitation minus runoff and evaporation from soil and transpiration from plants. They vary from 1% or less of annual precipitation in warm, dry climates to 40% or more in wet climates. Knowledge of natural recharge rates is essential for estimating sustainable pumping rates from aquifers. In many areas of the world, groundwater is pumped faster than the recharge rates, which is disastrous when the wells go dry and other water resources are not available. Knowledge of natural recharge rates also is important where radioactive or other waste disposed off on or near the surface can leach down to underlying groundwater. Natural recharge can be estimated with water balance, chloride balance, radioactive dating, tracer velocity, and water content profile studies. Other recharges are incidental, as from irrigated fields, septic tanks, and urban developments in dry climates where small showers which normally produce no runoff, begin to produce runoff from paved surfaces and roof tops that can recharge underlying groundwater. Induced recharge is obtained by pumping groundwater from aquifers in direct hydraulic connection with streams so that pumping pulls more stream water into the aquifer. Reducing evapotranspiration losses enhances recharge, as achieved by keeping ground water levels well below root zones, by replacing deep-rooted plants (trees) with shallow-rooted plants (grass), or by using plants that intercept less precipitation, allowing more water to infiltrate into the soil. |
