Non-renewable groundwater resources: a ... - unesdoc - Unesco

NON-RENEWABLE GROUNDWATER RESOURCESFigure 1Groundwater level drawdown in the Jwaneng Northern Wellfieldof Southern Botswana after 20 years production for mine and town water-supply94reducing abstraction. Borehole yields range from 31 m 3 /hr to 119 m 3 /hr and have averaged73 m 3 /hr.In 1994, remodelling was done to verify the capability of JNWF to supply approximately12 million cubic meters (MCM)/yr for a period of at least 15 years (i.e. 1994 to 2008) in line withdemand. The 1994 model revision concluded that the JNW is capable of supplying Jwaneng mineand township over a period of 1994 to 2008 an annual discharge of 13.29 MCM. The water rightfor JWF is 12 MCM per year.Modeled regional drawdowns during the period 1981–84 were about 2–3 meters greaterthan those actually observed despite the increase in pumping rates. This discrepancy betweenmodeled and actual water levels resulted in a re-examination and recalibration of the model.The model run was begun in 1980. Annual abstractions for the period from 1984 to 1990were assumed a constant 5,262,205 cubic meters. For the subsequent period (1990–95) abstractionswere increased by 10% to simulate increased water demand. After modifying aquiferparameters and running a series of sensitivity analyses, a revised model was used to compare1984 and the original drawdown predictions. The drawdown was revised to be 13 m after15 years, that is, up to 1995 of pumping at 22,262 m³/day.The 1984 recalibrated model ceased to be used as a management tool shortly after itsrelease since it was apparent from an increasing discrepancy between observed and predicteddrawdowns. Fortunately, the discrepancy was favourable.