Question 13.11: It is observed that the water table in an unconfined aquifer......
It is observed that the water table in an unconfined aquifer that extends over an area of 250 km² has an average annual rise of 5 m. The porosity of the material is estimated to be 12% and its specific yield 7%. (a) Determine the annual volume of recharge. (b) Estimate the maximum volume of water that potentially could be abstracted annually from the aquifer.
(a) The recharge volume = annual rise × area × porosity
= 5 × 250 × 10^{6} × 0.12
= 150 × 10^{6} m³ per annum
(b) The maximum volume that could be abstracted = annual rise × area × specific yield
= 5 × 250 × 10^{6} × 0.07
= 87.5 × 10^{6} m³ per annum
Note: it is unlikely (and undesirable) that the wells would be able to abstract all of this amount. Some flow through the aquifer downstream of the wells should be preserved, as should some groundwater discharge to rivers to maintain the baseflow.