Groundwater flow

Groundwater fills the pores and fractures of saturated ground below the water table. Layers that store and transmit useful amounts of water are aquifers: unconfined where the water table is free to rise and fall, confined where the aquifer is sealed above and below by less permeable layers and the water is under pressure. In India, groundwater supplies most irrigation and much drinking water, and over-pumping has lowered water tables across many districts.

Flow through an aquifer follows Darcy's law, velocity proportional to hydraulic gradient. Two properties describe an aquifer for well design: transmissivity, the hydraulic conductivity times the saturated thickness, which measures how easily water moves through the whole thickness; and storativity, the volume released per unit area per unit drop in head, which is small for confined aquifers and close to the specific yield for unconfined ones.

When a well pumps, the water level around it falls in a cone of depression. Under steady conditions, the Thiem equation relates the discharge to the drawdowns at two observation wells for a confined aquifer, and Dupuit's version handles unconfined aquifers using the saturated thickness. Unsteady pumping tests, interpreted by methods such as Theis and Cooper-Jacob, give both transmissivity and storativity.

Groundwater also matters for construction: dewatering excavations, uplift on basements and dams, seepage into tunnels and settlement caused by lowering the water table. Recharge structures, check dams and percolation tanks help restore aquifers.