A measure of how close air is to saturation with water vapour.

A measure of the volume of water that will flow through a unit cross-sectional area of rock per unit time, under a unit hydraulic gradient and at a specified temperature: K in the

Darcy’s law equation.

The slope of a water table.

Another name for the water cycle.

The quantity of water available from underground or from rivers in an area, given by the excess of precipitation over actual evapotranspiration.

The study of water movement on and beneath the ground, and the physics and chemistry of water.

The parts of the Earth that are mainly composed of water.