Transmissivity is the rate at which an aquifer passes water through its full saturated thickness. It equals hydraulic conductivity times thickness.
Transmissivity describes how easily water moves through the whole thickness of an aquifer. It equals the hydraulic conductivity K multiplied by the saturated thickness b. An aquifer with K of 5 m/day and a thickness of 30 m has a transmissivity of 150 m²/day. It is one of the most useful numbers for planning wells, because it largely controls how much water a well can give and how far its water level falls. As a rough guide, values under about 10 m²/day suit only small domestic supplies, while values above about 500 m²/day can support large production wells. Transmissivity is usually found from a pumping test, using methods such as Theis or Cooper-Jacob. These give an average value over the area the test reaches, which is more representative than a laboratory test on a small core. In a confined aquifer it stays nearly constant. In an unconfined aquifer it falls as the water table drops, since the saturated thickness shrinks. In hard rock, nearly all the flow may come from a few fractures, so values can vary greatly over short distances.