day per m2. Units are frequently shortened to metres per day or equivalent. Specific storage or storativity: a measure of the amount of water a confined aquifer will give up for a certain change in head; Transmissivity The rate at which water is transmitted through whole thickness and unit width of an aquifer under a unit hydraulic gradient. It is equal to the hydraulic conductivity times the thickness of an aquifer; Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include: Specific yield or drainable porosity: a measure of the amount of water an unconfined aquifer will give up when completely drained; Leakage coefficient: some aquifers are bounded by aquitards which slowly give up water to the aquifer, providing additional water to reduce drawdown; The presence of aquifer boundaries (recharge or no-flow) and their distance from the pumped well and piezometers. Analysis methods An appropriate model or solution to the groundwater flow equation must be chosen to fit to the observed data. There are many different choices of models, depending on what factors are deemed important including: leaky aquitards, unconfined flow (delayed yield), partial penetration of the pumping and monitoring wells, finite wellbore radius — which can lead to wellbore storage, dual porosity (typically in fractured rock), anisotropic aquifers, heterogeneous aquifers, finite aquifers (the effects of physical boundaries are seen in the test), and combinations of the above situations. Nearly all aquifer test solution methods are based on the Theis solution; it is built upon the most simplifying assumptions. Other methods relax one or more of the assumptions the Theis solution is built on, and therefore they get a more flexible (and more complex) result. Transient Theis solution The Theis equation was created by Charles Vernon Theis (working for the US Geological Survey) in 1935, from heat transfer literature (with the mathematical help of C.I. Lubin), for two-dimensional radial flow to a point sink in an infinite, homogeneous aquifer. It is simply where s is the drawdown (change in hydraulic head at a point since the beginning of the test in units of distance), u is a dimensionless parameter, Q is the discharge (pumping) rate of the well (volume per unit time), T and S are the transmissivity and storativity of the aquifer around the well (distance squared per unit time and dimensionless, respectively), r is the distance from the pumping well to the point where