S₁ and S₂ are the draw downs in an observation well at times t₁ and t₂ after pumping. For discharge Q and coefficient of transmissibility T, the relationship, is

For calculation of economical diameter D of a pipe in metres for a discharge Q to be pumped in cumecs, Lea suggested the empirical formula

The valve fitted closely in a recess against an opening in a pipe, is generally

For the same draw down in two observations wells at distances r1 and r2, the times after start of pumping are t1 and t₂ hours respectively. The relation which holds good is

In plain sedimentation tanks under normal conditions, impurities are removed upto