This is not an ordinary book on rainfall and runoff. All the general and working formulas in this book are theoretically derived. The formulas are therefore globally and eternally applicable, as long as the situations under consideration are within the assumptions and limitations of the theory. This epitomises the powerful nature of the physically-based approach in hydrology. This book covers formulas for flow depth; flow velocity; average flow velocity; wave celerity; average wave celerity; time of concentration; rising, equilibrium and falling phases of a hydrograph; forward characteristic; rising, equilibrium and falling phases of water surface profiles; duration of partial equilibrium discharge; and equilibrium detention storage for flow on an overland plane, and flow in nine different channel shapes, which are (i) circular, (ii) parabolic, (iii) rectangular (deep), (iv) rectangular (square), (v) rectangular (wide), (vi) trapezoidal with equal side slopes, (vii) trapezoidal with one side vertical, (viii) triangular, and (ix) vertical curb.