Water falling on a surface When water falls on a solid surface, a circle is made by it. It is different from ripples because ripples do not end in a certain radius, but when you see this circle, it has a certain boundary. Why does this happen? Can we calculate the radius?
 A: There are several phenomena that could fit the description in your question, but like suggested @emilio-pisanty and @qmechanic, you're certainly talking about the hydraulic jump!
The answer to your question is in fact quite complex since it hadn't been qualitatively addressed until 1993!  T. Bohr, P. Dimon and V. Putkaradge, J. Fluid Mech. 254 635(1993), available here, showed the radius should scale as $R_j \propto Q^{5/8}\nu^{-3/8}g^{-1/8}$, where $Q$ is the volume flux, $\nu$ is the kinematic viscosity and $g$ is the gravitational acceleration. A. Mukherjee, A. Datta and J. K. Bhattacharjee confirmed this scaling in an other paper in 2002, available here.
Your question could also refer to drop impacts. When a drop of fluid falls on a hydrophobic surface, a bead is formed at the periphery of the impacting drop. This deformation was studied in C. Clanet, C. Béguin, D. Richard and D. Quéré, J. Fluid Mech. 517 199(2004), available here, wherein a scaling of the maximal spreading is proposed.
