The present study investigates hydrodynamic effects of the strip roughness on the Surface-velocity Divergence (SD) value to breakthrough about physical modeling of gas transfer mechanism in natural rivers. Particularly, accurate PIV measurements were conducted in a computer-controlled laboratory flume varying water discharge and roughness spacing, systematically, in order to obtain the space and time distributions of surface velocity divergence, turbulent kinetic energy and dissipation rate. Finally, a new practical model for the surface velocity divergence was proposed considering turbulence micro scales.