The widely used weakly compressible variant of Smoothed Particle Hydrodynamics (SPH) method suffers from density oscillations and hence pressure oscillations. This is due to the particle Lagrangian nature of the SPH method in combination with weakly compressible assumption, explicit time scheme and that the approximation of the derivatives in the SPH method is central. There are two common strategies how to suppress these issues. One of them is to use numerical diffusive term which is added in the continuity equation in order to suppress the spurious oscillation on density field. The second option is to describe the particle-particle interaction in terms of Riemann problem and use Riemann solver, which provides numerical dissipation, to handle particle interactions. In our work, we deal with the relation between these two approaches. For the constant reconstruction and for the linear reconstruction we show that the usage of Riemann solvers is due to its intrinsic numerical viscosity equivalent to usage of diffusive terms based on even derivatives, with the difference that the Riemann solvers lead to significantly higher value of coefficient of numerical diffusion, then is used in case of standard diffusive terms.