  The spin-stiffness \rho_s of a 2D Heisenberg antiferromagnet depends
non-analytically on external magnetic field. We demonstrate that the
hydrodynamic relation between \rho_s, the uniform susceptibility \chi, and the
spin-wave velocity $c$ is not violated by such a behavior because similar
non-analytic terms from all three quantities mutually cancel out. In this work,
explicit expressions for the field-dependent spin stiffness and for the magnon
velocity of the 2D square lattice antiferromagnet are obtained by direct
calculation to order 1/S and in the whole range of magnetic fields.
