subroutine watersat_grad(T,qsat,dqsat)

  use watercommon_h, only: T_coup, RLVTT, RCPD
  implicit none

!==================================================================
!     Purpose
!     -------
!     Compute the water mass mixing ratio at saturation (kg/kg)
!     for a given pressure (Pa) and temperature (K)
!
!     Authors
!     -------
!     Robin Wordsworth (2010)
!
!==================================================================

!   input
  real T,qsat
  
!   output
  real dqsat

  if (T.lt.T_coup) then ! solid / vapour
     dqsat = RLVTT/RCPD*qsat*(3.56654/T             &
          +2484.896*LOG(10.)/T**2                   &
          -0.00320991*LOG(10.))
  else                 ! liquid / vapour
     dqsat = RLVTT/RCPD*qsat*LOG(10.)*              &
          (2948.964/T**2-5.028/LOG(10.)/T           &
          +25.21935*2999.924/T**2*EXP(-2999.924/T)  &
          +29810.16*0.0699382*EXP(-0.0699382*T))
  end if

  return
end subroutine watersat_grad