calculate_wrf_mvg_derivative Module Function

    module function calculate_wrf_mvg_derivative(self, h) result(dvw_dh)
        implicit none
        class(type_wrf_mvg), intent(in) :: self
        real(real64), intent(in) :: h
        real(real64) :: dvw_dh

        if (h < self%h_crit) then
            dvw_dh = (self%theta_s - self%theta_r) * &
                     self%alpha1**self%n1 * self%m1 * self%n1 * (-h)**(self%n1 - 1.0d0) * &
                     (1.0d0 + (-self%alpha1 * h)**self%n1)**(-self%m1 - 1.0d0)
        else
            dvw_dh = 0.0d0
        end if

    end function calculate_wrf_mvg_derivative