calculate_wrf_ko_derivative Module Function

    module function calculate_wrf_ko_derivative(self, h) result(dqw_dh)
        implicit none
        class(type_wrf_ko), intent(in) :: self
        real(real64), intent(in) :: h
        real(real64) :: dqw_dh

        if (h < 0.0d0) then
            dqw_dh = -(self%theta_s - self%theta_r) * &
                     exp(-(log(h / self%alpha1))**2.0d0 / (2.0d0 * self%n1**2.0d0)) / &
                     (sqrt(2.0d0 * pi) * h * self%n1)
        else
            dqw_dh = 0.0d0
        end if
    end function calculate_wrf_ko_derivative