GetVaporPressureDeficit Function

  function GetVaporPressureDeficit(TDryBulb, HumRatio, Pressure) result(VaporPressureDeficit)
    !+ Return Vapor pressure deficit given dry-bulb temperature, humidity ratio, and pressure.
    !+ Reference:
    !+ Oke (1987) eqn 2.13a

    real, intent(in)  ::  TDryBulb
      !+ Dry-bulb temperature in °F [IP] or °C [SI]
    real, intent(in)  ::  HumRatio
      !+ Humidity ratio in lb_H₂O lb_Air⁻¹ [IP] or kg_H₂O kg_Air⁻¹ [SI]
    real, intent(in)  ::  Pressure
      !+ Atmospheric pressure in Psi [IP] or Pa [SI]
    real              ::  VaporPressureDeficit
      !+ Vapor pressure deficit in Psi [IP] or Pa [SI]
    real              ::  RelHum
      !+ Relative humidity in range [0, 1]

    if (HumRatio < 0.0) then
      error stop "Error: humidity ratio is negative"
    end if

    RelHum = GetRelHumFromHumRatio(TDryBulb, HumRatio, Pressure)
    VaporPressureDeficit = GetSatVapPres(TDryBulb) * (1.0 - RelHum)
  end function GetVaporPressureDeficit