def avp_from_rhmin_rhmax(svp_temperature_min, svp_temperature_max, relative_humidity_min, rh_max):
    """
    Estimate actual vapour pressure (*ea*) from saturation vapour pressure and
    relative humidity.

    Based on FAO equation 17 in Allen et al (1998).

    :param svp_temperature_min: Saturation vapour pressure at daily minimum temperature
        [kPa]. Can be estimated using ``svp_from_t()``.
    :param svp_temperature_max: Saturation vapour pressure at daily maximum temperature
        [kPa]. Can be estimated using ``svp_from_t()``.
    :param relative_humidity_min: Minimum relative humidity [%]
    :param rh_max: Maximum relative humidity [%]
    :return: Actual vapour pressure [kPa]
    :rtype: float
    """
    tmp1 = svp_temperature_min * (rh_max / 100.0)
    tmp2 = svp_temperature_max * (relative_humidity_min / 100.0)
    return (tmp1 + tmp2) / 2.0