How to estimate power received by an antenna considering multipath effects?

I have the gain plots of a transmitting and receiving monopole antenna as shown in the image below. The axis of both antennas are coincident thus using the Friis Transmission Equation gives zero received power. In reality, would the power received be non-zero and is this caused solely multipath effects (eg path in orange)? How can I estimate the actual power received (assuming the surrounding medium is air)?