Why cavitation degrades the performance of a pump?
 

It is common sense that cavitation degrades the performance of a pump (causing the head of the pump to drop). But I seldom see any book clearly explains why this happen. Some books says that performance degrades is due to blockage of the blade passage by the bubbles.
As I know, the head of the pump can be determined by the euler equation

I wonder anybody have some idea explaining the cavitation induced head drop based on this equation.

You may want to take a look at the following book and the references therein:

http://authors.library.caltech.edu/25019/3/pumbook.pdf

For example, in Chapter 7 one possible explanation for the degradation of hydraulic performance is that voids located not on the suction side of the blades but rather under the shroud of a centrifugal pump may impact the angle with which flow leaves the impeller and significantly alter performance.

Thank you very much.