because water pressure decrease normal stress act on sliding plane and cohesion play  deterrent role in slope sliding.you can see in attached file a example about effect of water pressure and cohesion in SF.

Mr. Moradi,

Thanks for the contribution. In your opinion which one has more influence on the safety factor obtained by limit equilibrium analysis?

Thanks in advance.

In my opinion cohesion is material property and water pressure is site property,is not reasonable to compare material property by site property,in some situation may water pressure be dominant or may not.