Use specific category for every author on author page

I have written a quite extensive post on a similar subject that you can read here. Also take your time and read all linked posts.

In there I have explained why you should not be using query_posts and why you should not use custom queries in place of the main query

A few things here:

• Remember to create a child theme, do not make changes to a theme that you have not written. You will loose all your work if you update that theme

• It is real easy to create your own author archive page. Simply make a copy of your index.php and rename it author.php. It is that easy

• Use pre_get_posts to make changes to your author page. Do not make changes directly through a custom query on your author page. You can use the following in your functions.php

  function wpse166864_include_category( $query ) {
      if ( !is_admin() && $query->is_author() && $query->is_main_query() ) {
         $query->set( 'cat', '1' );
      }
  }
  add_action( 'pre_get_posts', 'wpse166864_include_category' );