Custom Map Makers

What does this error mean and how do I get rid of it?

ERROR: MAX_MAP_DRAWINDEXES
Your brush work is too complex. You need to simplify your design and try again.

http://www.urbanterror.info/forums/topic/10214-common-error-messages/

Can patch meshes cause this error? My brushwork hasn't changed since before I got this error, but I have done a lot with patch meshes.

I'd have to dig through the code to be sure, but I'll say yes.
Are you compiling with -patchmeta? If not, do so.

Also, if you're making terrain out of patch meshes, stop right now and use trisouping instead.

I assume that by trisouping you mean the method used on Riyadh? I think it looks inferior to patch meshes, and doing it has caused me a different error that required the -notjunc command to compile a map. I think the problem was that I had some irregular patch mesh shapes to make terrain curve around the entrance of a tunnel. This has been removed and the map is compiling again.

If it looks inferior you're doing it wrong, easy as that. You need a phong shader, btw.
Also alternate the resolution of the trisoup, you don't have to use the same triangle size everywhere in one terrain. Alternate between 128x128, 64x64 and 256x256 unit triangles depending on the distance/amount of detail required.

If you're getting errors with trisouping you're doing it horribly wrong. Please post some screenshots so we can have a look at it.
I have made an incredible large terrain for my Inception map which brought my NetRadiant to it's knees and increased BSP compile time by over an hour (vis and lighting increased it even further).
It did work perfectly fine though.

Using patch meshes for terrain is WRONG. Period.

Here is a view of the map as it is now. Is it good to do a rough draft with patch meshes and then replace them with trisouping?

Definitely replace everything terrain-related with tri- or at least quadsouping. Might want to mix both as well.
Imo drafts in Radiant are a waste of time, do it properly right from the start - if you need visual drafts, draw them on paper.

Also be aware that you have visible seams in the terrain right now - they will become way worse ingame. You can only get rid of them if you make the whole terrain out of a single patch mesh or replace it with brushes.
And check the water texture rotation, it looks like you rotated the brushes with texture lock enabled.

I am taking your advice and doing the trisouping. This has taken my brush count from 11500 to 14500, and I am only about halfway done...

Yes, I am in the progress of tri souping... I am less than a quarter done and have been working on it for about 8 hours