Elevation IV: Fixing Endorheic Basins

About 18% of the Earth's land does not drain to the ocean. These are called endorheic basins. I need to address them for two reasons.

First of all, if water never drains all the way out to the ocean, it's hard to get up enough drainage to make any kind of appreciable river. Second, the erosion model relies on these large drainage numbers to actually modify the terrain.

So how do I deal with these? Consider the following terrain (heights shown in feet).

All the (blue) hexes bordering the center (red) hex are higher, forcing all drainage into the central hex. How do we smooth this out?

How about the average of the neighbors (10769)? This converges to a solution when applied to the overall map, although it takes a while depending on the initial inputs. The problem is that it tends to erase deep valleys - I want deep valleys! It's much too much smoothing.

The key issue is that the hex is lower than all its neighbors. I could modify one of the neighbors to be lower. This is...unsatisfactory. Unless I was making something like a moonscape. Food for thought.

There are some other features in this map that I'll talk about in a future post.