Basic 7×7 block
The 7×7 block is the desert version of the 9×9. It is because in the desert, fountains only service a range of 3 tiles instead of 4.
The smaller block-size translates into more intersections. Couple that with the doubled rate of fire-risk in the desert, and it’s plain to see that your prefects will be doing it tough with this type of design in desert scenario.
Even if you like the 9×9 for central and northern provinces, I’d recommend that you learn to build something less fire-prone for desert scenarios. It’s worth your time, trust me!
The basic 7×7 design (desert maps)