Well, it's not all just desert; that's more in the south. In the middle it's mostly farmland (in fact, rather excellent farmland, thanks to the combination of good soil, plenty of sunshine, temperate weather, and - the recent droughts notwithstanding - decent water availability; there's a reason why California is the primary source of quite a few fruits and vegetables for the world, let alone the US), and in the north it's mostly forest.