You guys are relentless.  Well, the cynical answer is, “Because we couldn’t afford less.” The Los Angeles area locations in which we shot T-2 and T-3 were not available for T-4. We investigated shooting the whole movie out in the real desert (like Lone Pine, where we shot Tremors 1), but the budget would not allow it. So we had to find new land close to LA. The Polsa Rolsa ranch we selected has a lot more vegetation than even the ranch we used in T-2 — so that’s why.

However, a kindly fan has provided a different (and less cynical) answer below:

It could be as simple as a geological shift.  We know there are hot springs in the area, as well as many interesting geological hiccups and burps that the geology students were studying in modern day.  A slight shift could lower the water table in the Perfection area, thus leaving modern day Perfection much drier than it was 100 years ago!

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