Groundwater & Basement Water Intrusion in Glendale, AZ

Plumbing and appliances, by a wide margin. Supply lines, water heaters, and refrigerator and washer connections account for most of the interior water in a desert climate, and slab construction means those failures travel under the floor and surface late, often far from the break. The weather contribution is concentrated in a few intense monsoon cells. Knowing where the main shutoff is and how fast it can be reached matters more here than in most climates.

That is caliche. A cemented layer under thin desert soil absorbs very little, so an intense cell produces runoff at close to the full rainfall rate and the streets carry it. An inch of rain behaves like several inches would somewhere with absorbent ground. The annual total tells you how little rain arrives here; it says nothing about what happens to the rain that does.

It can, and it explains otherwise puzzling behavior. Much of this area was leveled and flood irrigated for decades, leaving engineered grades, ditches, and sometimes buried structures under what are now subdivisions. Those features can still route water, and they were built to spread it across fields rather than move it away from houses. Where a lot floods for no apparent reason, the historical land use is worth investigating.