How do you know is a region is a desert

It is an area with an annual rainfall of fewer than 25 centimeters. A desert is defined not by temperature but by the sparse amount of water found in a region. An area with an annual rainfall of fewer than 25 centimeters (9.75 inches) generally qualifies as a desert. Deserts can occur in colder areas too.If you add in a bit of wind to accelerate evaporation at the surface, and the continental regions below become extremely arid owing to the lack of available moisture. Deserts thus become dry.

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These islands/chinampas then provided rich soil for crops with easy access to water. i hope this is correct, im so sorry if its not !