Desert tortoises drink at temporary rain pools and can take in a large fraction of their body mass in one session. The water is stored in a capacious urinary bladder as very dilute urine. Through the dry season the animal reabsorbs water from it, allowing nitrogenous waste to concentrate to levels that would harm most vertebrates.
That physiology explains a standard field instruction. A handled or frightened tortoise often voids its bladder, and in a dry year the loss can be fatal, so wildlife agencies advise leaving tortoises alone unless one is on a road, in which case it is moved a short distance in the direction it was already travelling.
The rest of the year is spent out of sight. Individuals occupy burrows for most of each day and much of the year, which is why casual counts underestimate how many are present.