Survival skills
A human being can survive an average of three to five days without the intake of water. The issues presented by the need for water dictate that unnecessary water loss by perspiration be avoided in survival situations. The need for water increases with exercise.
Nirjala Ekadashi
While on other ekadashis abstinence of food is observed, on Nirjala Ekadashi, an absolute fast is observed, without partaking even water. The water-less fast is considered extremely difficult to follow as the day falls in the hot Indian summer and thus, it is deemed as very pious austerity. The fast is observed 24 hours from sunrise on Nirjala Ekadashi to sunrise the next day. Some observe it from sunrise to sunset. On the day before Nirjala Ekadashi, the devotee performs the evening prayer (Sandhyavandanam) and takes only one meal, without rice - as rice eating is prohibited. The devotee is however permitted to have a single tiny drop of water as part of the "Achamana" purification ritual. Water more than that equals breaking the vow.