Storing enough water for drinking is easy, say 2 liters a day per person for 100 days is only 200 liters, easily stored in the form of ready bottled water.

Storing enough water for basic domestic needs other than drinking is entirely doable, but takes up a fair bit of space. Say 25 liters a day at a minimum, 50 would be better.
For 100 days that is several tons, fairly easily stored in purpose made tanks, cisterns or barrels.

Storing enough water for a farm or large garden though not actualy impossible is unlikely to be viable due to the costs.
An outdoor reservoir 2 meters deep and with a plan area 10 meters by 10, would store about 200 tons, a great deal by domestic standards but not really much for a farm. Also obtaining enough water to initialy fill a 200 ton store might be problematic in areas that are continually arid.
If however water is plentiful at certain times of year, then filling a 200 ton store from a natural water source is easy with a suitable pump.
Remember that significant volumes of water can be lost to evaportation from uncovered reservoirs.

A combination of water saving, and storage should suffice.
Use a low flush toilet normally, and in times of drought consider an outdoor pit latrine that uses no water at all.

Use a low flow shower head, and try to take short showers.