Tayammum should be performed instead of Wudhu or Ghusl in the following seven circumstances:
When it is not possible to procure sufficient water for performing Wudhu or Ghusl.
655. If a person happens to be in a populated area he should make his best efforts to procure water for Wudhu or Ghusl till such time that he loses all hope. And if he happens to be in a desert, he should search for water on the way or at nearby places. And if the land is uneven, or densely wooded, and it is difficult to walk, he should search for water in all the four directions for a distance covered by one or two flings of an arrow. (A fling is equal to about two hundred steps).
656. If out of the four directions, some are even and others are uneven, one should search for water in the even direction to the extent of two arrow flings, and on the side which is uneven to the extent of one arrow fling.
657. It is not obligatory for a person to search for water in the direction where he is sure that water is not available.
658. If the time left for Namaz is not short, and if he is sure or feels sure that water is available at a farther place, he should go there to procure water, provided that going there is not extremely difficult, and that the distance is not unusually long. And if he has mere suspicion about water being there, then it is not necessary for him to go.
659. It is not necessary that a person should go himself in search of water. He can send a reliable person for this purpose. And it is sufficient if one person goes on behalf of many.
660. If a person feels that there might be some water in the provision he carries or at the place of encampment or even in the convoy, he should search for it thoroughly, till he is satisfied that there is no water or he becomes hopeless.