Tayammum is the obligatory alternative instead of ghusl or wudhu in certain situations, like:
1. When there is not sufficient water for either ghusl or wudhu.
2. When there is sufficient water, but there may be a barrier to getting, or using it.
3. When there may be a possibility that using water for ghusl or wudhu might deplete your supply of water, so much so that there may not be sufficient quantity of water to quench one’s thirst, including members of your family, relatives, community and even the livestock.
4. When there is not enough time to perform ghusl and pray an obligatory prayer in full and within its time.
5. When obtaining water could lead to putting oneself in an untenable situation.
6. When water has changed in both colour and property to the extent that you find yourself in real in a state of revulsion when using it.
7. When your responsibility is to use the limited quantity of water to remove the najasah from the mosque.
8. When you fear you become sick if you use water for ghusl or wudhu, or if using water could worsen your ill health. However, this should be the case when the matter does not come under the subject of wiping (mas’h) over a jabirah (lit. splint: a generic name used for any form of dressing any part of the body after injury or illness), especially when there is a risk of worsening the condition of the injury or ailment if the dressing is removed, and the part of the body is not among that which is covered by the ruling concerning.