Purity: General Rule for Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Tayammum must be performed in place of wudu or ghusl in seven situations [which are set out below]:
Not having water
Not having access to water
Using water is harmful
Hardship (haraj) and excessive difficulty (mashaqqah)
Needing water to quench thirst
Performing wuḍūʾ or ghusl conflicts with another legal responsibility that is more important or just as important
Shortage of time
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)