Is it permissible to perform istinja' from urine and feces with very little water, and is there a specific limit to the scarcity of water used in istinja'?

Istinja' is the purification of the site of impurity with water, or with a tissue and the like, or with both together, which is preferable. If one confines oneself to water alone, it is obligatory to remove the impurity itself and its traces until the area returns to its original state. The determining factor is the removal of the impurity, not the scarcity or abundance of water.