The theme of this year's World Water Day - marked annually on 22 March - is "Nature for Water", which "explores nature-based solutions (NBS) to the water challenges we face in the 21st century" according to the United Nations Environment Programme.The challenges are tough as the UN notes that currently 2.1 billion people "lack access to safely managed drinking water services," while an estimated 1.8 billion "use an unimproved source of drinking water with no protection against contamination from human faeces." Business Standard takes a look at the water challenges faced in India.