What is water scarcity and what are its main causes?

Water scarcity occurs when the availability of water is less than 1000 m3 per person per day, according to Swedish expert.
The causes for water scarcity are :
(i) Amount of precipitation.
(ii) Size of population.
(iii) Unequal access to water among different social groups.
(iv) Uneven distribution of water in terms of space and time.