Water scarcity can be because of a lot of people depending on the same source of water, so the level of water will come down quickly. So, overpopulation is an issue which leads to over utilisation of water.
Very less rainfall or uneven rainfall throughout the year can be another possible reason for water scarcity.
Water pollution is another reason behind water shortage. The water may be so polluted that it is impossible to use it no matter how much water is present.