To accommodate the rapid human population growth, the water supply should increase by

20% in 15 years.

Explanation:

As the population of humans has grown to over an exponential level the issues of the fresh and clean drinking water is becoming a problems day by day as many place on earth that considered to be the storehouse of the fresh waters are depleting fast and the due to the rise in the global demands and temperature over the oceans the melt of the glaciers and the rise of seawater.

20% over a 15-year period