Tank irrigation accounts for more than one-third of total irrigated area in Andhra Pradesh, Karnataka and Tamil Nadu states in India. Tank water supplies fluctuate randomly from year to year and within a year. Using 40 years of rainfall data, it was estimated that in five out of 10 years, tanks will be experiencing deficient supply, in three years the tank will fail and in two years the tank will have full supply. The poor performance of the tanks resulted in heavy dependence on groundwater supplementation.