Natural wetlands are disappearing globally due to a multitude of factors. In contrast, manmade artificial wetlands (irrigation tanks) have proliferated due to agricultural expansion. Constructing irrigation tanks is an ancient practice widespread in India, and elsewhere around the world. The semiarid plains of south India have the largest network of irrigation tanks and support diverse flora and fauna, especially waterbirds. However, research on waterbird diversity and abundance has been restricted to a few specific water bodies, and landscape scale data is lacking. In this study we analyse the trends in waterbird abundance based on total counts of birds done in January each year by over 100 volunteers across twelve years from 2011 onwards in 133 irrigation tanks of the Tamiraparani river basin, south India. We performed similar counts in the dry season for 4 years. We developed a novel approach of using total counts and abundance of species showing declining trends to prioritize tanks for conservation in this region. Trend analysis indicated insignificant variation in species richness, while total waterbird abundance declined marginally. Fourteen out of the 50 species showed declines, of which 12 were residents and 2 were winter migrants. At the family level ducks, jacanas, terns, wagtails and waders showed a significant negative trend and guild analysis indicated a decline in the abundance of species using wetland vegetation for food and foraging. We also observed that large reservoirs in the river basin serve as a refuge for waterbirds during periods of severe drought. Some tanks in summer provide vital foraging resource for birds during water scarcity. We prioritised a cluster of seven tanks in summer and winter respectively for future conservation efforts.