In the period 1990–2006, the decrease in water consumption of Tallinn, the capital of Estonia, determined both the input and output of the water balance of Lake Ülemiste, the drinking water reservoir of the city as well as its hydraulic retention time (HRT). In 2006, the city's water consumption accounted for only a quarter of that in 1990.The role of regulated inflow in the water balance decreased and that of catchment run-off increased. Lake HRT increased four-fold. Before the 1990s, Lake Ülemiste resembled a river, now it is more lake-like. Changes in the water regime were correlated with the decline in concentrations of organic matter and chloride suggesting a likely causal relationship between them. Managerial challenges turned on finding new functions for upstream reservoirs and on the opportunity to refill the lake with better quality raw water.
Keywords: city water consumption, drinking water reservoir, hydraulic retention time, operating rules, water balance, water quality