Of all the non-structural measures, flood forecasting and warning is the most widely accepted and has been used since the latter half of the 20th century. It supplements almost all other structural as well as non-structural measures. Flood forecasting involves estimating when a flood is likely to cause damage or loss of life, what its magnitude will be (usually in terms of its maximum stage at a given location) and how long it will last. Flood forecasts are formulated and issued with a certain lead-time allowing concerned authorities to take preventative and emergency measures. Authorities can respond appropriately with dam operations, opening and closing the gates of various flood management structures, anticipatory releases to increase reservoir storage capacity, etc. The effectiveness of a flood forecast and warning system is as much a function of the accuracy, timeliness and outreach of the forecast as of the response behavior and preparedness. Inflow forecasts for reservoirs, detention basins, bypass channels, etc. play an important role in flood peak alleviation. It is important to draw reservoir operation guidelines covering various scenarios and effect managed flood releases based on these forecasts. (WMO 2006a)