These warnings give people time to move equipment and stored goods and reinforce flood protection measures. Forecasts often are based on complex mathematical models that use data from river gauge readings from which hydrologists estimate flow rate. Water from other streams and reservoirs in the flood path also are considered in forecasting downstream flooding. For ungauged watersheds, hydrologists make predictions considering watershed area, length of stream channels, soil types, slopes and land uses. Mapping flood-prone areas provides fair warning of the risks of building in such places and should discourage unwise land use.
Communities use such maps to plan and design public structures and highways.
These maps also are used in drafting zoning and building regulations to prevent land use that could increase the risk of flooding. The federal government uses the maps to determine those eligible for federal flood insurance. Flood hazard maps may show several zones according to estimated frequency of flooding. Hydrologists use statistical analyses of streamflow data and computer models of the effects of various combinations of meteorological and hydrological conditions to prepare these maps. Aerial photographs and soil maps also are useful for defining flood-prone areas.