Define Unit Hydrograph
Hydrograph of one inch or 1cm of precipitation over a drainage basin area (watershed). It is the base hydrograph from which other hydrographs can be determined by ratio when other precipitation quantities( < or > l inch or 1 cm) are known for the same drainage basin area. Also, calculating the "storm", or "surface water", or "overland flow" hydrograph and knowing the precipitation quantity, this method can be used to determine the plot points for a unit hydrograph for the same drainage basin area.