hydrograph

Graph showing how the discharge of a river varies with time (generally over a matter of days). By studying hydrographs, water engineers can predict when flooding is likely and take action to prevent it.

A hydrograph shows the lag time, or delay, between peak rainfall and the resultant peak in discharge, and the length of time taken for that discharge to peak. The shorter the lag time and the higher the peak, the more likely it is that flooding will occur. The peak flow is equal to the groundwater flow plus the storm flow. Factors likely to give short lag times and high peaks include heavy rainstorms, steep slopes, deforestation, poor soil quality, and the covering of surfaces with impermeable substances such as tarmac and concrete. Actions taken by water engineers to increase lag times and lower peaks includes planting trees in the drainage basin of a river.