Hydrographs

A hydrograph plots discharge at a point on a stream against time. A storm hydrograph has a characteristic shape: a rising limb as runoff from the catchment arrives, a peak, and a longer recession limb as the catchment drains. Its shape reflects both the storm and the catchment. Small, steep, compact catchments give sharp, early peaks; large, flat or elongated ones give lower, later and broader ones. Urbanisation sharpens peaks.

A hydrograph is made of direct runoff, the quick response to the storm, sitting on top of base flow, the steady contribution from groundwater. Separating them is the first step in analysis. Simple methods draw a straight line from the start of rise to a point on the recession a fixed number of days after the peak, or extend the pre-storm recession under the peak; there is no unique answer, but consistency matters.

Key time parameters are the lag time (from centroid of rainfall to the peak), time of concentration (time for water from the most remote part of the catchment to reach the outlet), time to peak and base time. Rainfall excess, the part that becomes direct runoff, is found by subtracting losses using a φ-index (a constant loss rate) or infiltration equations. The area under the direct runoff hydrograph, divided by catchment area, equals the depth of rainfall excess, a useful check on any analysis.

Gauging stations that record water level continuously convert it to discharge using a rating curve, checked by periodic current-meter or acoustic Doppler measurements.