Important Definitions to understand in Hydrograph Theory

• HYDROGRAPH: Hydrograph is the graph of Flow vs. Time. The unit of flow could be cfs, m3/s, etc and the unit of time could be hours, days, months etc. The area under a hydrograph between two points (time interval) represents the volume of water passing through point of interest in that time interval. The total area under the hydrograph represents total runoff volume for that storm event.
• DIRECT RUNOFF HYDROGRAPH: Direct runoff hydrograph is the hydrograph excluding the contribution from the groundwater flow or baseflow.
• BASEFLOW It is the groundwater discharge into the streams. Baseflow is very important during dry period. It provides the very important flow to sustain the stream ecology and biology during rainfree time period.
• TIME OF CONCENTRATION: It is the time required by the entire drainage area to contribute to the runoff. If the rainfall duration is not enough for the entire drainage to contribute to the flow at a point of interest, then tc can be estimated as time from the from the start of excess rainfall to the inflection point on the recession limb.
• LAG TIME: It is the time from center of mass of excess rainfall to the center of mass of direct runoff hydrograph. Some variation in this interpretion is common, but the whole point is to interpret the position of runoff hydrograph relative to the rainfall causing the runoff. One estimate of lag time as provided by NRCS is that: tl = 0.6*tc
• SHEET FLOW: It is the flow of water over the plane surface as a more or less uniformly thick film or sheet of water.
• UNIT HYDROGRAPH: Unit Hydrograph is the hydrograph of one inch of excess rainfall distributed uniformly over the drainage area for a given duration 'D'. This is known as Unit Hydrograph for duration D.