A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in units of cubic meters per second (m³/s) or cubic feet per second (cfs). Hydrographs often relate changes of precipitation to changes in discharge over time. The term can also refer to a graph showing the volume of water reaching a particular outfall, or location in a sewerage network. Graphs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers.
Terminology Other related terms include:
Approach Segment the river flow before the storm (antecedent flow). Rising limb The rising limb of the hydrograph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event. Peak discharge the highest point on the hydrograph when the rate of discharge is greatest. Recession (or falling) limb The recession limb extends from the peak flow rate onward. The end of stormflow (a.k.a. quickflow or direct runoff) and the return to groundwater-derived flow (base flow) is often taken as the point of inflection of the recession limb. The recession limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. Lag-1 autocorrelation method to compare streamflow data to itself by shifting or "lagging" initial discharge dataset 1 time unit. A Lag-10 would mean the initial data is shifted 10 days, then is compared to an unshifted version of the data. Not to be confused with lag time. Lag time the time interval from the maximum rainfall to the peak discharge. Time to peak time interval from the start of rainfall to the peak discharge. Time of concentration the time from the end of the precipitation period to the end of the quick–response runoff in the hydrograph.
Types Types of hydrographs include:
Stream discharge hydrographs Stream stage hydrographs Precipitation hydrographs Storm hydrographs Flood hydrographs Annual hydrographs a.k.a. regimes Direct Runoff Hydrograph Effective Runoff Hydrograph Raster Hydrograph Lag-1 Hydrograph Storage opportunities in the drainage network (e.g., lakes, reservoirs, wetlands, channel and bank storage capacity)
Baseflow separation A stream hydrograph is commonly determining the influence of different hydrologic processes on discharge from the subject catchment. Because the timing, magnitude, and duration of groundwater return flow differs so greatly from that of direct runoff, separating and understanding the influence of these distinct processes is key to analyzing and simulating the likely hydrologic effects of various land use, water use, weather, and climate conditions and changes. However, the process of separating “baseflow” from “direct runoff” is an inexact science. In part this is because these two concepts are not, themselves, entirely distinct and unrelated. Return flow from groundwater increases along with overland flow from saturated or impermeable areas during and after a storm event; moreover, a particular water molecule can easily move through both pathways en route to the watershed outlet. Therefore, separation of a purely “baseflow component” in a hydrograph is a somewhat arbitrary exercise. Nevertheless, various graphical and empirical techniques have been developed to perform these hydrograph separations. The separation of base flow from direct runoff can be an important first step in developing rainfall-runoff models for a watershed of interest—for example, in developing and applying unit hydrographs as described below.
Unit hydrograph
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