Hydrograph
Encyclopedia
A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
It 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.
Types of hydrograph can include:
hydrology, a hydrograph is a time record of the discharge
of a stream
, river
or watershed
outlet. Rainfall and/or snowmelt is typically the main driver of watershed discharge; a hydrograph records how a watershed responds to these and other drivers of watershed hydrology.
A watershed's
response to rainfall depends on a variety of factors which affect the shape of a hydrograph:
The separation of baseflow from direct runoff in a hydrograph is often of interest to hydrologists, planners, and engineers, as it aids in 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.
That said, 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.
A UH is specific to particular watershed, and specific to a particular length of time corresponding to the duration of the effective rainfall. That is, the UH is specified as being the 1-hour, 6-hour, or 24-hour UH, or any other length of time up to the time of concentration of direct runoff at the watershed outlet. Thus, for a given watershed, there can be many unit hydrographs, each one corresponding to a different duration of effective rainfall.
The UH technique provides a practical and relatively easy-to-apply tool for quantifying the effect of a unit of rainfall on the corresponding runoff from a particular drainage basin. UH theory assumes that a watershed's runoff response is linear and time-invariant, and that the effective rainfall occurs uniformly over the watershed. In the real world, none of these assumptions are strictly true. Nevertheless, application of UH methods typically yields a reasonable approximation of the flood response of natural watersheds. The linear assumptions underlying UH theory allows for the variation in storm intensity over time (i.e., the storm hyetograph) to be simulated by applying the principles of superposition and proportionality to separate storm components to determine the resulting cumulative hydrograph. This allows for a relatively straightforward calculation of the hydrograph response to any arbitrary rain event.
An instantaneous unit hydrograph is a further refinement of the concept; for an IUH, the input rainfall is assumed to all take place at a discrete point in time (obviously, this isn't the case for actual rainstorms). Making this assumption can greatly simplify the analysis involved in constructing a unit hydrograph, and it is necessary for the creation of a geomorphologic instantaneous unit hydrograph.
The creation of a GIUH is possible given nothing more than topologic data for a particular drainage basin. In fact, only the number of streams of a given order, the mean length of streams of a given order, and the mean land area draining directly to streams of a given order are absolutely required (and can be estimated rather than explicitly calculated if necessary). It is therefore possible to calculate a GIUH for a basin without any data about stream height or flow, which may not always be available.
), a hydrograph is a record of the water level (the observed hydraulic head
in wells screened across an aquifer
).
Typically, a hydrograph is recorded for monitoring of heads in aquifers during non-test conditions (e.g., to observe the seasonal fluctuations in an aquifer). When an aquifer test
is being performed, the resulting observations are typically called drawdown
, since they are subtracted from pre-test levels and often only the change in water level is dealt with.
It can also refer to a graph showing the volume of water reaching a particular outfall
Outfall
An outfall is the discharge point of a waste stream into a body of water; alternatively it may be the outlet of a river, drain or a sewer where it discharges into the sea, a lake or the like....
, or location in a sewerage network, graphs are commonly used in the design of sewerage
Sewerage
Sewerage refers to the infrastructure that conveys sewage. It encompasses receiving drains, manholes, pumping stations, storm overflows, screening chambers, etc. of the sanitary sewer...
, more specifically, the design of surface water
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
sewerage systems and combined sewers.
Terminology
The discharge is measured at a certain point in a river and is typically time variant.- Rising limb: The rising limb of hydrograph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event
- Recession limb: The recession limb extends from the point of inflection at the end of the crest segment to the commencement of the natural groundwater flow (baseflowBaseflowBaseflow is the portion of streamflow that comes from "the sum of deep subsurface flow and delayed shallow subsurface flow"...
). It represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. - Peak discharge: the highest point on the hydrograph when the rate of discharge is greatest
- Lag time: the time interval from the center of mass of rainfall excess to the peak of the resulting hydrograph
- Time to peak: time interval from the start of the resulting hydrograph
- Discharge: the rate of flow (volume per unit time) passing a specific location in a river or other channel
Types of hydrograph can include:
- Storm hydrographs
- FloodFloodA flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
hydrographs - Annual hydrographs aka regimesRiver regimeRiver regime can describe one of two characteristics of a reach of an alluvial river:* The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration, and drainage basin characteristics * A series of characteristic power-law...
- Direct RunoffRunoffRun-off or runoff may refer to:* Surface runoff, the flow of water, from rain, snow melt, or other sources, over land* Runoff model , a mathematical model describing the rainfall-runoff relations of a rainfall catchment area or watershed...
Hydrograph - Effective Runoff Hydrograph
Surface water hydrography
In surface waterSurface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
hydrology, a hydrograph is a time record of the discharge
Discharge (hydrology)
In hydrology, discharge is the volume rate of water flow, including any suspended solids , dissolved chemical species and/or biologic material , which is transported through a given cross-sectional area...
of a stream
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
, river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
or watershed
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
outlet. Rainfall and/or snowmelt is typically the main driver of watershed discharge; a hydrograph records how a watershed responds to these and other drivers of watershed hydrology.
A watershed's
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
response to rainfall depends on a variety of factors which affect the shape of a hydrograph:
- Watershed topographyTopographyTopography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
and dimensions - GeologyGeologyGeology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
(e.g. bedrockBedrockIn stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
permeability and transmissivity) - The area of a basin receiving rainfall
- Land use and land cover(e.g. vegetation types and density, impervious areas)
- Drainage densityDrainage densityDrainage density is the total length of all the streams and rivers in a drainage basin divided by the total area of the drainage basin. It is a measure of how well or how poorly a watershed is drained by stream channels...
- Duration of rainfall and precipitation intensity and type
- EvapotranspirationEvapotranspirationEvapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
rates - River geometrics
- The seasonSeasonA season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
- VegetationVegetationVegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
type and cover - River conditions (e.g. dams)
- Initial conditions (e.g. the degree of saturation of the soil and aquifers)
- SoilSoilSoil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
permeability and thickness - Storage opportunities in the drainage network (e.g. lakes, reservoirs, channel storage capacity)
Baseflow Separation
A stream hydrograph is commonly conceptualized to include a baseflow component and a direct flow component. The former represents the relatively steady contribution to stream discharge from groundwater return flow, while the latter represents the additional streamflow contributed by surface runoff, typically as the result of a storm event.The separation of baseflow from direct runoff in a hydrograph is often of interest to hydrologists, planners, and engineers, as it aids in 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.
That said, 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
A unit hydrograph (UH) is the hypothetical unit response of a watershed (in terms of runoff volume and timing) to a unit input of rainfall. It can be defined as the direct runoff hydrograph (DRH) resulting from one unit (e.g., one cm or one inch) of effective rainfall occurring uniformly over that watershed at a uniform rate over a unit period of time. As a UH is applicable only to the direct runoff component of a hydrograph (i.e., surface runoff), a separate determination of the baseflow component is required.A UH is specific to particular watershed, and specific to a particular length of time corresponding to the duration of the effective rainfall. That is, the UH is specified as being the 1-hour, 6-hour, or 24-hour UH, or any other length of time up to the time of concentration of direct runoff at the watershed outlet. Thus, for a given watershed, there can be many unit hydrographs, each one corresponding to a different duration of effective rainfall.
The UH technique provides a practical and relatively easy-to-apply tool for quantifying the effect of a unit of rainfall on the corresponding runoff from a particular drainage basin. UH theory assumes that a watershed's runoff response is linear and time-invariant, and that the effective rainfall occurs uniformly over the watershed. In the real world, none of these assumptions are strictly true. Nevertheless, application of UH methods typically yields a reasonable approximation of the flood response of natural watersheds. The linear assumptions underlying UH theory allows for the variation in storm intensity over time (i.e., the storm hyetograph) to be simulated by applying the principles of superposition and proportionality to separate storm components to determine the resulting cumulative hydrograph. This allows for a relatively straightforward calculation of the hydrograph response to any arbitrary rain event.
An instantaneous unit hydrograph is a further refinement of the concept; for an IUH, the input rainfall is assumed to all take place at a discrete point in time (obviously, this isn't the case for actual rainstorms). Making this assumption can greatly simplify the analysis involved in constructing a unit hydrograph, and it is necessary for the creation of a geomorphologic instantaneous unit hydrograph.
The creation of a GIUH is possible given nothing more than topologic data for a particular drainage basin. In fact, only the number of streams of a given order, the mean length of streams of a given order, and the mean land area draining directly to streams of a given order are absolutely required (and can be estimated rather than explicitly calculated if necessary). It is therefore possible to calculate a GIUH for a basin without any data about stream height or flow, which may not always be available.
Subsurface hydrology hydrograph
In subsurface hydrology (hydrogeologyHydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
), a hydrograph is a record of the water level (the observed hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
in wells screened across an aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
).
Typically, a hydrograph is recorded for monitoring of heads in aquifers during non-test conditions (e.g., to observe the seasonal fluctuations in an aquifer). When an aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
is being performed, the resulting observations are typically called drawdown
Drawdown
Drawdown has four distinct meanings:* Drawdown , a lowering of a reservoir or a change in hydraulic head in an aquifer, typically due to pumping a well.* Drawdown , decline in the value of an investment, below its all-time high....
, since they are subtracted from pre-test levels and often only the change in water level is dealt with.
See also
- Surface waterSurface waterSurface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
- Runoff model (reservoir)Runoff model (reservoir)A runoff model is a mathematical model describing the rainfall - runoff relations of a rainfall catchment area, drainage basin or watershed. More precisely, it produces the surface runoff hydrograph as a response to a rainfall hydrograph as input...
- HydrogeologyHydrogeologyHydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
- Aquifer testAquifer testAn aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
- BaseflowBaseflowBaseflow is the portion of streamflow that comes from "the sum of deep subsurface flow and delayed shallow subsurface flow"...
- Hydrograph separation
External links
- The U.S. Geological Survey (USGS) offers real-time streamflow data for thousands of streams in the United States.
- SCS Dimensionless Unit Hydrograph.