Analysis of Stream Hydrographs
A hydrograph is the time-series record of water level, water flow or other hydraulic properties, and can be used to gain insights into the relationships between rivers and aquifers. Typically, a stream hydrograph shows the fluctuations in stream flow through time and is a commonly available dataset routinely measured to support the management of water resources. For a gaining stream, where groundwater is contributing to stream flow, analysis of the stream hydrograph can indicate the magnitude and timing of this contribution.
Hydrograph separation is probably the most widely used technique to solve many hydrological problems. This method aims to separate the observed hydrograph into two components of:
- quickflow - the direct response to a rainfall event including overland flow (runoff), lateral movement in the soil profile (interflow) and direct rainfall onto the stream surface (direct precipitation); and
- baseflow - the longer-term discharge derived from natural storages, mostly assumed to be groundwater discharge from the shallow unconfined aquifer.
Analysing the stream hydrograph to separate out the baseflow component provides information on the characteristics of the natural storages feeding the stream. Groundwater discharge from the shallow unconfined aquifer is commonly assumed to be the main contributor to baseflow. For this to be a significant process, the unconfined aquifer needs to be adequately replenished (typically on a seasonal basis), have a shallow watertable that is higher than the stream water level, and have adequate water storage and transmission properties to maintain flow to the stream (Smakhtin, 2001). For a gaining stream, where the underlying aquifer satisfies this criteria and groundwater contributes to stream flow, analysis of the stream hydrograph can indicate the magnitude and timing of this contribution.
However, in certain catchments baseflow may not be dominated by groundwater discharge from the shallow unconfined aquifer. Other storages such as connected lakes or wetlands, snow, glaciers, caverns in karst terrains, or temporary storage within the river bank following the passage of high-flow events (bank storage) can also contribute to the baseflow regime of a stream (Griffiths and Clausen, 1997).
Another complication is that baseflow is also influenced by any water losses from the stream. The hydrographic record essentially represents the net balance between gains to and losses from the stream. These losses include direct evaporation from the stream channel or from any connected surface water features such as lakes and wetlands, transpiration from riparian vegetation, evapotranspiration from source groundwater seepages, leakage to the underlying aquifer, or rewetting of stream bank and alluvial deposits (Smakhtin, 2001). These processes are often aggregated into a transmission loss for the reach of the stream.
Also, water use or management activities can significantly affect the baseflow regime. Many streams have highly modified flows due to the development and use of water resources. Overextraction can mean that streams that were naturally perennial due to prolonged baseflow, can become intermittent. Major regulated systems such as the River Murray have artificially high flows during the summer due to releases to supply irrigation and urban users. Specific activities that can influence baseflow include:
- stream regulation where flow is controlled by infrastructure such as dams, locks or weirs. Releases from surface water storages for downstream users can make up the bulk of streamflow during dry periods. Baseflow analysis should be undertaken in unregulated reaches, or at least the regulated catchment area should be no more than 10% of the catchment area of the streamflow gauge (Neal et al, 2004);
- direct pumping of water from the stream for consumptive uses such as irrigation, urban supply or industry;
- artificial diversion of water into or out of the stream as part of inter-basin transfer schemes;
- direct discharges into the stream, such as from sewage treatment plants, industrial outfalls or mine dewatering activities;
- seasonal return flows from drainage of irrigation areas;
- artificial drainage of the floodplain, typically for agricultural or urban development, which can enhance rapid runoff and reduce delayed drainage;
- changes in land use, such as clearing, reafforestation or changes in crop type, which can significantly alter evapotranspiration rates; and
- groundwater extraction, sufficient to lower the watertable and decrease or reverse the hydraulic gradient towards the stream.
Careful consideration of the overall water budget and management regime for the stream is required before the assumption that baseflow equates to groundwater discharge can be made.
Analysing the baseflow component of the stream hydrograph has had a long history of development since the early theoretical and empirical work of Boussinesq (1904), Maillet (1905) and Horton (1933). Several useful reviews have been written including Hall (1968), Nathan and McMahon (1990), Tallaksen (1995) and Smakhtin (2001) to map this development. The multitude of methods that have evolved can be conveniently categorised into three basic approaches of:
Advantages
- Uses stream flow data that is commonly collected and publicly available.
- Provides information of the variation in groundwater discharge through time.
Disadvantages
- Baseflow separation only applicable for gaining stream conditions.
- Base flow conditions assumed to be entirely groundwater discharge which may not always be valid. Method cannot be applied in rivers that are regulated or have significant diversions or extractions, or have large natural storages such lakes or wetlands.
- Characterises the groundwater discharge regime only at the stream gauging sites. The technique provides information on the temporal changes but not the spatial distribution of groundwater inputs along a stream.
Data Availability
Refer Hydrology Data
Case Studies
Richmond flow percentiles
Murray-Darling Basin baseflow study
Relevant Links
ASTHyDA Project : European project focussing on tools for assessing low surface water and groundwater flows
US Geological Survey HYSEP : hydrographic separation program based on the fixed-interval, sliding-interval and local-minimum methods
USGS Geological Survey PART: program for estimating baseflows using a stream partitioning method
US Geological Survey PULSE:analytical solutions to estimate groundwater discharge and baseflow of stream based on recession-curve-displacement method
US Geological Survey RECESS: method for analysing streamflow recession to determine the master recession curve
US Geological Survey RORA: program for estimating groundwater recharge using the recession-curve-displacement method
US Bureau of Reclamation BFI: software for determining a Base Flow Index using a local minimum approach
CEH Wallingord Low Flows 2000: Decision support tool for estimating low flows in the United Kingdom
CRC Catchment Hydrology RAP : River Analysis Package including baseflow analysis
References
Boussinesq J, 1904. Recherches theoretique sur l'ecoulement des nappes d'eau infiltrees dans le sol et sur le debit des sources. J. Math. Pure Appl. 10 (5): 5-78.
Griffiths GA and Clausen B, 1997. Streamflow recession in basins with multiple water storages. Journal of Hydrology 190, 60-74.
Hall FR, 1968. Base flow recession - a review. Water Resources Research 4(5): 973-983.
Horton RE, 1933. The role of infiltration in the hydrological cycle. Transactions of the American Geophysical Union 14:446-460.
Maillet E, 1905. Essais d'hydraulique souterraine et fluviale. Librairie Sci. Hermann Paris, 218pp.
Nathan RJ and McMahan TA, 1990. Evaluation of automated techniques for baseflow and recession analysis. Water Resources Research. 26(7):1465-1473.
Neal BP, Nathan RJ, Evans R, 2004. Survey of baseflows in unregulated streams of the Murray-Darling Basin. Proceedings 9th Murray-Darling Basin Groundwater Workshop, Bendigo
Smakhtin VY, 2001. Low flow hydrology: a review. Journal of Hydrology 240:147-186.
Tallaksen LM, 1995. A review of baseflow recession analysis. Journal of Hydrology 165, 349-370.