WHAT IS STREAM FLOW ANALYSIS?
Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The dischargestream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.
ANALYSIS OF STREAM HYDROGRAPHS
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:
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:
- 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.
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.