Measuring Stream Flow
The volume of water flowing in a stream generally cannot be continuously recorded. In order to calculate the volume of stream flow, a gaging station is established. The gage obtains a continuous record of water level in the stream.
| A stream gaging station can provide a continuous record of stream conditions. |
A mathematical relationship must be established between the water level (stage) and stream flow volume (discharge) at the site. This relationship is called a rating curve. A field hydrologist will directly measure discharge at the gage at different water levels, particularly during flood flows. Using these measurements, a rating curve is developed. This table of related stage/discharge values can be used to estimate the amount of water discharged for any given stage reading.
Some major watersheds contain a continuous stream gaging station to quantify the flow regime other watersheds must extrapolate information from an adjacent watershed. (see map)
Rating curves must be revised if upstream land use changes significantly or if the stream cross-section changes at the gage site. Many gage sites are established where channel geometry is relatively stable such as a bridge or other stream crossing.
In addition to current stream flow, historic stream flow data are required to develop hydrologic time series analysis and if needed, water budgets. Stream flow records for gaged streams are available from the U.S. Geological Survey (USGS). If stream flow data have not been gathered over a sufficient period of record, several methods can be used to estimate hydrology (Bovee et al. 1998; Wurbs and Sisson 1999). Hydrologic simulation models (e.g., HEC-HMS, WMS) use watershed characteristics, precipitation, and runoff patterns to synthesize or extend a streamflow record.
Furthermore, streamflow data from gages in the same region can be used to synthesize runoff patterns for another watershed by establishing a statistical relationship. Accurate synthesis from one river system to another is only feasible if watershed characteristics such as soil, area, topography and precipitation patterns are similar.
Flow Regime Measurements
Scientists have described river flows in detail, identifying five (5) aspects:
- Magnitude – discharge value
- EXAMPLE: How large was the flood? What stage and discharge values were reached?
- Duration – length of time flows remain at the level of interest
- EXAMPLE: How long did the river stay above floodstage?
- Timing - seasonal and diurnal pattern of the event
- EXAMPLE: What time of year did the river flood? What time of day did flows peak?
- Frequency – number of events within a given timeframe
- EXAMPLE: How often did the river reach floodstage last year?
- Rate of change – rate at which water level of interest is obtained
- EXAMPLE: How quickly did the water level rise?
Discharge
Discharge (also known as stream flow, flow or flow rate) is expressed as volume of water over a given time period. A variety of units are used to describe flow from near instantaneous terms such as cubic feet per second (cfs) to long time intervals such as acre-feet per year (afy). Cubic feet per second (cfs) is a measurement of stream flow rate that represents one cubic foot of water moving past a given point in one second; whereas acre feet per year (afy) is the volume of water necessary to cover one acre of surface area to a depth of one foot (43,560 cu ft) that moves past a given point in a year.
Exceedance Values
The exceedance value is the magnitude of discharge over a period of time. Using historic flow records, the Q value reflects the percentage of time stream flow has been found at that Q level. For example, Q50 is average flow; 50% of the time flows have been measured at or above that level and 50% of the time flows have been at or below that level. The Q90 value indicates that 90% of the time, stream flow has been greater than that value. In other words, the stream flow has only been that level or below 10% of the time. Q90 is considered protected low flow level in Minnesota and is used for suspending water appropriation permits.
Hydrologic records of flow regimes are critical for understanding and investigating stream components other than flow. A hydrologic record is needed to assess:
- Habitat changes
- Hydraulic functions
- Water quality factors
- Channel maintenance
- Riparian and valley forming processes
Limitations and Constraints for Using Hydrologic data
There are limitations and constraints to consider when using hydrologic data. One constraint is the availability of adequate hydrologic data (e.g., streamflow and precipitation records). Data that is incomplete in space or time limits assessment of current conditions and prediction of potential change. Further, it should be understood that past records of precipitation or streamflows may not reflect current or future conditions and therefore may have limited applicability. Additionally, global climate change will introduce new uncertainty to extrapolation of past conditions. Changing patterns in precipitation, temperature, prevailing winds and storm events are examples of uncertainties that will need consideration in predicting future hydrologic data.