Streamflow is the flow rate or discharge of water at a specified location on a natural stream channel. In general terms, streamflow is the residual of the precipitation that falls on the area upstream of the selected stream channel reference point. The dominantly horizontal character of streamflow establishes its roles in climate of the first kind and the terrestrial branch of the hydrologic cycle as the transfer function for returning excess continental liquid moisture to the oceans. Streamflow at any particular time integrates all of the hydroclimatic processes and storages occurring upstream relative to the selected reference point.
Modern methods for measuring streamflow developed in the nineteenth century are described in Chapter 1. The nature of streamflow measurements is responsible for streamflow data being very expensive, difficult to obtain, and commonly restricted to selected rivers and streams (Ward and Elliot, 1995). An additional consideration is that many gauged streams are affected to some extent by reservoir regulation and water diversions. Although the number of hydrological stations expanded greatly at the international level in the closing decades of the twentieth century, all continents other than Europe and North America have relatively low density hydrological networks. Europe has a high density hydrological network that supports the longest continuous records. Streamflow was recorded on many large European rivers in the nineteenth century, and in Britain the longest continuous flow measurements started on the River Lea in 1879 (Arnell, 1996). However, the earliest continuous stream-flow records for North American rivers begin around 1900. An extensive network of streamflow measurements for the United States is maintained by the U.S. Geological Survey in cooperation with state water agencies. Many national computer archives of hydrologic data can be accessed through the Hydrological Operational Multipurpose System of the WMO (Mosley and McKerchar, 1993).
The hydrologic balance (Equation 2.13) illustrates that the single parameter unique to the hydrologic cycle is runoff. Runoff is that part of precipitation that eventually becomes the water in river and stream channels, but it is generated over an area. Consequently, runoff is never measured directly and it is unlikely that it ever will be measured directly. Rather runoff is determined using a series of intermediate steps. First it must be recognized that runoff refers to a depth of water spread evenly over a specified area. This is the major basis for explaining why runoff is not measured directly. This point may also aid in clarifying why confusion exists over the difference between runoff and stream discharge or streamflow. The water in a river or stream channel is both runoff and discharge. Streamflow is runoff from an area contributing water to the stream channel, and this area is called the watershed, drainage area, drainage basin or catchment. Streamflow is also the stream discharge at a specific point on the river or stream. The runoff phenomenon is treated in detail in Chapter 6. The quantification of streamflow and runoff is addressed here.
Stream discharge is a volumetric measure for a specific time interval and is commonly reported as m3s_1. For periods longer than a day, the meaning becomes less precise. Discharge for longer periods is expressed as km3.
Quantification of runoff is aided by the natural collection system represented by the watershed or the area contributing the water that passes through a given river or stream cross-section. This area is commonly topographically defined, and the boundary delimits a watershed divide. Delineation of the watershed area is important for defining the paths and rates of water movement between the time it arrives as precipitation on the watershed and when it arrives at the watershed outlet. A complex array of space and time variations in physical watershed variables is integrated by the runoff process. The result is that runoff provides the space and time averaging that is difficult to achieve when individual hydroclimatic parameters are measured and averaged over an areal unit. Runoff is the volume of water quantified by streamflow, but it is also the depth of water that would occur by spreading the streamflow volume evenly over the area upstream of the stream gauge.
Streamflow is the only phase of the hydrologic cycle for which reasonably accurate measurements are made of the volumes involved. However, stream-flow at a given location is seldom measured in a single, relatively simple manner, and the measurement procedures are subject to introduced errors over time. For most rivers and streams, there is a specific relationship between the flow at a particular location along the channel and the water level in the channel. This relationship is due to the physical characteristics of the watershed and the stream channel. Measurement errors are commonly related to changes in these physical characteristics. Nevertheless, the assumed relationship between the water level and the magnitude of discharge is the underlying basis for determining streamflow. Typically, streamflow data are available for a much shorter duration than climate data.
In practice, three steps followed in developing streamflow data are measuring the water level, measuring stream discharge, and identifying the relationship between water level and discharge. River stage refers to the water-surface elevation or water level at a point along a stream relative to an arbitrary datum or fixed reference point of known elevation. The principal devices for determining the water level are the staff gauge and a variety of mechanical instruments for measuring, recording, and transmitting water-level data (Mosley and McKerchar, 1993; Herschy, 1997). The vertical staff gauge is the simplest type of manual gauge, and it represents the river stage concept clearly (Fig. 4.16). The gauge is fixed to a bridge pier or piling and has a graduated scale that includes all possible water levels that may occur at the location. An observer determines visually the water level at the time of observation. This method is most appropriate for large rivers where the river stage changes slowly. Since observations are done only once a day, a major disadvantage of the manual gauge is that the water level may change significantly between gauge observations.
Recording gauges overcome the problem of water-level changes between observations by providing a continuous water-level record. Both float-actuated and pressure-actuated recording gauges can be installed in a shelter over a well that is connected by open pipes to the river or stream. Water-stage fluctuations in the channel coincide with the water level in the stilling well, and a continuous water-level record is produced. Modern telemetry permits real-time retrieval of data. Figure 4.17 portrays daily streamflow for a large river at the beginning of the snowmelt season.
Stream discharge is the basic measurement for a watershed, but discharge is a derived term expressing the volume rate of water movement past a given point
1-May 11-May 21-May 31-May 10-Jun 20-Jun 30-Jun
Fig. 4.17. Daily streamflow for the Columbia River at the International Boundary for two months during the 2006 high flow season. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)
with respect to time. The actual measurements involved in determining stream discharge are stream velocity and the cross-section area of the stream through which the water is passing. Stream velocity is the linear rate of water movement, but the highly variable conditions of streamflow prevent the adoption of a universal velocity measurement method. However, various mechanical current meters are the most widely used method for measuring stream velocity. The stream cross-section at the point where velocity is measured defines the area for deriving discharge. Therefore, stream discharge (Q) is calculated as where v is stream velocity and Ac is the cross-sectional area of flow.
For all but the smallest streams, multiple measurements of stream velocity and a corresponding cross-section area are needed. The river cross-section is divided into a number of predetermined equal-area subsections ranging from 20 to more than 30 depending on the physical characteristics of the stream channel and the measurement method employed. No subsection should account for more than 5% of the total discharge. Therefore, the spacing of the subsections will be closer together where the flow is fast and deep and farther apart where flow is slow and shallow.
Cross-sections are negotiated by wading, by specially constructed cable ways, by boat, or from bridges. The flow in each subsection is measured with a current meter and a sounding is taken to determine the water depth. Because of expected velocity distributions with depth across a stream channel, velocities are usually measured at 0.2 and 0.8 of the total depth in each river subsection when the depth exceeds 0.76 m (Mosley and McKerchar, 1993). The mean of these two velocities
closely approximates the average velocity in a vertical section in most free-running streams. When only one measurement is made, the velocity is measured at 0.6 of the subsection depth. Total discharge is the sum of the individual subsection discharges. Alternative discharge measurement methods are described by Moseley and McKerchar (1993) and Herschy (1997).
The direct measurement of stream discharge is difficult and time consuming. However, a plot of measured discharge against water level at the time of discharge measurement usually defines a smooth curve known as the stage-discharge relation or a rating curve. The rating curve and a continuous waterlevel record at the gauge serving as the basis for the curve permits computation of a continuous streamflow record. Periodic discharge measurements are employed to verify that the stage-discharge relation has not changed. The rating curve can be altered by changes in the stream bed, stream velocity or aquatic vegetation. It is critical to recognize that discharge is an instantaneous measure that is related to a specific day and hour. A continuous record of the river stage permits quantification of discharge for periods as long as 24 hours.
Streamflow quantification is a necessary first step in determining the depth of runoff. The volumetric expression of stream discharge represents both streamflow and runoff. Recall that runoff is the volume of water produced by a specific drainage area. Runoff and streamflow are both expressed in km3, but runoff is also expressed in cm or mm depth over the drainage area. Consequently, the computation of stream discharge produces the volumetric expression of runoff, but runoff expressed as water depth over the drainage basin requires an additional computation.
Runoff in cm (Rcm) at the gauge site is calculated by
where Qis discharge in km3, Ad is the area above the streamflow measurement point in square kilometers (km2), and 0.00001 is the km3 of water produced by 1 cm of water covering 1 km2. Runoff expressed in cm or mm has the advantage that it is comparable to the commonly used measures of precipitation and evapotranspiration.
The streamflow response to precipitation at varying time scales is determined by the manner in which watershed processes partition precipitation into runoff and streamflow. The varied nature of these individual processes is examined in Chapter 6. Estimates of the quantity and volume of runoff are derived using models and equations intended to represent these processes and transform precipitation into a quantitative description of the stream response. The underlying basis for the models and estimating equations is the mass conservation of water expressed in the hydrologic balance (Equation 2.13). Watershed models are discussed in Section 6.15.
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