Assessing the National Streamflow Information Program (2004) / Chapter Skim
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4 Streamflow Network Design
4 Streamflow Network Design
Pages 68-99
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From page 68... ...
and elsewhere have relied largely on statistical methods, most commonly based on the standard error in estimating regional discharge at ungaged sites. Although statistical procedures offer numerical precision for network design supporting regional hydrologic estimation, these approaches do not support the many other goals and uses of site-specific streamflow data.
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From page 69... ...
Measurement nodes in the stream network provide estimates of fluxes or concentrations of particulate and dissolved constituents. Streamgage networks may be driven by the need for information at a specific location, such as concentrations or fluxes where a river enters a waterbody or crosses an in-ternational boundary, or a critical flood warning site.
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From page 70... ...
in designing a streamgage network. An important class of management problems requires streamflow data from gages that sample "representative" locations, to support regional modeling, estimation, and trend detection.
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From page 71... ...
Thus, the most appropriate role for these methods for NSIP is supporting the analysis of incremental refinements to local and regional hydrologic networks, within the broader context of the NSIP network design. Within this formality, distinct variations of this decision problem have been described and applied in network reduction, network expansion, and network refinement.
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From page 72... ...
All of these approaches require continuous, well-defined information metrics that can be expressed as a function of the number and/or location of gages. In well-defined networks with limited objectives, statistical approaches for network design can be used to evaluate incremental decisions to add or eliminate individual gages within a local gage network serving narrow, well-defined goals, such as estimating flows at ungaged sites.
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From page 73... ...
Streamflow Network Design 73 · Major flow -- obtain flow rates and volumes in large streams · Outflow from the state -- account for streamflow leaving the state · Streamflow conditions assessment -- assess current conditions with regard to long-term data and define temporal trends in flow As shown in Figure 4-1, in 1996, Texas had 329 streamflow of which 312 stations were continuous flow recorders and 17 were peak flow stations. The number of continuous flow recorders reached a maximum of about 420 gages in 1972 and declined thereafter.
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From page 74... ...
(2001) concluded that · stations on the steepest part of the curve offered the most valuable regional hydrologic information relative to basin characteristics; · sampling error increased to the west where the climate is more arid: -- sampling error for mean annual flow was 6.6 to 114.3%, and -- sampling error for 25-year peak flow was 9.9 to 28.5%; · there was greater variability in error between regions than was introdcued by changing the number of stations within a region; and · there was much less error in regression equations for the 25-year peak flow than for the mean annual flow in arid regions.
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From page 75... ...
. FIGURE 4-3 Sampling error for planning horizons of 5, 10, and 25 years for 25-year peak flow as a function of number of available stations in a hydrologic region.
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From page 76... ...
The two numbers, however, are not directly comparable because the statistical study applies to gages in natural watersheds while the NSIP study applies to all watersheds. This study illustrates both the strengths and the limitations of the statistical approach to network design.
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From page 77... ...
Some gages have more information content, others have less, but it is difficult to know how much information content is enough to justify the existence of a gage. Statistical methods for stream network design should be used to justify incremental decisions to add or eliminate individual gages within a local gage network serving narrow, well-defined goals (such as hydrologic regionalization)
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From page 78... ...
Any set of points on a stream network can be used to subdivide a watershed into subwatersheds. Figure 4-9 shows several subwatershed divisions of the Guadalupe basin in Texas for flooding, water quality, and water supply.
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From page 79... ...
Streamflow Network Design 79 FIGURE 4-5 Coverage of a continuous spatial phenomenon by measurements at points. FIGURE 4-6 NEXRAD radar rainfall locations and coverage of radar station KEWX, Austin-San Antonio, Texas.
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From page 80... ...
studies, where water quality management segments are defined on the principal reaches of the Guadalupe River, and the subwatersheds are the areas draining to these segments. The lower left panel shows the subwatersheds defined for water availability modeling in which the outlet of each subwatershed is a point at which the Texas Commission for Environmental Quality
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From page 81... ...
In contrast to network designs used to monitor continuous surfaces, fluxes, or fields (e.g., air quality, solar radiation, contaminated groundwater; see Figure 4-5) , streamgage locations are confined to the stream network (Figure 4-8)
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From page 82... ...
in network design. Where clear accepted service standards can be defined (e.g., insurance standards defining acceptable standards for fire protection)
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From page 83... ...
The design for a national gage network is therefore much more complex than the traditional network design problem that has historically been defined by the narrower problem of hydrologic regionalization. Pragmatically, traditional statistical methods based on marginal information value will continue to support incremental decisions and continual improvement in locating new streamgages as the NSIP plan is implemented.
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From page 84... ...
The NSIP gage network resulting from the five NSIP goals results in a subwatershed dataset for the nation. In effect, this NSIP subwatershed dataset subdivides the nation into water resources sampling units, each measured by a gage at its outlet.
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From page 85... ...
permits; · stage and discharge information for rivers used for canoeing, kayaking, or rafting; · streamflow data for rivers draining parcels of federal land of >100 square miles; · streamflow data for all major rivers with surface water diversions that exceed 25 percent of the river's mean annual flow; · discharge data for the inflow and outflow of all reservoirs with >50,000 acre-feet of total storage; · streamflow data for coastal rivers that support a migratory fish population; and · stage or discharge information on rivers that support commercial navigation. Like the NSIP network design, a metric was defined for each of these additional nine goals, and the number of gage sites needed to meet these goals was evaluated.
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From page 86... ...
station network to monitor streamflow and act as sentinel watersheds to evaluate altered rainfall-runoff relations induced by changes in climate or weather. The ICWP also recommended what it called "a new concept: defining a national network through watershed coverage." This would involve subdividing the landscape into HUC-8 and HUC-10/11 subwatersheds and siting gages funded by the Cooperative Water Program at or near the terminus of each HUC-8 subbasin and, within these subbasins, have gages placed as a function of the localized water management need for such information.
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From page 87... ...
Applications of this concept are further developed later in this chapter. ICWP recommendation 1: "Provide stage and discharge data at each National Weather Service and Natural Resource Conservation Service forecast or service location for the purposes of flow forecasting (flood, normal and drought)
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From page 88... ...
ICWP Recommendation 4: "Use the existing Hydrologic Benchmark station network to monitor streamflow and act as sentinel watersheds to evaluate altered rainfall-runoff relations induced by changes in climate or weather." The HBM network is a set of 73 gage locations in pristine environments intended to monitor flows in undisturbed watersheds. The sentinel watershed goal of the NSIP generates 874 gage site locations representative of the nation's ecological and hydrologic regimes.
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From page 89... ...
NETWORK DESIGN GOALS: CONTRASTING NSIP WITH STATE-DESIGNED STREAMFLOW NETWORKS During the 1980s the USGS sponsored several state-level studies assessing the adequacy of the state's streamgage networks (e.g., Fontaine et al., 1984; Medina, 1987)
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From page 90... ...
Nevertheless, in considering national needs supported by the NSIP network, valuable local and regional goals such as specific watershed research or operational needs should not play an overriding role in national network design. Another area that is often mentioned as a candidate for the NSIP is urban hydrology.
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From page 91... ...
based metric for gaging small streams were to be developed similar to the other five goals in NSIP, it would require a high-resolution digital representation of the stream network of the nation. At present, the best representation of the digital stream network of the nation is the National Hydrography Dataset (NHD)
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From page 92... ...
NSIP NETWORK DESIGN: FROM DATA TO INFORMATION As noted earlier in this chapter, there is a sharp distinction between sets of gaging points (i.e., sites at which streamflow is measured) and sets of information points (i.e., sites at which streamflow information is generated)
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From page 93... ...
. For example, real-time streamflow information can be a critical component in flash flood warning and response, yet the marginal value of gage information cannot be quantified independently from the warning, dissemination, and emergency response plans that collectively determine the effectiveness of any flash flood warning system (Drabek, 1999; Gruntfest and Handmer, 2001; Handmer et al., 1999)
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From page 94... ...
The NSIP should also support data collection for less certain future needs for expanded data collection, such as enhanced streamflow information in coastal zone streams discharging to estuaries or ephemeral streams in the Great Basin. The USGS should create a mechanism to institutionalize adaptive management of the nation's likely future needs for streamflow information and provide a mechanism to support these likely emerging needs as part of the core federally funded gage network.
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From page 95... ...
as well as measurements that characterize the form and function of the riparian corridor and floodplain. This richer data collection stream requires a data management system with the capacity to handle very diverse data formats, ranging from remotely sensed digital imagery to four-dimensional velocity fields derived from acoustic Doppler current measurements over a river reach.
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From page 96... ...
96 Data Collection Streamflow Information · Stream gages · Flow, · Intense data collection · Velocity, width, depth, during extreme events Streamflow · Sediment, N, P, bacterial loads · Water quality Gaged Information point Cycle Ungaged point Data Management Information Dissemination · Tabular, statistics, unit values · Internet (Water Watch, Streamstats) · Quality assurance · Reports · Aerial photography, satellite imagery · Telemetry using radio, satellite Information Generation · Regional regression equations · Data assimilation and modeling · Estimation at ungaged sites FIGURE 4-10 Streamflow information cycle: from data to information.
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From page 97... ...
The streamflow information cycle is then "closed" by continuous feedback and the recurring systematic evaluation of current and emerging information needs. Generating streamflow information with quantitative confidence limits helps both in its interpretation as well as in linking the quality of the information to its value for individual users and the nation.
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From page 98... ...
Overall, the proposed design of the NSIP streamgage network represents a sound and well-reasoned foundation to support this continuous process. The use of a coverage model to design the national gage network to meet the five NSIP goals represents a sound approach to designing a robust data collection network for the NSIP.
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From page 99... ...
The program should support a continuous streamflow "information cycle" of data collection, data management, information generation, and information dissemination. This richer data collection stream requires a data management system with the capacity to handle very diverse data formats, ranging from remotely sensed digital imagery to four-dimensional velocity fields.
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