Assessing the National Streamflow Information Program (2004) / Chapter Skim
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3 Selection of NSIP Base Gage Locations
3 Selection of NSIP Base Gage Locations
Pages 47-67
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From page 47... ...
2. Flow Forecasting (sites needed for validation and improvement of forecasts where the National Weather Service and other federal agencies carry out flood or water supply forecasts; 3,244 gage sites)
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From page 48... ...
. Of the 4,424 base NSIP gage sites, 2,796 or 63 percent are active USGS gaging locations; 307, or 7 percent, are active gage sites operated by other agencies for which the USGS wants to assume the full costs of operation; 837, or 19 percent, are inactive gages (sites where a gage once operated but no longer does)
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From page 49... ...
THE FIVE CRITERIA FOR SITING NSIP STREAMGAGES Goal 1. Meeting Legal and Treaty Obligations on Interstate and International Waters Provide river discharge information to meet the operational requirements of river basin compacts and Supreme Court decrees at each point where major rivers cross international or state boundaries.
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From page 50... ...
In contrast to the view of the ICWP, the committee believes that the border gage sites proposed by the USGS should be retained as part of the NSIP base gage network.
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From page 51... ...
During the course of the study, the USGS requested that the committee title this goal "Flow Forecasting" rather than "Flood Forecasting" in order to be more inclusive of other needs, such as water supply forecasts, navigation, agriculture, recreation, and drought response. This change is appropriate.
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From page 52... ...
These locations are all USGS gage locations in the western United States where water supply forecast needs are most critical. Of the 576 NRCS gage sites, 321 are already in the NSIP base gage network, so the addition of the NRCS sites would add 255 sites to the 3,244 NWS sites currently identified for the flow forecasting goal, an increase of 8 percent.
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From page 53... ...
to improve short-range streamflow forecasts. Hydrologists at the RFCs produce the numerical guidance for issuing river forecasts and flood warnings by running NWSRFS models for river basins within their area.
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From page 54... ...
NWS forecast points and USGS streamgages are collocated so that measured streamflow data can be used to calibrate and initialize forecast models. Accurate and reliable forecasts require both real-time streamflow information for model initialization and a historical record of streamflow information over many years (or decades)
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From page 55... ...
Despite the complexity of forecast models, model predictions diverge from actual conditions due to uncertainties in the measurement of precipitation and other weather input variables or the inherent simplifications associated with computational modeling of watershed and river processes. Real-time observations provide the "ground-truth" needed to continuously make reliable forecasts for rapidly changing river conditions.
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From page 56... ...
Since USGS gage sites were selected as forecast points early on in the river forecasting process at the NWS, the hydrologic models used by the NWS have been designed around the long-term streamgage network. For example, the delineation of model elements (subcatchments)
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From page 57... ...
on streamflow and biota, and others. To support such an accounting the USGS has identified the six-digit hydrologic cataloging units as the regional water planning units that such a water balance will require and has located an NSIP gage site near the outlet of each of these six-digit hydrologic regions.
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From page 58... ...
. FIGURE 3-7 Gage locations for sentinel watersheds (874 gage sites of which 71 percent are solely for sentinel watersheds and 29 percent also serve other purposes)
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From page 59... ...
. One consideration in evaluating the NSIP base gage network and sentinel gage subset is the coverage in terms of spanning a representative range of basin sizes.
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From page 60... ...
Michael Norris, USGS, personal communication, June 2003. FIGURE 3-9 Gage locations for water quality (210 gage sites of which 58 percent are solely for water quality and 42 percent also serve other purposes)
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From page 61... ...
ASSESSMENT OF THE DISTRIBUTION OF GAGE SITE LOCATIONS The selection of gage sites using the five NSIP criteria reflects a process of assessment within the USGS as to which goals out of all those used to justify installing a gage site are appropriately national or federal goals and which goals are better left for state and local interests. Among the five criteria, the flow forecasting criterion to support the river forecast operations of the NWS results in many more site locations than any other criterion (Figure 3-10)
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From page 62... ...
Of course, some site locations support more than one goal; only 4,424 site locations are needed to support the 5,293 locations selected independently by the five criteria. One way of assessing the result of this selection process is to examine the distribution of gage sites across the country.
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From page 63... ...
. FIGURE 3-12 Density of NSIP gage locations measured in square kilometers of land area per gage.
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From page 64... ...
In such circumstances, raingages are more effective for flood warning, exemplified by Las Vegas' local alert service. There are 28 NRCS water supply forecast sites in Nevada, and of these, 11 are already in the NSIP base gage network.
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From page 65... ...
The principle of adaptive management should be incorporated explicitly into the NSIP to periodically reevaluate the network goals and criteria to ensure that the network meets present and anticipated future needs for streamflow information. This periodic reassessment of emerging needs may also support gaging of streams in small watersheds or coastal plains where there is a perception of insufficient coverage and common sense dictates the inclusion of gage sites that may not be included by rigidly applying the five current criteria.
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From page 66... ...
This periodic reevaluation of network goals and criteria will ensure that future needs for streamflow information, including ephemeral streams and possibly small watersheds and coastal plains, are met by the network. The NSIP gage site program is very much attuned to the site locations of National Weather Service river forecast points, and vice versa.
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From page 67... ...
Selection of NSIP Base Gage Locations 67 coordination mechanism should be established between the NWS, NRCS, and USGS for the selection of flow forecast NSIP base gages. This coordination should consider the national NSIP coverage model proposed in the following chapter.
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