Physiographic and Land Cover Attributes of the Puget Lowland and the Active Streamflow Gaging Network, Puget Sound Basin, Washington

Abstract


Geospatial information for the active streamflow gaging network in the Puget Sound Basin was compiled to support regional monitoring of stormwater effects to small streams. The compilation includes drainage area boundaries and physiographic and land use attributes that affect hydrologic processes. Three types of boundaries were used to tabulate attributes: Puget Sound Watershed Characterization analysis units (AU); the drainage area of active streamflow gages; and the catchments of Regional Stream Monitoring Program (RSMP) sites. The active streamflow gaging network generally includes sites that represent the ranges of attributes for lowland AUs, although there are few sites with low elevations (less than 60 meters), low precipitation (less than 1 meter year), or high stream density (greater than 5 kilometers per square kilometers). The active streamflow gaging network can serve to provide streamflow information in some AUs and RSMP sites, particularly where the streamflow gage measures streamflow generated from a part of the AU or that drains to the RSMP site, and that part of the AU or RSMP site is a significant fraction of the drainage area of the streamgage. The maximum fraction of each AU or RSMP catchment upstream of a streamflow gage and the maximum fraction of any one gaged basin in an AU or RSMP along with corresponding codes are provided in the attribute tables.


Introduction


The Regional Stormwater Monitoring Program (RSMP) is being developed by the Puget Sound Stormwater Work Group to provide stormwater managers with information about natural resources affected by stormwater and about the efficacy of stormwater management actions at multiple scales. The Puget Sound Stormwater Work Group recommended using the existing streamflow gages in Puget Sound for monitoring the status and trends of small streams (Puget Sound Stormwater Work Group, 2010), but acknowledged some outstanding issues regarding the adequacy of the active streamflow gaging network for this purpose. An initial assessment indicated that the network covers much of the Puget Lowland (US Environmental Protection Agency, 2011), but most areas are covered by streamflow gages on large rivers that might not provide useful information for addressing small streams (Konrad and Voss, 2012). 


Purpose and Scope


This report compiles spatial information about the Puget Lowland and the active streamflow gaging network that can be used to advance regional monitoring of small streams, particularly with regard to stormwater effects. Two types of information are provided: (1) physiographic and land use attributes that influence runoff processes, and (2) coverage and resolution of the active streamflow gaging network. Information on physiographic and land use attributes can be used to identify types of streams that might not be adequately represented by the active streamflow gaging network. The coverage and resolution of the active streamflow gaging network could be used in future applications to identify areas where the network likely is adequate for monitoring streamflow and areas where there are gaps in the coverage or resolution of the network.


Spatial Framework


Three types of spatial units were used for summarizing spatial information: (1) Puget Sound Watershed Characterization analysis units (AU), (2) drainage areas for active streamflow gages, and (3) catchments for the initial set of 100 sites selected for water-quality monitoring as part of the RSMP (Collyard, 2011). Boundaries for AUs are available from the Washington Department of Ecology (2012). Drainage areas for streamflow gages were previously digitized (Konrad and Voss, 2012) and, as part of this project, were visually inspected and edited to provide more consistency with AU boundaries. The drainage areas are available as an ArcGIS shape file. Catchments for RMSP sites were digitized using the ArcHydro watershed delineation tool (Djokic and others, 2011). Discrepancies among the AU, RSMP, and active streamflow gaging basin boundaries exist, particularly in urban areas where engineered drainage systems cross topographic divides. High resolution topography and accurate drainage (natural and engineered) networks would be required to resolve these inconsistencies. 


Physiographic and Land Cover Attributes


Processes that regulate the amount and timing of runoff from a catchment (stormwater production) are affected by physiographic and land cover attributes of the catchment. Thirteen attributes were compiled for the Puget Sound Basin from publicly available sources (table 1). The attributes are summarized for each AU (table 2), each active streamflow gage (table 3), and each RSMP catchment (table 4).


Active Streamflow Gaging Network


Streamflow gages that share drainage area with AUs or RSMP sites provide useful information for regional stormwater monitoring. The value of the information from a streamflow gage, however, is limited in cases when the gaged area is much larger than the area of interest. For example, a streamflow gage on a large river will not be useful for assessing stormwater generated from a small tributary. The fraction of drainage area shared between two nested sites generally indicates the correlation between streamflow at those sites. Konrad and Voss (2012) determined that the correlation of monthly mean streamflow was relatively strong (Kendall’s tau greater than 0.7) when the drainage area of the upstream streamflow gage was at least 25 percent of the drainage area of the downstream streamflow gage. As a result, the fraction of an AU or RSMP drainage area that is gaged and, conversely, the area of an AU or RSMP site as a fraction of the drainage area of streamflow gage, are important for assessing whether a streamflow gage provides useful information about stormwater from an AU or at an RSMP site. 


An overlay of the drainage basins for active streamflow gages on AUs was done using the Arc Tool “Identity.” This tool divides each AU into polygons that correspond to areas that overlap with active streamflow gages. The results are summarized in columns Q–S of table 2 in terms of (1) the area of AU that is actively gaged; (2) the maximum area of the AU in the drainage area of any one streamflow gage; (3) the code for the streamflow gage that covers the largest area of the AU; and (4) the gaged area of the analysis unit as a fraction of the drainage area of the streamflow gage. The same procedure was applied to identify streamflow gages that overlap with RSMP catchments. This information was summarized in table 4 in terms of (1) the area of the RSMP catchment that is actively gaged; (2) the maximum area of the RSMP catchment in the drainage area of any one streamflow gage; (3) the code for the streamflow gage that covers the largest area of the RSMP catchment; and (4) the gaged area of the RSMP catchment as a fraction of the drainage area of the streamflow gage. In cases where the boundaries between the basin of a streamflow gage is not aligned with an AU or RSMP catchment, the gaged part of the AU or RSMP catchment will be less than its area, but the difference represents errors in the GIS coverages rather than actual ungaged parts of the AU or RSMP catchment.


Table 2 can be used to identify areas in the Puget Sound Basin where streamflow gages are likely to be particularly useful for regional monitoring of small streams. Values in column S (Gaged area of an AU as a fraction of the drainage area of the gage) in table 2 indicate the fraction of the drainage area of a streamflow gage that is within the listed AU. The streamflow gage is identified by the code in column Q (Gage Code) of table 2. A streamflow gage will provide better spatial resolution of streamflow for streams in an AU where the values in column S of table 2 is closer to 1 (the drainage of the streamflow gage is wholly within the AU) than when the value is closer 0 (the drainage area of the streamflow gage is much larger than the AU).


Table 3 can be used to identify streamflow gages with particular combinations of attributes for either assessing regional gaps or locating representative streamflow gages. A comparison of the attributes of AUs, summarized in table 5, and attributes of streamflow gages, summarized in table 6, indicates that streamflow gaging network generally represents the attributes of the Puget Lowland, except for low elevation areas with high drainage densities (table 6).


Table 7 can be used to identify streamflow gages that cover sites in the RSMP. If the drainage area of an RSMP site represents a relatively large part of the gaged drainage area, it may be appropriate to integrate streamflow information from the streamflow gage into the RSMP.


Summary


Physiographic and land cover attributes of the Puget Lowland, the active streamflow gaging network, and RSMP catchments were tabulated to assess the coverage and resolution of the active streamflow gaging network for regional monitoring of small streams. The streamflow gaging network covers much of the Puget Lowland and, in many cases, lowland areas represent a significant fraction of the drainage area of streamflow gages. Streamflow gages with drainage areas that are primarily within one AU are likely to be useful for monitoring small streams. Streamflow gages that include RSMP sites will also be useful provided the drainage area of the gages is not much larger than the drainage area of the RSMP site.


References Cited


Collyard, S., 2011, 2012 Status and trends stormwater monitoring and assessment strategy for small streams—Addendum to quality assurance project plan—Status and Trends Monitoring for Watershed Health and Salmon Recovery: Washington State Department of Ecology, Publication No. 06-03-203, 32 p., accessed June 10, 2013, at http://www.ecy.wa.gov/programs/wq/psmonitoring/ps_monitoring_docs/
SWworkgroupDOCS/SmallStreamMonitQAPPfinalDraft102011.pdf. 


Djokic, Dean, Ye, Zichuan, Dartiguenave, Christine, 2011, Arc Hydro Tools Overview—Version 2.0: Esri, 18 p., accessed June 10, 2013, at http://downloads.esri.com/blogs/hydro/AH2/Arc_Hydro_Tools_2_0_Overview.pdf.


Konrad, C.P., and Voss, F.D., 2012, Analysis of streamflow-gaging network for monitoring stormwater in small streams in the Puget Sound Basin, Washington: U.S. Geological Survey Scientific Investigations Report 2012-5020, 16 p., https://pubs.usgs.gov/sir/2012/5020/.


Puget Sound Stormwater Work Group, 2010, Stormwater monitoring and assessment strategy for the Puget Sound Region: Puget Sound Stormwater Work Group, 90 p., accessed November 25, 2013, at http://www.ecy.wa.gov/programs/wq/psmonitoring/ps_monitoring_docs/SWworkgroupDOCS/2010SW.pdf.


Vaccaro, J.J., Hansen, A.J., and Jones, M.A., 1998, Hydrogeologic framework of the Puget Sound aquifer system, Washington and British Columbia: U.S. Geological Survey Professional Paper 1424-D, 77 p., http://pubs.er.usgs.gov/publication/pp1424D.


U.S. Environmental Protection Agency, 2011, Level III and IV ecoregions of the continental United States: U.S. Environmental Protection Agency, accessed November 28, 2011, at http://www.epa.gov/wed/pages/ecoregions/level_iii_iv.htm.


Washington Department of Ecology, 2013, Puget Sound Watershed Characterization Project: Washington Department of Ecology, accessed October 18, 2013, at http://www.ecy.wa.gov/services/gis/data/pugetsound/characterization.htm.

First posted January 13, 2014