Introduction 


The SPARROW (Spatially Related Regressions on Watershed attributes) model was used to estimate mean annual total nitrogen (TN) and total phosphorus (TP) loads in surface-waters across the Pacific Northwest (PNW) region of the United States. This modeling effort used the National Hydrography Dataset Plus (NHD) (Horizon Systems, 2013) as a hydrologic framework and serves as a refinement of SPARROW models of the PNW that used the RF1 (River Reach File 1) hydrologic framework (Brakebill and others, 2011; Wise and Johnson, 2011). In addition to the hydrologic network, many of the catchment attributes that were compiled for the RF1 SPARROW models were refined for the new models. Using the NHD as the hydrologic framework for SPARROW provided predictions for a greater number of stream reaches compared to the RF1 hydrologic network. The PNW NHD SPARROW models included predictions for 232,811 stream reaches compared to 12,039 stream reaches for the PNW RF1 models.


Surface-water nutrient enrichment has been identified as a water-quality problem across the PNW (Wise and Johnson, 2011) and modeling plays a central role in water-quality management by providing a means for predicting water-quality conditions and assessing the effectiveness of proposed improvement strategies (National Research Council, 2001). The results from the PNW NHD SPARROW models will allow environmental managers and other stakeholders to identify the sources that contribute the largest amounts of nutrients to downstream waters and to evaluate nutrient reduction scenarios. The inclusion of smaller streams and headwater reaches in the model simulation results compared to the RF1 models also will allow for refined analyses of water‑quality issues.


Purpose and Scope


This report describes the methods that were used to assess surface-water nutrient conditions in the PNW and to present the results from that assessment. The objectives of the assessment were (1) to calibrate TN and TP SPARROW models for the PNW using the NHD stream network; (2) to use the calibrated models to estimate mean annual nutrient conditions; and (3) to quantify the relative contribution of different nutrient sources to instream nutrient loads.


Description of the Modeling Domain


The domain of the PNW NHD SPARROW models covered about 708,000 km², but did not include the area of the Columbia River basin (about 106,000 km²) that drains the Canadian part of the river basin. In the United States, the domain covered parts of eight states and included five major regions (fig. 1; table 1): Pacific Coast, West Side Basins, Columbia River Basin, Snake River Basin, and Oregon Closed Basins, and 22 level three, six-digit Hydrologic Unit Code (HUC6) watersheds (Seaber and others, 1987). The Pacific Coast region is characterized by steep, forested watersheds that drain directly to the Pacific Ocean. The West Side Basins region lies between the Cascade Range and the Coast Range (including the Olympic Mountains in Washington). Although most of this region is forested, it contains most of the population in the PNW model domain (including Seattle and Portland) as well as large areas of agricultural land. The Columbia River Basin and Snake River Basin regions are dominated by sparsely vegetated, rocky areas, high desert steppe, semi-arid forests, and areas of intensive agricultural production. The Oregon Closed Basins region is characterized by alternating regions of narrow, uplifted mountains, flat arid plains, playas, and alkali lakes. This is the least populated region in the PNW model domain and contains little forestry or agricultural activity. In 2001, scrub and grassland covered 43 percent of the modeling domain, forest covered 40 percent, agriculture covered 9 percent, and developed land covered 7 percent. The remaining 1 percent was various minor land cover types (Homer and others, 2004).


First posted July 17, 2013