Precipitation-Runoff and Streamflow-Routing Models for the ...

EXPLANATIONMainstem basins modeledTributary basins modeledBasins not modeledWillamette River Basins1 Sandy River2 Portland (only Johnson Creek was modeled)—See Figure 153 Clackamas River—SeeFigure174 Tualatin River—See Figure 195 Salem-Portland—See Figure 356 Molalla River—See Figure 217 Yamhill River—See Figure 238 Albany-Salem—SeeFigure339 Rickreall Creek10 Mill Creek11 Luckiamute River12 Harrisburg-Albany—See Figure 3113 North Santiam River—See Figure 2514 South Santiam River—See Figure 2515 Jasper-Harrisburg—SeeFigure2916 Marys River17 Calapooia River18 Long Tom River19 McKenzie River—SeeFigure2720 Coast Fork Willamette River21 Middle Fork Willamette River71116184598 1012 171520261419211331Figure 5. Major drainage basins of the Willamette River Basin, Oregon.map (McFarland, 1983). The data layer included fiveclasses of aquifer units found in the Willamette RiverBasin, each with varying permeability rates (table 4).Tertiary-Quaternary sedimentary deposits, located inthe flatter regions of the basin near the river, have thehighest permeability rates and yield as much as 2,000gallons per minute to wells. Tertiary rocks of the CoastRange have the lowest permeability rates and yieldless than 10 gallons per minute to wells. Figure 6shows the geology classifications for the Molalla RiverBasin.Soils—The soils data layer was digitized from a1:500,000-scale Natural Resource Conservation Service(NRCS; formerly the Soil Conservation Service)general map of Oregon soil series (Soil ConservationService, 1986). The data layer uses the NRCS fourgroupclassification for infiltration properties, rangingfrom greater than 0.45 inch per hour (Group A) to lessthan 0.05 inch per hour (Group D). Soil series withinthe study area contain all four groups. Six classes inthe soils data layer are listed (A, B, B+C, C, C+D, andD) in table 3 and defined in table 5. Figure 6 shows thesoil classifications for the Molalla River Basin.Hydrologic response units—The land-use, slope,aspect, geology, and soils data layers were merged(overlain) to create an HRU data layer for each majorbasin and each subbasin in the Willamette RiverBasin. Figure 7, the HRU data layer for the MolallaRiver Basin, is an example showing the many categoriescreated by merging the data layers. New polygonscreated from the merged data layers contain a sum ofattributes that already exist with the five contributingdata layers. A four-digit code that identified the combinationof land-use, slope and aspect, geology, andsoils classes within the polygon was assigned to allpolygons (table 3). Polygons that have a unique aspectof north, east, south, or west were created only forareas with slopes greater than 30 percent. For polygonswhere the slope was less than 30 percent, a basindominantvalue of aspect was assigned as an attribute.Aspect, as a parameter value used in PRMS, is lesssensitive and less important in areas of lower reliefthan it is in steeper areas. More than 1,000 individualHRU’s were created for the Willamette River Basin.Later, INFO tables were created that assigned PRMSparameter values to each individual HRU category asdescribed by their codes. Small polygons of less than15