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

June measurements were made after a dry winterfollowed by an unusually wet spring and probablyreflect slightly higher than normal base flows for earlysummer. An Acoustic Doppler Current Profiler(ADCP) was used to measure discharge in the mainstemWillamette River and major tributary inflows.The ADCP provided better accuracy (+1.8 percent)than a mechanical current meter and the capability tomake more measurements in a given time. Dopplertheory and accuracy is described in a report by Simpsonand Oltmann (1993). A water-use inventory wasnot made during the June measurements because therewas little irrigation during this period. Major municipaland industrial users were accounted for in all estimatesof gains and losses. No attempt was made toaccount for evaporation from the river surface or forground-water withdrawals for agricultural and domesticuse.Measurements to determine gains and lossesshould be made when the flow is steady or nearly so,but this is rarely possible. Arrangements were made inAugust 1992 with the USACE for steady releases fromreservoirs under their control; however, no sucharrangements were made with the Eugene Water andElectric Board (EWEB). In August, the EWEB filledtheir reservoirs on the McKenzie River daily from2200 to 0600, diverting about 300 ft 3 /s. In June, flowwas receding from recent rains and continuing snowmelt.In order to compare measured flows made at differentlocations and times with a flow that waschanging with time, the changing flow, as recorded ata stream-gaging station, was routed to the measurementlocation. DAFLOW was used to route a flowhydrograph down the main stem. Tributary inflowsand water-use withdrawals were added or subtractedfrom the routed flow, and the routed discharge wasthen compared to the measured discharge in estimatinga gain or a loss (fig. 14). Differences greater thanthe error of the individual measurement and any routingerror were considered to be significant. For example,measurements made in August from RM 72.0 toRM 60.0 indicated a loss, but the loss was smallerthan the estimated accuracy; therefore, the loss maynot have been real. In contrast, the loss at RM 55.0in August of about 300 ft 3 /s was real to within + 120ft 3 /s, the measurement accuracy.Gain-loss estimates identified (1) the seasonalityof ground-water inflow to the main stem and (2) themagnitude and general location of the ground- andsurface-water interactions. Tables in Appendix 4 listthe locations, measured discharges, and gain-lossresults of these measurements. Figure 14 shows themeasured gains and losses for two representative,but different flow regimes (summer low flow, andspring/early summer base flow) on the main stem of theWillamette River.Measurements made during the drought inAugust indicated very little water contribution from theground-water system between RM 195.0 and RM 60.0on the Willamette River main stem—an indication thatthe river was contributing to the ground-water systemin the lower reach between RM 60.0 and RM 55.0(fig. 14). All municipal, industrial, and agricultural surface-waterwithdrawals from the river were accountedfor in the analysis; however, no attempt was made toaccount for ground-water withdrawals that would interceptwater naturally flowing to the river. It was estimatedthat an average of 100 ft 3 /s was being withdrawnfrom the ground-water system in the Willamette Valleyduring the time of the measurements (Broad and Collins,1996).Measurements made in June, after an exceptionallywet spring, indicated an approximate 2,000 ft 3 /sground-water contribution from about RM 140.0 nearPeoria to RM 84.0 at Salem (fig. 13). Measurements inSeptember of the same year indicated that the groundwatercontribution continued from RM 84.0 to RM40.0 (fig. 14). Large increases were noted adjacent tothe alluvial fans of the Santiam and Molalla Rivers.The upper main-stem Willamette River is a systemof braided streams with many islands, sloughs, andgravel bars. Gain-loss measurements indicate that substantialhyporheic flow probably occurs between RM195.0 and 140.0. The word “hyporheic” means “underriver,” and the hyporheic zone is defined as the subsurfacearea where stream water and ground water mix.From a water-quality standpoint, important chemicaland biological processes can occur in the hyporheiczone. Even though flows were higher during the Junemeasurements than during the August measurements, abetter flow picture emerges because more measurementsand more accurate measurements were made inJune (fig. 14). As much as 1,000 ft 3 /s or 15 percent ofthe total river flow can be in the hyporheic flow zone.NETWORK-ROUTING APPLICATIONSIn order to model a stream network, an inflowhydrograph at the upstream boundary of the network35