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01957 ' Tt'.Jlifl1{ 11l-{('r-based .fen.Jors /0,. r:aminl/OlLf in Silu ",ani/orlnK ofsu.f!wnded .fed,fflNlf 3 --CA.lIBFV.TEOlISST100BPOINTIoIE.OSIJRt"MENT (a> o CROSS-SECTlONAlLVINTffil'lATEDWJTHo.nSAMPtEl'(b) -OlSCHARGEOf WATER ~ ,~ ~ - uo :t ~ "" ~o ~ "" ;; n g '00 . '00 = =. z ~ 0 w m ~ ~ , ~ ~ ~,- 0 , " 0 ~ , z ~ ~ '00 o 0 y 0 rP 0 '00 ~ ~ ~ \ ' ''''''''' . , , . ~ '" , . , 0 "" 12.00 000 "00 000 000 "'" 0'" '"'' 000 1.1~2CIOl '.20-:2'001 1.\9-2'001 '-20-2001 Fig. 2 (a) Comparison of sand concenlralions and (b) silt & day concenlralions measured 31 the Grand Canyon gage using L1SST.lOOB and a D-77 bag sampler during the one-day July 2001 test. Discharge data are from Ihe Grand Canyon gauge. variable. ranging from about 50 to 60 mg rl during the diurnal peak (Fig. 2(b)). This is likely the result of a high degree of turbulence al the test site that keeps the fines well mixed and sampled more consistently, relative to sand.sized particles. A second field test was conducted from 22 September, 200 I to 8 February, 2002, to explore perfonnance characteristics of both the USST-looB and a LlSST-25 during longer, continuous deployments required for long term monitoring. Both the LlSST- 1008 and the L1SST-25 measure only the volumetric concentration and grain size of suspended particles. However, mass concentration Can be estimated by the user once a suitable density conversion is gravimetrically determined. The L1SST -25 tested has a size range similar to the L1SST-IOOB, however, the L1SST-25 provides only a sauter mean diameter (the ratio of particle surface area to volume) rather than a size distribu- tion, a~ plOvided by the L1SST-looB. During autumn 2001, the L1SST-looB was fitted with a path-reduction module (PRM) to expand the instrument's concentration range by almost a factor of four (optical path of 5 cm reduced to I em). Although path reduction does allow for higher-concentration measurements (by reducing the sample volume and related number of particles that attenuate laser transmission), the optical accessory used for testing turned out to be flawed and altered the raw d.lta in ways thai were not trivial 10 resolve. The PRM's influence was most pronounced on scattering related to the sand-sized particles (inner nngs of the detector). It is therefore essential that each PRJt.1 used with LlSST be tested and carefully evaluated prior to field deployment. In contrast to the L1SST-looB with the PRM,lhe L1SST-25 (fixed optical path of 2.5 cm) measured higher concentrations of suspended sediment with no discer- nable complications over the 4-month long tesl. Despite complications introduced by the faulty PR~t. the 10-18 January, 2002, concentration data ohtained from the L1SST-JOOB also compared well with cableway samples for sand (Fig. 3(a)) and silt & clay (Fig. 3(h)) C()I!t:clcd WIth the D.77 bag: sampler.