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1\' -..J 1-- --J irrigation season encom passes about 59 percent of the year. thc nonirrigation season about 41 per- cent of the year). The reason for using a seasonally weighted mean is because of the large differences in streamfiow and dissolved-solids loads between the irrigation and nonirrigation seasons in many Grand Valley streams and washes. [fthe periodic sampling was biased to one of the seasons (for example. if most of the samples were col. lected in the irrigation season). then an arithmetic mean on all the samples might result in an unrealis- tic mean dissolved-solids load. if that mean is assumed to represent the dai Iy load throughout the year. Using a seasonally weighted mean removes seasonal sampling bias. Once the seasonally weighted mean is calculated. the resultant mean load is assumcd tl) represent the mean annual dissolved.solids load and is multiplied by 365.25 to estimate the total annual dis. solved-solids load for a multi-year period in which the sampling was done. To determine if annual dis. solved.solids loads cotnputed using only periodic data are repre- sentative of actual loads. the loads for 1973-83 also were computed using another melhl>u called SLOAD. A computer program named SLOAD (Salt LOAD) was developed by LiellCnnann and others (1987) for estimating dis- solved-solids loads in streams and rivers. In the SLOAD method. daily weamfiow. daily specifie- conductance. and periodic dissolved-solids data are used to estimate loads. Two equations are computed usmg the periodic data. One equation rdates dissolved. solids load to streamflow. and the second equation relates dissolved. solids load to streamflow and specific conductance. The equa- tions then arc used to compute daily loads for the time period of interest using the daily streamfiow record and. if available, daily spe- cific conductance. The daily loads arc summcd to give monthly and annual dissolved-solids loads. Dissolved-solids loads computed using the SLOAD method should be a more accurate estimate of dissol ved.solids loads when com. pared to the periodic method because much ml>rc detailed data (daily streamflow and specific conductance) are used instead of only periodic dissolved-solids data. A comrarison of the two methods is provided in the next section of this fact sheet. Comparison of Methods Annual dissolved-solids loads calculated by the two meth. ods are compared in table 2. The loads are applicable to the periods when (he g;tging swtions were ore rated at the lour sites during 1973-83 (table I). The ditTerences in annual loads rangcd from ;lI1out 4 to 8 percent (table 2). Using the reriodic method, the annual dis. solved-solids loads for Leach Creek and Big Salt Wash were less than the annual loads eompuled using the SLOAD method: the orposite was true for Reed Wash and Salt Creek (table 2). For the four stations. the annual loads based only on periodic data are not substantially different than the annual loads computed using SLOAD. The implication is th;lt the periodic dissolved.solids.load data were collected over the oen. '" eral range of streamllow and dis. solved-solids concentrations lor several years at these four sites and that fairly reliable estimates or annual dissolved-solids loads can be obtained without having daily slrcamnow data. A disadvanrage of using only periodic samrles is that runolf from large storms may be missed. In the Grand Valley. such storms arc relatively mfrequent and orten of shan duratIon. At least for the reriods when the four gaging stations were operated, resulls indi- cated that using only periodic data for computing dissolved.solids loads provides reasonable cst,. males of the annual loads. Table 2 Comparison 01 annual dissolved-solids loads computed uSing two methods 11'<.:rll"'-.tI": rTl~'lh(>;j. 'lnl~ l~nodic wal~r-quJ.lily ;mJ ,lr.:-allllh"... J,lla ;H~ u~J 10 C'[11I1:II,' I(lad,_ SL(lt\1) IIIClh(x!. daily ...lre.ullllnv.-' and ;,rcc1fic conductancl" ;UL' 1I".J 1\'C\WlI;llt: luad...: I""d, tllf L';Jch Crcd..: .1l1J Rig SaIl W~~l:\ :w: h;\....:d on data colleclI:d durin.~ Igl.1._ll. ;mJ 1\\;0,0... h'.l" Re:cll W~hh Jnd SaIl Crc-L'K :.lIe h,hl'J lm J<lla clllk":leJ Junng 19H-8.\.loads are: in ton:- per year I Annual dissolved- Difference in loads, Stream solids load periodic minus SLOAD Periodic SLOAD Load Percent Leach Crt.'d, 41.400 44.9011 -J.500 -7 X Big Sail \\':J~h XJ.~OO 8!t~OO ,5.(XJ() -).7 Reel! \\'a~h 1.11.).('100 1.\0.000 9.61Xl 7" Sal! Cn:d IJ I.JOO I :!5,)OO 5.HOIJ -U,