Laserfiche WebLink
<br />wcrc cffcctive. there could be mea. <br />surable decreases in dissolved.sol. <br />ids loads in the streams in these <br />areas. <br /> <br />During 1973-8:1. the USGS. <br />in cooperation with the BOR. <br />l\.0perated several gaging stations on <br />~elected streams in the Grand <br />6;Valley. Daily streamflow and spe- <br />cinc conductance were recorded. <br />and water-quality samples were <br />collected at some of the stations. <br />All the gaging stations operated by <br />the USGS. where water.quality <br />data had been collected. were dis- <br />continued by October 198:1. [n <br />1991. the National [rngation Water <br />Quality Program (NIWQP) of the <br />U.S. Depanment of the Interior <br />began a study in thc Grand Valley. <br />Thc focus of the N[WQP study was <br />on selenium sources and pathways <br />and effects of sclennllll on hsh and <br />[mds (Butlcr and others. 1996). <br />Although no g;lging staliolls were <br />reactivated in support of the <br />NIWQP study. watCl' samples for <br />chemical analyses that Included <br />dissolved solids were collected <It <br />seveml sites where the gaging sta- <br />tions had been located III 1973-8:1. <br />Data collected for NIWQP were to <br />be used to estimate sdenium and <br />dissolved-sohds loads at outllow <br />sites on the major slre<lms <lnd <br />washes draining irrigated 41reas in <br /> <br />the Grand Vallcy. By [996. suffi. <br />cient streamflow and chemical d<lta <br />had been collected \0 estim<lle <br />dissolved-solids loads for four of <br />the sites (fig. I) at fOflllcr gaging <br />stations: Leach Creek (site LC). <br />Big Salt Wash (site BSW), Reed <br />Wash (site RW). and Salt Creck <br />(site SC). Dissolved-solids loads <br />during 1973-8:1 then could be <br />compared to dissolved.solids <br />loads for 1991-96 for the four <br />sites to determine the effects of <br />salinity-control projects in the <br />Grand Vallcy. <br />This fact sheet describes <br />changes in dissolved-solids loads <br />in four streams in the Grand Valley <br />between 1973-~n and 1991-96 <br />and the relation between changes <br />in loads and implemcntation of <br />s<llinity.control projects. The fact <br />sheet also descrlhes the mcthods <br />used to compute dissolved.solids <br />loads and thc ,ationale for making <br />Ihe load compaflsons bctween <br />1973 -8:1 and 199 1-96. <br /> <br />Methods Used To Estimate <br />Dissolved-Solids Loads <br /> <br />Daily slreamllo\\'. daily <br />specific conductancc. and periodic <br />water-quality data that were col- <br />lected allhc four ~itcs are summa- <br /> <br />rized in table [. It is more <br />appropriate to compare dissolved. <br />solids loads between time periods <br />if loads were calculated using the <br />samc method. Because only peri. <br />odic water-quality and streamtiow <br />data were collected during 1991- <br />96 (table I) at the four sites. only <br />periodic data were available for <br />computing dissolved.solids loads <br />for 1991-96. Therefore. to com. <br />pare the loads for 1973-83 to loads <br />for 1991-96. the loads for 1973-8:1 <br />also necded to be computed using <br />only periodic data. <br />The method for estimatlllg <br />loads using only periodic water. <br />quality and streamllow data is <br />referred to as the periodic method. <br />[n this method. loads are computed <br />for individual samples by using the <br />dissolved.solids concentrallon <br />frolllthc chemical analysis of water <br />samples and the strcamllow mea <br />sured when the sample was col. <br />lected. The individual dissolvcd- <br />solids loads then are separated into <br />irrigalion-season and nonirriga- <br />tion.season samples. and means are <br />computed for each set of seasonal <br />data. A mean annual load is deter. <br />mined by computing a weighted <br />mean of the two seasonal mean <br />loads. The weighting factors are <br />based on the fraction of the year <br />cach scason encompasses (the <br /> <br />[POR. ~noJ ,'1 r":C\lrll. N. llunlt~r(lf'i1mpk~1 <br /> <br />Table 1. Sampling sites and summary of daily and periodic data collected in 1973-83 and 1991-96 <br /> <br /> Period of record. daily <br />Sampling USGS streamflow and <br />site station Stream name conductance <br />(fig. 1) number <br /> 1973-ll3 1991-96 <br />LC (lI)1:'~(i~O LC::.II:hCrcc\.; (14/7:L09/77 NOlle <br />BSW Oil I." \~7(J Big SJII Wash 0317.1-09/77 :'\llll1l.' <br />RW O(JI:i.nO() Rct.'u Wash n4n)-M/X, Nllll~' <br />SC {I'Jlh;..W(J Sail Creek fJ.l(73-09/;S-' \:,,1\\": <br /> <br />Periodic waler-quality sampling <br /> <br />1973-ll3 <br />POR --rr-- <br />OX17.l-07177 2X <br />OIV7l....07177 ~lJ <br />ORn.'-[O/~~ -1X <br />08/7 '-OYfX.\ .;;:. <br /> <br />1991-96 <br />POR <br />0.1191-12/90 <br />OV91-I~N6 <br />03191-12/96 <br />(HI4I-l\{Il(\ <br /> <br />.l9 <br />.\0 <br /> <br />N <br />32 <br />20 <br />