Estimated Decreases in Dissolved-Solids USGS Loads in Four Tributaries to the Colorado River science for a changing world in the Grand Valley, Colorado, 1973-96 The Colorado River is used extensively for municipal, agricul­ tural, and industrial purposes in seven Western States. Since water development began in the 1870's, the average dissolved-solids con­ centrations in the lower Colorado River has more than doubled when compared to pre-development con­ centrations (U.S. Department of the Interior, 1995). The dissolved- -^^^- solids concentration, commonly p DENVER COLORADO referred to as salinity, is a measure HJBS?=» . Colorado springs of salts, such as sodium chloride, Pueblo calcium bicarbonate, and calcium sulfate, that are dissolved in water. Dissolved solids can have adverse 0 5 KILOMETERS effects on agricultural, municipal, Figure 1. Location of the Grand Valley, extent of irrigated area, and locations of study sites. and industrial uses, especially in the lower part of the basin. The fields. The salinity control project Valley were reported by Butler Colorado River Basin Salinity in the Grand Valley consisted of (1996) However, dissolved-solids Control Act, passed by the U.S. two parts: (1) The BOR lined parts concentrations and loads also Congress in 1974, authorized the of the Government Highline Canal had significant downward trends in Bureau of Reclamation (BOR) and (fig. 1) and placed some laterals in the Colorado River upstream the U.S. Department of Agriculture pipe to decrease distribution-sys­ from the Grand Valley, which (USDA) to plan and construct tem losses, and (2) the USDA introduced some uncertainty in salinity-control projects to (through the Natural Resources determining the effect of the Grand decrease dissolved-solids loading Conservation Service) was respon­ Valley salinity-control projects on to the Colorado River. One of the sible for on-farm improvements to the dissolved-solids trends in the authorized projects was a salinity- decrease water losses from on-farm Colorado River. control project to address dis­ ditches and to decrease deep perco­ Examining dissolved-solids solved-solids loading from irri­ lation from fields. loads in tributary streams and gated agricultural lands in the In 1994, the BOR requested washes that dissect the Grand Grand Valley (fig. 1). Because the U.S. Geological Survey Valley (fig. 1) could be a more ground water that was recharged (USGS) to examine trends in dis­ direct measure of effects of salin­ primarily from irrigation practices solved-solids data for the Colorado ity-control projects than are dis­ was the major source of dissolved- and Gunnison Rivers to examine solved-solids loads in the Colorado solids loading from the Grand the effectiveness of the salinity- River. These streams and washes Valley (Bureau of Reclamation, control projects in the Grand Valley carry much of the irrigation- 1978), the salinity-control project (U.S. Department of the Interior, induced dissolved-solids load was designed to decrease the 1995). Significant downward resulting from subsurface irriga­ recharge to this ground water from trends in dissolved-solids concen­ tion return flow to the Colorado canal, lateral, and ditch leakage and trations and loads in the Colorado River. If salinity-control projects in from deep percolation in irrigated River downstream from the Grand these relatively small watersheds were effective, there could be mea­ the Grand Valley. By 1996, suffi­ rized in table 1. It is more surable decreases in dissolved-sol- cient streamflow and chemical data appropriate to compare dissolved- ids loads in the streams in these had been collected to estimate solids loads between time periods areas. dissolved-solids loads for four of if loads were calculated using the During 1973-83, the USGS, the sites (fig. 1) at former gaging same method. Because only peri­ in cooperation with the BOR, stations: Leach Creek (site LC), odic water-quality and streamflow operated several gaging stations on Big Salt Wash (site BSW), Reed data were collected during 1991- selected streams in the Grand Wash (site RW), and Salt Creek 96 (table 1) at the four sites, only Valley. Daily streamflow and spe­ (site SC). Dissolved-solids loads periodic data were available for cific conductance were recorded, during 1973-83 then could be computing dissolved-solids loads and water-quality samples were compared to dissolved-solids for 1991-96. Therefore, to com­ collected at some of the stations. loads for 1991-96 for the four pare the loads for 1973-83 to loads All the gaging stations operated by sites to determine the effects of for 1991-96, the loads for 1973-83 the USGS, where water-quality salinity-control projects in the also needed to be computed using data had been collected, were dis­ Grand Valley. only periodic data. continued by October 1983. In This fact sheet describes The method for estimating 1991, the National Irrigation Water changes in dissolved-solids loads loads using only periodic water- Quality Program (NIWQP) of the in four streams in the Grand Valley quality and streamflow data is U.S. Department of the Interior between 1973-83 and 1991-96 referred to as the periodic method. began a study in the Grand Valley. and the relation between changes In this method, loads are computed The focus of the NIWQP study was in loads and implementation of for individual samples by using the on selenium sources and pathways salinity-control projects. The fact dissolved-solids concentration and effects of selenium on fish and sheet also describes the methods from the chemical analysis of water birds (Butler and others, 1996). used to compute dissolved-solids samples and the streamflow mea­ Although no gaging stations were loads and the rationale for making sured when the sample was col­ reactivated in support of the the load comparisons between lected. The individual dissolved- NIWQP study, water samples for 1973-83 and 1991-96. solids loads then are separated into chemical analyses that included irrigation-season and nonirriga- dissolved solids were collected at tion-season samples, and means are several sites where the gaging sta­ Methods Used To Estimate computed for each set of seasonal tions had been located in 1973-83. Dissolved-Solids Loads data. A mean annual load is deter­ Data collected for NIWQP were to mined by computing a weighted be used to estimate selenium and Daily streamflow, daily mean of the two seasonal mean dissolved-solids loads at outflow specific conductance, and periodic loads. The weighting factors are sites on the major streams and water-quality data that were col­ based on the fraction of the year washes draining irrigated areas in lected at the four sites are summa­ each season encompasses (the Table 1. Sampling sites and summary of daily and periodic data collected in 1973-83 and 1991-96 [FOR, period of record; N, number of samples] Period of record, daily Sampling USGS streamflow and Periodic water-quality sampling site station Stream name conductance (fig-1) number 1973-83 1991-96 1973-83 1991-96 POP N POP N LC 09152650 Leach Creek 04/73-09/77 None 08/73-07/77 28 03/91-12/96 32 BSW 09153270 Big Salt Wash 03/73-09/77 None 08/73-07/77 29 03/91-12/96 20 RW 09153300 Reed Wash 04/73-09/83 None 08/73-10/82 48 03/91-12/96 39 SC 09163490 Sail Creek 04/73-09/83 None 08/73-09/83 53 03/91-11/96 30 irrigation season encompasses solids load to streamflow, and the in annual loads ranged from about about 59 percent of the year, the second equation relates dissolved- 4 to 8 percent (table 2). Using the nonirrigation season about 41 per­ solids load to streamflow and periodic method, the annual dis­ cent of the year). The reason for specific conductance. The equa­ solved-solids loads for Leach using a seasonally weighted mean tions then are used to compute Creek and Big Salt Wash were less is because of the large differences daily loads for the time period of than the annual loads computed in streamflow and dissolved-solids interest using the daily streamflow using the SLOAD method; the loads between the irrigation and record and, if available, daily spe­ opposite was true for Reed Wash nonirrigation seasons in many cific conductance. The daily loads and Salt Creek (table 2). For the Grand Valley streams and washes. are summed to give monthly and four stations, the annual loads If the periodic sampling was biased annual dissolved-solids loads. based only on periodic data are not to one of the seasons (for example, Dissolved-solids loads computed substantially different than the if most of the samples were col­ using the SLOAD method should annual loads computed using lected in the irrigation season), be a more accurate estimate of SLOAD. The implication is that then an arithmetic mean on all the dissolved-solids loads when com­ the periodic dissolved-solids-load samples might result in an unrealis­ pared to the periodic method data were collected over the gen­ tic mean dissolved-solids load, if because much more detailed data eral range of streamflow and dis­ that mean is assumed to represent (daily streamflow and specific solved-solids concentrations for the daily load throughout the year. conductance) are used instead of several years at these four sites and Using a seasonally weighted mean only periodic dissolved-solids data. that fairly reliable estimates of removes seasonal sampling bias. A comparison of the two methods annual dissolved-solids loads can Once the seasonally weighted is provided in the next section of be obtained without having daily mean is calculated, the resultant this fact sheet. streamflow data. A disadvantage of mean load is assumed to represent using only periodic samples is that the mean annual dissolved-solids runoff from large storms may be load and is multiplied by 365.25 Comparison of Methods missed.