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Studies of Sources and Transport of Dissolved Solids (Salt) in the Colorado River Basin using the Spatially Referenced Regressions on Watershed Attributes (SPARROW) Model

The Upper Colorado River Basin (UCRB) encompasses about 112,000 mi2 and discharges more than 6 million tons of dissolved solids (salt) annually to the lower Colorado River Basin. It has been estimated that between 32 and 45 percent of this salt originates from irrigated agricultural land sources, which compose less than 2 percent of the land area of the UCRB (Kenney and others, 2009; Miller and others, 2017).  High salt concentrations in the Colorado River are a concern because they result in substantial economic damages to water users, primarily in reduced agricultural crop yields and corrosion of water systems for irrigation and public supply. Damages due to salinity in the Colorado River are estimated to be greater than 300 million dollars annually. Multiple studies, starting in 2007, have used the SPARROW model to help identify sources and transport od dissolved solids in the Colorado River Basin. 

Figure 1. Study area and stream-monitoring sites in the Upper Colorado River Basin (UCRB).

Figure 1. Study area and stream-monitoring sites in the Upper Colorado River Basin (UCRB). (Public domain.)

The occurrence and distribution of dissolved solids (often simply referred to as ‘salt’) in surface and ground water of the Upper Colorado River Basin (UCRB) has been extensively studied and characterized in a number of investigations completed in the 1970s and 1980s. Increasing dissolved-solids concentrations in the Lower Colorado River Basin and their associated adverse economic impact led to the enactment of the Colorado River Basin Salinity Control Act in 1974 and the establishment of water-quality criteria for salinity in the Colorado River system (Colorado River Basin Salinity Control Forum, 2005). This in turn, spurred many studies of dissolved solids in the UCRB (U.S. Department of the Interior, 2003). 

As a result of these early studies, a solid conceptual understanding of the sources and transport mechanisms of dissolved solids in the basin has been developed. (see figure 2 from Kenney and others, 2009)  

Conceptual model illustrating the processes by which dissolved solids are generated and transported.

Figure 2. Conceptional model illustrating the processes by which the dissolved-solids are generated and transported to streams in the Upper Colorado River Basin (UCRB). (Public domain.)

A regional study of dissolved solids in surface water of the southwestern U.S. by Anning and others (2007), included a dissolved-solids Spatially Referenced Regressions on Watershed Attributes (SPARROW) surface-water quality model and provided a framework for a finer-scale SPARROW modeling effort specific to the UCRB.

Figure 3. Irrigated agricultural lands in the Upper Colorado River Basin (UCRB) and associated lithologic classification group

Figure 3. Irrigated agricultural lands in the Upper Colorado River Basin (UCRB) and associated lithologic classification group

(Public domain.)

 

The SPARROW model applied statistical analysis to the dissolved-solids supply and transport within the UCRB. Model results indicated that of the seven geologic source groups defined, the high-yield sedimentary Mesozoic rocks had the largest yield of dissolved solids, about 41.9 tons/mile2 (fig.3). Irrigated, sedimentary-clastic Mesozoic lands had an estimated yield of 1,180 tons/mile2. In this study, the dissolved-solids contribution of irrigated agricultural lands and natural sources were found to be about 45 and 57 percent, respectively at Colorado River at Lees Ferry, Arizona, the downstream monitoring site at the boundary of the Upper and Lower Colorado River Basins. (Kenney and others, 2009) 

Between 1988 and 2012, the Colorado River Basin Salinity Control Program spent between $10 million and $60 million annually on salinity control projects aimed at reducing salinity loads in surface waters of the Colorado River Basin (U.S. Bureau of Reclamation, 2013). Because optimal management and (or) mitigation of salinity requires a sound understanding of the spatial distribution of salinity sources, load accumulation, and transport mechanisms, the UCRB dissolved-solids SPARROW model was updated and further enhanced with streamflow and dissolved data concentrations collected during 1984−2012. The updated model also defined basin attributes not available for use in the previous SPARROW model, including delineation of irrigated lands by irrigation type (sprinkler or flood). (Miller and others, 2017) 

Table 1. Estimated annual total dissolved-solids loads from sources in the Upper Colorado River Basin.

Table 1. Estimated annual total dissolved-solids loads from sources in the Upper Colorado River Basin. (Public domain.)

Eleven sources of dissolved solids in the basin were included in the model: seven geologic source groups, three irrigated agricultural land source groups, and one point source associated with saline springs (table 1). Seventy-eight landscape transport characteristics representing climatic, physical drainage basin, land cover, and soil characteristics that conceptually, may play a role in the delivery of

 

dissolved solids from sources to streams, were tested for potential inclusion in the model. The larger yields from irrigated agricultural lands relative to geologic sources, and the larger yields from flood irrigated lands relative to sprinkler irrigated lands are consistent with the conceptual understanding of dissolved-solids sources in the UCRB. (Miller and others, 2017)

Table 2. Estimated annual total dissolved-solids loads and yields from watersheds in the Upper Colorado River Basin.

Table 2. Estimated annual total dissolved-solids loads and yields from watersheds in the Upper Colorado River Basin. (Public domain.)

The enhanced SPARROW model estimated that approximately 6.4 million tons/year of dissolved solids are delivered from the UCRB to the Lower Colorado River Basin (table 2). Yields generated from irrigated agricultural lands were found to be substantially greater than those from geologic sources, with sprinkler irrigated lands generating an average of approximately 150 tons/mi2 and flood irrigated lands generating between 770 and 2,300 tons/mi2 depending on underlying lithology. At the basin scale, the model estimated that 32 percent of the dissolved-solids loads are from irrigated agricultural land sources that compose less than 2 percent of the land area in the UCRB. Notably, study results indicate that the conversion from flood irrigated agricultural lands to sprinkler irrigation on agricultural lands is a likely process contributing to a decrease in dissolved-solids loads from irrigated lands. (Miller and others, 2017)

 

 

Results of these studies involving the SPARROW model are being used by Federal, State, and local resource managers to reduce the economic and ecological impacts of salinity in the Colorado River. New SPARROW modeling work is underway to estimate time-variable salinity loading from irrigated agricultural lands.  Salinity of the Colorado River is one of the most complex water issues in the west. Unraveling the mysteries of this dynamic system will require forward-thinking scientists and ongoing studies to find meaningful ways to reduce the salinity for its many users.