Substantial Declines in Salinity Observed Across the Upper Colorado River Basin During the 20th Century, 1929 to 2019

Abstract:

Salinity has a major effect on water users in the Colorado River Basin, causing an estimated $300 to $400 million per year in economic damages. In response, the Colorado River Basin Salinity Control Program implements and manages projects to reduce salinity (dissolved solids) loads, investing millions of dollars per year in improved irrigation infrastructure, vegetation management, and other mitigation strategies. To inform and improve mitigation efforts, there is a need to better understand long-term changes in salinity, and in particular, characterize trends in salinity that took place prior to the implementation of salinity control projects. This study uses long-term datasets of dissolved-solids and specific-conductance measurements, collected as early as 1929, to explore long-term trends in dissolved solids in streams of the Upper Colorado River Basin. A statistical trend analysis using weighted regressions on time, discharge, and season (WRTDS) was used to evaluate changes in dissolved-solids loads and concentrations at 14 stream sites using data from 1929 to 2019, including time periods prior to the construction of large dams and prior to the implementation of salinity control projects. Results indicate that substantial and widespread downward trends in dissolved-solids loads and concentrations have occurred over the last 60 to 90 years. For 12 of the 14 sites (86%), decreases in flow-normalized dissolved-solids loads and concentrations were observed, with median declines of -40% (range of -9 to -65%) and -38% (range of -14 to -57%) for load and concentration, respectively. The steepest and most widespread declines in dissolved-solids loads and concentrations occurred from 1980 to 2000, possibly resulting from salinity mitigation efforts that began in the 1980s. Notably, the pace and extent of these decreases declined sharply between 2000 to 2019, even though salinity control activities were ongoing. This, in addition to observed decreases in dissolved solids beginning as early as the 1940s at some sites, suggests that dissolved solids are not only affected by salinity control efforts, but also respond to broad, regional changes in land cover, land use, and/or climate-related factors.