Intra-annual Freshwater Salinization Regimes in Rivers Draining the Arid and Semi-arid Western United States

Abstract:

The salinization of rivers is increasing across the globe due to non-point source loading of base cations, anions, and carbonates that originate from accelerated weathering and anthropogenic activities such as agricultural and urban land use. Although we are gaining a better understanding of decadal trends in river water quality, our understanding of intra-annual regimes -- namely the timing, magnitude, and duration of characteristic patterns within a year -- of salinization in rivers draining diverse dryland landscapes and their relationship to catchment land-use is limited. Here we classify intra-annual salinization regimes in rivers draining arid and semi-arid landscapes through a combination of dynamic time warping and hierarchical agglomerative clustering of publicly available time series of continuous specific conductance (a proxy for dissolved salts) and discharge as well as point measurements of water chemistry. We compiled publicly-available data from U.S. Geological Survey (USGS) gages with both discharge (Q) and specific conductance (SC) daily time series, as well as nearly 60,000 USGS and >3 million Water Quality Data Portal (WQP) point measurements of SC, Mg, Cl, Na, K, alkalinity, pH, salinity, and Q across 57,213 locations in the Great Basin and Colorado River Basin from 1900 to 2019. Using daily SC data from 166 USGS gages from 1979-2020, we found that 42 sites reached maximum annual SC in December of most years, followed by 18 sites reaching maximum SC in both October and November, and ≤ 17 gages reaching maximum SC in any other month of the year. We further investigate the relationship between salinization regimes determined by data-based hierarchical clustering and variation in the composition and configuration of land-use in the contributing watersheds. Advancing our understanding of the dynamics of salinization in rivers will improve our ability to predict and mitigate the effects of excess salinity on lotic freshwater ecosystems and the ecosystem services they provide.