Quantifying the impact of climate extremes on salt mobilization and loading in non-developed, high-desert landscapes...

Excess salt loading from watershed landscapes into river systems acts as a chemical stressor in water bodies and can have significant impacts on downstream water quality. High salinity threatens sustainable crop production globally and is especially prevalent in semi-arid and arid regions. However, relatively little research has been conducted to evaluate salt movement and loadings in natural, high-desert catchments in the face of climate change and extreme climate events. In this study, we use the watershed model SWAT and a newly developed salinity module to simulate the mass transport of 8 major salt ions, SO₄^2-, Cl^-, CO₃^2-, HCO₃^-, Ca²⁺, Na⁺, Mg²⁺, and K⁺, in the soil-aquifer-stream system of the Purgatoire River Watershed (PRW) (Colorado, USA, 8935 km²) via major hydrologic pathways (surface runoff, percolation, recharge, soil lateral flow, groundwater upflux, groundwater discharge) and quantify changes in predicted salt loads with possible future increasing storm intensity. The PRW is susceptible to high salt transport due to high topographic slopes, dry climatic conditions, and sparse vegetation, and loads to the Arkansas River, a major source of irrigation water in the Arkansas River Basin. From study results we conclude that 99% of salt in the Purgatoire River originates from subsurface water pathways (soil lateral flow, groundwater flow), composed primarily of SO₄^2-, Ca²⁺, and HCO₃^-. If intensity of large storms increases by 5% and 35%, the total salt mass exported from the watershed increases by 12% and 73%, respectively, indicating large influxes of legacy salt from the soil-aquifer system. For baseline and storm intensity scenarios, the PRW contributes significant salt loads to agricultural regions via the Arkansas River, highlighting the need for basin-wide salt management strategies to include upland desert regions. We expect these results, and associated consequences for salt management, to be similar for other upland desert catchments worldwide.