Quantifying the Impact of Climate Extremes on Salt Mobilization and Loading in Non-Developed, High Desert Landscapes

Abstract

Excess salt loading acts as a chemical stressor in water bodies and can have significant impacts on water quality. High salinity threatens sustainable crop production globally and is especially prevalent in semi-arid and arid regions. For this reason, salt transport in irrigated semi-arid and arid regions has been intensively studied. However, comparatively little research has been conducted to evaluate the salinity contributions of dominantly non-irrigated basins, and to my knowledge, no previous research has evaluated the changes in salt loads from upland semi-arid catchments in the face of climate change and extreme climate events.

This research utilizes the Soil and Water Assessment Tool (SWAT) and a coupled salinity module (SWAT-Salt), applied to a natural watershed, to fill this knowledge gap. SWAT-Salt simulates the reactive transport of 8 major salt ions, SO₄^2-, Cl^-, CO₃^2-, HCO₃^-, Ca²⁺, Na^+, Mg²⁺, and K⁺, in the soil-aquifer-stream system of a watershed, with salt mass transported via major hydrologic pathways (surface runoff, percolation, recharge, soil lateral flow, upflux, and groundwater discharge). Specifically, this study has two major research objectives: 1) develop an accurate SWAT-Salt model that can estimate salinity loads from a largely undeveloped, upland desert catchment, the Purgatoire River Basin in Colorado, USA; and 2) quantify changes in predicted salt loads in the Purgatoire River Basin with increasing storm intensity.

The SWAT-Salt model developed in this study was used to evaluate the contribution of salt to the Arkansas River from the Purgatoire River, a dominantly non-irrigated desert catchment in southeastern Colorado. The ~8,935 km² Purgatoire River basin is susceptible to high salt transport due to very high topographic slopes, dry climatic conditions, and sparse vegetation. Much of the natural salt in this region has been deposited from 20,000 years of weathering the Mancos Shale formation. Calibration and validation of the salinity module was evaluated through comparisons of measured and simulated in-stream loads of individual salt ions during the period 1990-2010. Model results indicate that 76% of the salt in the Purgatoire River comes from groundwater sources, and ~24% of the salt comes from landscape soil lateral flow. Sulfate, calcium, and bicarbonate account for ~56%, ~20%, and 14% of the total salt load, respectively. The impact of climate change on salt transport and mobilization was evaluated through model scenarios of increasing storm intensity (5% and 35% increases in daily precipitation for the most extreme storms) congruent with global climate models.

Results suggest that if the largest storm events increase in intensity by the maximum predicted value of 35%, the total salt mass exported from the Purgatoire River watershed would increase by 73%. If the largest storm events increase in intensity by the median predicted value of 5%, the total salt mass exported would increase by 12%. Similar results are expected but should be evaluated for other upland desert catchments. From this thesis, I conclude that: 1) natural, largely undeveloped basins can export significant salt loads to downstream agricultural regions; 2) Future increasing storm intensity with changing climatic conditions can have a large impact on salt exports from high-desert landscapes; and 3) process-based models such as SWAT-Salt can be valuable in evaluating salt loadings from high-desert watersheds and can be applied to other watersheds worldwide.