CLIMATE VARIABILITY AND SALINIZATION OF THE PRAIRIES: SNOWMELT VS RAINFALL

The semi-arid glaciated plains of the North American continent, also known as the prairies, are a complex hydrological system characterized by hummocky terrain, where wetlands containing seasonal or semi-permanent ponds, occupy the numerous topographic depressions. The prairie soils and water bodies contain high salt concentrations; mainly sulfate salts and due to limited hydraulic connection between the wetlands, the salts are usually cycled within the wetland between the wetlands ponds and uplands. In this research we explored salt transport processes under extreme wet climatic conditions. The period between 2000 and 2012 was characterized by an unusual degree of climatic variability, including severe floods and droughts, and this had a marked effect on the spatial distribution, water levels and chemical composition of the ponds.

We used field observations from the St. Denis National Wildlife Area, Saskatchewan, taken over the last 20 years, to quantify the salt transport processes. Measurements include meteorological data, soil moisture, salinity and chemical composition, groundwater levels and pond water volume, salinity, and chemical composition. The record includes periods of exceptional snow and periods of exception summer rainfall, both of which resulted in unusually high pond water levels. Measurements indicated that severe pond salinization only occurred in response to extreme summer rainfall. It is hypothesized that since rainfall water infiltrates through the soil towards the depressions, while snowmelt water flows mainly as surface water over frozen soils, they have markedly different impacts on salt transport and pond salinization. Under rainy summer conditions hydraulic head gradients were reversed and subsurface water infiltrated and washed salts from the uplands to the ponds. Moreover, it was observed that for extremely wet conditions subsurface hydraulic connections may be formed between adjacent ponds which result in salt transport from the upper to the lower lying ponds. All of these processes were observed in various temporal and spatial scales, emphasizing how important it is to understand these processes to better estimate the risk and mitigate the impacts of future soil and water salinization.