Predicting combined effects of land use and climate change on river and stream salinity

Abstract:

Agricultural, industrial, and urban development have all contributed to increased salinity in streams and rivers, but the likely effects of future development and climate change are unknown. We developed two empirical models to estimate how these combined effects might affect salinity by the end of this century. The first model predicts natural background specific electrical conductivity (EC, a measure of salinity) from static (i.e., geology, long-term atmospheric deposition, and soils) and dynamic (i.e., climate and vegetation) environmental factors. This model explained 78% of the variation in EC among minimally impacted sites. We applied this model to 2001 sites chosen probabilistically from all streams in the conterminous US and compared estimated background EC to current measurements. At 34% of these sites EC was > 50% greater than the predicted natural background. The second model predicts deviation of EC from natural background as function of land use and other static and dynamic environmental factors. This model explains 60% of the variation in deviation from natural background. We then predicted the effects of climate change on EC at end of the century by replacing dynamic variables (climate and land use) in both models with published projections of future conditions based on the A2 emissions scenario used in global climate models. By end of century, EC is expected to increase 234 µS/cm on average with over 50% of streams having > 50% increases in EC and 35% more than doubling their EC. However, most of the change is due to development, with climate change accounting for less than 10% of the increase. Previous changes in EC have occurred primarily in the Midwest and Plains, but future changes shift to the South. In extreme cases, increased salinity may make water unsuitable for agricultural or municipal use. However, widespread moderate increases are a greater threat to stream biota due to the elimination of low EC habitats.