Trends in nutrient and soil loss in Illinois rivers, 1978–2017

Nutrient and soil loss, defined herein as the loss of nutrients or soil to streams and other downstream receiving waters, affect watersheds around the globe. Although governments make large investments mitigating nutrient and soil loss through watershed management efforts, the efficacy of these efforts is often difficult to assess, in part because streamflow variability obscures the effects.

This study investigates the effects of watershed management on nutrient and soil losses in the State of Illinois during two periods: 1978 to 2017, and 2008 to 2017. The former period provides an important test case for assessing the efficacy of major Federal programs like the Clean Water Act and the Conservation Reserve Program at mitigating nutrient and soil loss, whereas the latter spans the years after these policies were well established, thereby providing an assessment of whether these programs have kept pace with ongoing trends in climate and watershed management.

The effect of interannual streamflow variability on long-term nutrient and soil loss trends was removed using an extension of the Weighted Regressions on Time, Discharge, and Season methodology, called generalized flow normalization. This process also partitions trends into components attributable to long-term changes in streamflow and watershed management. The Weighted Regressions on Time, Discharge, and Season trend analysis indicated significant, widespread trends in nutrient and soil loss in Illinois since 1978. From 1978 to 2017, improvements in watershed management reduced nitrogen and soil loss from watersheds within Illinois, but this effect was partially or entirely negated by increasing losses due to changing streamflow. During the same period, phosphorus loss also increased owing to a combination of inadequate management efforts and changing streamflow. During 2008–17, however, nutrient and soil losses have all accelerated, threatening to undo previous reductions if the current trends continue.