Impacts of cold region hydroclimatic variability on phosphorus exports: Insights from concentration-discharge relationship

Abstract

In lakes of the Northern Great Plains, algae blooms are of great concern due to the recent increase of streamflow resulting in influx of nutrients, particularly phosphorus (P). The cold sub-humid climate and snowmelt hydrology exert strong controls on P concentration, export and supply in the basin. To date, few studies have explored the impacts of hydroclimatic conditions on P concentration, export and concentration-to-streamflow relationships in cold region agricultural basins, and particularly in the headwater catchments of the Devils Lake Basin. This study gains a better understanding of the impacts of hydroclimatic variation on concentration-to-streamflow relationships between two headwater catchments (Mauvais Coulee: 1032 km2 and Tributary 3: 160 km2) draining to Devils Lake during the 2017–2018 period. High-resolution P observations during the first flush of snowmelt runoff are presented. The controlling factors of P exports using both field observations and hydroclimatic variability detected by physically-based hydrologic simulations are investigated. Total P is dominated by dissolved P, with little contribution from particulate P, which is presumed to be locked in an ice matrix in frozen soils. The Mauvais Coulee Basin expresses near chemostatic concentration-to-streamflow relationships in the rising limb of the hydrograph, while concentration correlates positively to streamflow in the descending limb. Our study period has two contrasting water years; a wet year (2016–17) and a dry year (2017–18). The wet year experienced high SWE, snowfall, maximum frozen soil conditions and a thick basal ice layer resulting in high snowmelt streamflow, while the dry year is characterized by lower SWE and absence of frozen soil/basal ice causing very little streamflow. During the wet year lower mean P concentration are observed compared to the dry year due to the presence of frozen soil/basal ice during melt period. However, the P export is much higher in wet year due to large snowmelt streamflow volume. Both chemostatic and flushing streamflow-to-concentration relationships are observed in both years. Thus, the impacts of hydroclimatic conditions to the concentration-to-streamflow relationship are inconclusive. Field observations of more extended time periods will be useful to better understand the influence of hydroclimatic variability on the concentration-to-streamflow relationship.