Date of Award

January 2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geological Engineering

First Advisor

Taufique H. Mahmood

Abstract

In Devils Lake ND, a terminal lake in the Northern Great Plains, algae blooms are of great concern due to the recent increase of streamflow and subsequent elevated concentrations of nutrients, particularly phosphorus. To date, very few studies explore P concentration to streamflow relationship in cold region agricultural basins, specially 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 2016-2018 period. This study presents high-resolution P observation data during the first flush of snowmelt runoff while identifying the controlling factors of P exports using both field-based observations and hydroclimatic variability detected by physically based hydrologic simulations. United States Geological Survey provided the streamflow measurements, and I collected water samples (filtered and unfiltered) three times daily during the spring snowmelt seasons of 2017 and 2018 and analyzed for P. Based on P concentration in soil and accumulated snow, the soil is the most likely source of observed P in the stream waters. 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 (2016-17 and 2017-18) expresses near chemostatic concentration to streamflow relationships in the rising limb of the hydrograph, while concentration to streamflow relationship correlate positively in the descending limb. The poor correlation in the rising limb of the hydrograph suggests that the excessive amount of snow water equivalent and the existence of basal ice limit the contact time between meltwater and soil

resulting in inconsistent P export. In 2018, Tributary 3 basin showed a linear and positive concentration to streamflow relationship in both rising and descending limbs suggesting extensive flushing of P from soil. Frozen soil conditions simulated by a physically based model show strong correlations with the observed concentration to streamflow relationships suggesting that the extent of frozen soil conditions can partly explain the extent of chemostatic behavior. Higher annual export loads per unit basin area were observed in Tributary 3 in both field seasons partially due to the heavily cultivated and tilled soil of agricultural lands. Tillage indexes suggest that there are more exposed soil and less crop residues in Tributary 3 basin, while the Mauvais Coulee basin has more crop residue and less exposed soil. This suggests that the volume of runoff and the land management practices are contributing factor in total load export.

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