Date of Graduation

5-2025

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Speir, Shannon

Committee Member

Brye, Kristofor R.

Second Committee Member

Daniels, Michael B.

Third Committee Member

Muenich, Rebecca

Keywords

Biogeochemistry; Pastureland; Stormflow; Unpaved roads; Water Quality

Abstract

Climate change is expected to increase flow variability and intermittency in streams, changing the magnitude and timing nutrient and sediment delivery. This may exacerbate water quality issues in agricultural landscapes, where nutrient and sediment inputs are often elevated. Elevated nutrients can drive eutrophication in downstream waterways, and excess sediment loss can decrease primary productivity. Non-perennial headwater streams are particularly vulnerable to the impacts of climate change and agriculture as they serve as key sites for nutrient cycling at the terrestrial-aquatic interface. Despite their importance, the interactive effects of agriculture and stream intermittency, as well as unpaved roads in rural watersheds are understudied.

Here, we sampled the Brush Creek watershed (5,165 ha), a non-perennial tributary of Beaver Lake Reservoir, Northwest Arkansas’s primary drinking water source. In Brush Creek, ~45% of land cover is dedicated to poultry production and pastureland, and there are 190 km of unpaved roads. I conducted monthly baseflow sampling for nitrate (NO3--N) and soluble reactive phosphorus (SRP) export from October 2023-September 2024. While both NO3--N and SRP increased moving downstream as surface connectivity increased, I found that only NO3--N loads increased simultaneously with percent of upstream sites flowing. I also found NO3--N loads were higher in subwatersheds with greater %pasture/hay, suggesting that fertilizers may be a potential NO3--N source in the watershed. My results indicate surface connectivity in Brush Creek controls the timing and spatial patterns of nutrient loss, where %pastureland controls the magnitude of NO3--N loss. Overall, my findings suggest management should focus on periods of hydrologic reconnection and in subwatersheds with more pasturelands to effectively protect downstream water quality.

Beginning in February 2024, I also sampled at baseflow monthly for total suspended sediments (TSS) and collected samples opportunistically during four storm events to understand local (i.e., type of road crossing) and watershed-scale (i.e., land cover/use) controls on sediment loss from unpaved roads. Surprisingly, mean TSS yields downstream vs. upstream of road crossings were comparable, especially at bridge and culvert sites, indicating these types of road crossings may not be critical sources of TSS to streams. At the watershed scale, we found that TSS load increased as the total length of unpaved roads and area of pastureland in a subwatershed increased (Simple Linear Regression; R2=0.24, p<0.001 for both). Finally, we documented higher TSS yields during stormflow than at baseflow (26.87 ± 6.82 vs. 0.38 ± 0.04 kg km-2 day-1; unpaired t-test, p<0.01). Our findings indicate that unpaved road networks are key contributors to sediment export in rural watersheds. As such, BMP implementation should focus on unpaved roads to effectively protect downstream water quality in regions with extensive unpaved road networks.

My thesis highlights the importance of targeted BMPs in agricultural areas with intermittent streams. These practices should focus on periods of hydrologic reconnection, pasture-dominated subwatersheds, and managing sediment loss from unpaved roads to mitigate both nutrient and sediment loss. My research supports the importance of proper land management for protecting downstream water quality as climate change magnifies flow variability and intermittency.

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Available for download on Wednesday, June 17, 2026

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