Headwater stream condition and nutrient runoff: relating the soil and water assessment tool (SWAT) to empirical ecological measures in an agricultural watershed in pennsylvania

Authors: HIRT, CLAIRE - Pennsylvania State University; Veith, Tameria - Tamie; COLLICK, AMY - University Of Maryland Eastern Shore (UMES); YETTER, SUSAN - Consultant; BROOKS, ROBERT - Pennsylvania State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2020
Publication Date: 3/9/2020
Citation: Hirt, C.C., Veith, T.L., Collick, A.S., Yetter, S.E., Brooks, R.P. 2020. Headwater stream condition and nutrient runoff: relating the soil and water assessment tool (SWAT) to empirical ecological measures in an agricultural watershed in pennsylvania. Journal of Environmental Quality. 1-20. https://doi.org/10.1002/jeq2.20032.
DOI: https://doi.org/10.1002/jeq2.20032

Interpretive Summary: Stream health is often measured by the numbers and types of larval insects found in the stream waters. Improving the stream conditions for these insects requires land managers of upslope fields to control and reduce losses of excess nutrients and sediment from the fields since these nutrients and sediments can enter the streams and lower the stream water quality. Unfortunately, the exact ways in which these nutrients and sediments harm the insects are unknown. That is, the connection is unclear between high quality stream health and the timings, combinations, and amounts of nutrients and sediments entering the stream. In this study, nutrient and sediment losses from agricultural practices were modeled using a detailed computer program. Insects were sampled at multiple sites throughout the watershed. Results of the computer modeling and insect sampling were compared and evaluated to better understand the connections between nutrient and sediment loadings into the stream and the health of the stream insect populations. Findings are important in helping us maintain and improve the quality of our agricultural streams.

Technical Abstract: Managing nonpoint sources of nutrients and sediments are the primary challenges for improving conditions within the Susquehanna-Chesapeake Basin. Aquatic macroinvertebrates are widely-used indicators of stream ecological integrity, but the relationship between nutrient runoff and macroinvertebrate response remains indistinct. Logistical and financial hurdles hinder collection of high-resolution empirical nutrient data, but landscape-based models like the Soil and Water Assessment Tool (SWAT) offer a more practical approach. SWAT was used to simulate nutrient runoff in a small, upland, agricultural Pennsylvania watershed that drains into Mahantango Creek, a tributary to the Susquehanna River. We used three levels of ecological assessment to interpret the SWAT modeling results. Macroinvertebrate communities (intensive reach assessment) were sampled at 14 sites in 2014 and described using an Index of Biotic Integrity (IBI). Biological integrity was moderately degraded in many reaches. The Stream–Wetland–Riparian (SWR) Index (rapid buffer assessment) and landscape metrics (remote-sensing assessment) were used to assess environmental stressors, and both indicated prevalence of agricultural stressors. Baseflow nitrate grab samples, collected once per season, showed no significant relationship with IBI score, indicating that sporadic nutrient sampling is inadequate for assessing ecological impacts. Thirty spatiotemporal scales of nutrient data were extracted from SWAT, and three nutrients were considered: phosphorus, nitrate, and organic nitrogen. Best Subsets Regression was performed on IBI scores using SWAT (five representative variables), land cover, and SWR variables. Modeling results were highly significant (p < 0.001) with high R2 values (84.8 and 86.2), showing a significant negative relationship between instream nutrient concentration and IBI score. This study demonstrates the viability of SWAT as an alternative to in-field nutrient sampling, the value of spatiotemporal scale in model outputs, and the importance of site condition variables in relating nutrients to stream ecological health.