Farmers can implement conservation practices, such as changing winter land cover by planting cover crops, to reduce nutrient losses from fields and prevent eutrophication of downstream water bodies. Yet, we currently lack an understanding of how the impacts of conservation practices manifest across scales. For example, in the Shatto Ditch Watershed (SDW; Indiana) the planting of cover crops decreased field-scale nitrate (NO3-N) loss from subsurface tile drains by 69-90%; however, watershed NO3-N export only decreased by an average of 13%.
In this study, I explored how seasonal groundwater contributions to streamflow may influence the impacts of cover crops (e.g., annual ryegrass) at the watershed-scale in the SDW. I sampled five times from November 2018 to September 2019, collecting discharge measurements, stable isotope samples, and nutrient samples from tile drains, longitudinal stream sites, and groundwater wells. I analyzed oxygen and hydrogen stable isotope signatures of stream and tile drain water and used end-member mixing models to determine the proportion of groundwater versus precipitation in each sample. Using the discharge measurements, I also developed a water budget to estimate the groundwater contribution to streamflow. Finally, I analyzed water samples for NO3-N and ammonium (NH4+-N) to estimate the amount of nitrogen (N) coming from groundwater via a nutrient budget approach.
With isotope mixing models, I found that streamflow primarily consisted of groundwater, and groundwater contributions to streamflow varied seasonally (mean=83%, range=71-90%). I also observed seasonal variation in the groundwater contribution to streamflow with the water budget (mean=44%, range=14-70%). I created a hybrid model, combining stable isotope and water budget results, to estimate the amount of tile drain flow derived from groundwater, revealing the total groundwater input to the stream (mean=89%, range=83-97%). Results from the N mass balance indicated high groundwater NH4+-N concentrations may contribute to stream NO3-N export via nitrification. Although groundwater made up the majority of the streamflow, the majority of the NO3-N came from the tile drains. Further exploration revealed three large, county drains that were excluded from the original analysis contribute between 28-76% of total NO3-N to streamflow and may account for the lack of NO3-N reductions perceived at the watershed scale. My study shed light on the complex interactions between nutrient and water sources and biogeochemical transformations in streams and provided an expanded understanding of the effect of scale on detecting the impacts of conservation efforts.